Residuos plásticos: estudio de la viabilidad de su reciclado en materiales compuestos para aplicaciones constructivas. Propuesta de productos de yeso con residuos plásticos para sistemas de tabiquería y revestimiento

TESIS DOCTORAL / PHD DISSERTATION PLASTIC WASTE: Study of its recycling viability in composite materials for construction applications. Proposal of gypsum-based product with plastic waste content for partition walls and covering systems. Estudio de la viabilidad de su reciclado en materiales compuestos para aplicaciones constructivas. Propuesta de productos de yeso con residuos plásticos para sistemas de tabiquería y revestimiento. RESIDUOS PLÁSTICOS Autora / Author: Mª Isabel Romero Gómez Sevilla, 2023 Directores / Supervisors: Dra. Paloma Rubio de Hita / Dra. Carmen Rodríguez Liñán / Dr. Diego Frustaglia RESIDUOS PLÁSTICOS: Estudio de la viabilidad de su reciclado en materiales compuestos para aplicaciones constructivas. Propueta de productos de yeso con residuos plásticos para sistemas de tabiquería y revestimiento. PLASTIC WASTE: Study of its recycling viability in composites materials for constructions applications. Proprosal of gypsum-based composites with plastic waste content for partition walls and covering systems. Mª Isabel Romero Gómez. Sevilla, 2023 Tesis doctoral realizada en el seno del Departamento de Construcciones Arquitectónicas I y del grupo de investigación PAIDI TEP-205 “Análisis y Evaluación de Sistemas Constructivos y Estructurales en la Arquitectura”, adscrito al Instituto Universitario de Arquitectura y Ciencias de la Construcción (IUACC) de la Universidad de Sevilla. Los grandes cambios nacen de pequeñas acciones... AGRADECIMIENTOS Antes de comenzar con el cuerpo principal de la presente tesis doctoral, me gustaría dedicar unas palabras de agradecimiento a aquellas personas que en mayor o menor medida han contribuido a que esta investigación llegue a buen término: En primer lugar, a mis directores de tesis, especialmente a la Profesora Paloma Rubio de Hita, por darme la oportunidad e iniciarme en el maravilloso mundo de la investigación. Así como por sus útiles consejos a nivel personal y profesional, despertando en mí el gusto por la docencia. A mis compañeros del grupo TEP 205 por su cariñosa acogida, en especial a Alejandro y Mª Jesús, por el persistente apoyo emocional, asistencia y cercanía. A los profesores del Insituto Superior Técnico de Lisboa Jorge de Brito, Inês Flores-Colen, Rui Vasco Silva y Manuel Franciasco Costa Pereira por el asesoramiento, apoyo y ayuda ofrecida, así como por la confianza establecida en mí. Sin duda, trabajar con vosotros ha sido una de las experiencias más enriquecedoras de mi carrera investigadora. Al personal del Laboratorio de Construcciones Arquitectóncias, por las incalculables horas de trabajo, en las que sin vuestra ayuda, esta investigación no habría sido posible. A mis amigas y compañeras de fatiga en esta durísima etapa predoctoral, sin cuyo apoyo emocional, consejos y oxigenación hubiése sido imposible concluir este trabajo. A mis chalecos salvavidas, gracias por ser claridad en mis tormentas. A esa persona que sin haber estado desde siempre, se ha cruzado en mi vida, complementándome y haciendo de ella una continua aventura, por sus incansables muestras de cariño y ánimo. Por último, y no menos importante, a mi familia, especialmente a mis padres, pilar fundamental de mi vida, por su incansable apoyo en cada caída y alegría inconmensurable con cada uno de mis logros. No puedo olvidar tampoco a esas estrellas que ya no estan con nostros, pero que desde el cielo guían cada uno de mis pasos. Por cada granito de arena, eternamente GRACIAS. FINANCIACIÓN Se desea expresar un agradecimiento especial a las diferentes instituciones públicas que han financiado la presente investigación. En primer lugar, citar que la autora de este trabajo ha sido beneficiaria, durante cuatro años, de un contrato predoctoral de Formación del Profesorado Universitario (FPUFPU 18/02405) concedido por el Ministerio de Ciencia, Innovación y Universidades dentro del Programa Estatal de Promoción de Talento y su Empleabilidad (convocatoria 2018). Igualmente, el citado contrato permitió la concesión de dos Ayudas a la movilidad para estancias breves y traslados temporales para beneficiarios del Programa de Formación del Profesorado Universitario del Ministerio de Ciencia, Innovación y Universidades. Con ellas, se realizaron dos estancias de invetigación predoctorales, de 3 meses de duración cada una, en el Civil Engineering Research and innovation for Sustainability unit (CERIS) del Instituto Superior Técnico (IST) de la Universidad de Lisboa (Lisboa, Portugal), durante los meses 05/05/2021 a 05/08/2021 (EST21/00483) y 03/09/2022 a 03/12/2022 (EST22/00447). Además, la trayectoria predoctoral de la autora ha estado ligada desde sus inicios a diferentes proyectos de investigación de alta afinidad con la investigación doctoral desarrollada: el proyecto I+D+I del Plan Estatal 2021- 2023 - Proyectos Investigación Orientada “Rehabilitación acústica ecoeficiente de centros escolares” (PID2021- 123192OB-I00); y el proyecto CERIS Seed Project 2020- 2023 - Acçoes exploratórias externas conducentes ao desenvolvimiento de projectos transversais “CAL-TEX: Durability of lime-based plasters with textile waste under wet/UV cycles simulating the impact of climate change” (UIDB/04625/2020), en los cuales se participa como parte del equipo de trabajo. Por último, haciendo mención a los antecedentes de la citada investigación, a partir del Trabajo Fin de Grado desarrollado por la autora, el cual inicia el interés en la materia de construcción sostenible y la propuesta de nuevos materiales de construcción a partir de residuos plásticos, cabría citar que la autora disfrutó de dos becas de Iniciación y Acceso a la investigación, provista por el VI Plan Propio de Investigación, y Transferencia, Universidad de Sevilla, así como otra de Colaboración en tareas de Investigación, financiada por el Ministerio de Educación y Formación Profesional. Ambas previas a la etapa predoctoral (período como estudiante de Grado y Máster). 14 RESUMEN La devastadora situación generada a nivel mundial por la producción incontrolada de plásticos y la ineficiente gestión de estos residuos, especialmente, los de un único uso, requiere de soluciones inminentes. En este sentido, se ve en el sector de la construcción, puntero dentro de la economía mundial e implicado en las distintas fases del ciclo de vida de los plásticos, la oportunidad de fomentar medidas que combatan la alarmante situación desencadenada por la contaminación plástica. Por ejemplo, potenciando la implementación de residuos plásticos en la fabricación de nuevos materiales ecoeficientes. Tomando como referencia la literatura existente, el principal objetivo de esta tesis es avanzar en el análisis de la viabilidad de desarrollo de nuevos compuestos a base de yeso, reemplazando parcialmente la matriz conglomerante por cinco tipos de residuos plásticos. Pese a tener distinto origen, los residuos estudiados comparten un balance negativo entre producción y reciclaje, así como su falta de inclusión en el sistema de reciclaje actual. Por tanto, representan focos de contaminación plástica de potencial crecimiento. Las fracciones de residuos seleccionados fueron: cápsulas de café monodosis (PPT), bolsas de plástico ligeras (LDPE), toallitas húmedas (PPF), colillas de cigarrillo (CA) y redes de pesca (PA6). En función de la composición y propiedades de cada tipo de plástico, se organizaron en dos grupos: partículas trituradas (PPT, LDPE) y fibras de refuerzo (PPF, CA y PA6). Los dos primeros bloques de esta investigación se enfocan en la caracterización de las propiedades físico-mecá- nica, térmica y de resistencia al agua de ambos grupos de compuestos. Se aplicaron porcentajes de reemplazo de hasta 10% para los plásticos triturados y hasta 3.5% para aquellos usados como fibra de refuerzo, ambos en relación al peso de yeso (wt%). Concluida la campaña de caracterización, se seleccionaron los compuestos y dosificaciones óptimas. En este caso, las muestras que contenían 7.5wt% PPT consiguieron las mejoras más significativas respecto al material de referencia, en tanto en cuanto, resistencia a compresión y reducción de la absorción de agua, simultáneamente. Así como, en el caso de las fibras plásticas recicladas, se seleccionaron los compuestos con un 2.5wt% PA6 como óptimos, por conseguir la mejora generalizada de las propiedades analizadas, respecto al material de referencia. Además, la adición de fibras PA6 generó un comportamiento post-fractura más dúctil, evitando la fragmentación completa de los compuestos. 15 En una tercera etapa de este trabajo, se evaluó el comportamiento de los compuestos óptimos ante situaciones de elevada temperatura (condiciones de servicio hasta 300ºC y fuego hasta 1000ºC). Como principal conclusión, se determinó la mayor afección de los procesos de deshidratación del yeso en el desgaste de las propiedades físico-mecánicas de los compuestos sometidos hasta 300ºC de exposición, en comparación con la influencia del contenido plástico. Además, en términos de reacción al fuego, los nuevos compuestos de yeso con contenido plástico (PPT y PA6) fueron clasificados como materiales no inflamables-Euroclase A2, de acuerdo con la normativa aplicable. Finalmente, en la última etapa de esta tesis, se proponen dos tipos de productos a base de yeso, a partir de los compuestos óptimos seleccionados. Por un lado, se desarrolla un prototipo de bloque de yeso alveolar (AGB) usando 7.5wt% partículas PPT y 2.5wt% fibras PA6 de reemplazo, respectivamente. Estos bloques fueron comparados con soluciones comerciales, obteniendo resultados alentadores. Mientras que el uso de PPT condujo a un producto de similares prestaciones al bloque de control, aunque con una significativa reducción de la permeabilidad al agua, la adición de fibras PA6 dio lugar a importantes mejoras respecto a la resistencia mecánica de referencia. Asimismo, una destacable reducción de la conductividad térmica fue registrada en ambos casos. Por otro lado, atendiendo al buen desempeño de las fibras PA6 para mejorar el comportamiento a flexión de los compuestos de yeso, se propone una nueva aplicación para estos, como placa de yeso para falso techo registrable (FCP). El uso de 2.5wt% PA6 de reemplazo, permitió obtener placas que cumplían todos los requerimientos de la normativa, incluso con valores de rotura a flexión superiores a la referencia, resistencia al impacto mejorada, así como una reducción significativa de la absorción de agua. Por tanto, se concluye la viabilidad de desarrollar nuevos productos ecoeficientes a base de yeso, con propiedades físico-mecánicas, térmicas, de resistencia al agua y medioambientales mejoradas respecto al material de referencia, así como un correcto comportamiento al fuego, mediante la adición de 7.5wt% PPT y 2.5wt% PA6, cumpliendo todos los requisitos establecidos por normativa. Al mismo tiempo, se propone, un nuevo medio de reciclaje alternativo a los existentes, para promover una gestión y eliminación eficiente de residuos plásticos altamente contaminantes. 16 ABSTRACT The rampant production of plastic and poor waste management, particularly with single-use plastics, have caused a global crisis. To address this issue, alternative measures must be taken, especially in the construction industry, which plays a significant role in the plastic life cycle. It is crucial to promote new recycling methods that can combat plastic pollution. One solution is to use various plastic waste to create environmentally friendly materials for building projects. Based on the previous literature, the main objective of this thesis is to make progress in analysing the viability of developing new gypsum-based composites, partially replacing the binder matrix with five types of plastic waste. Despite those residues having different sources, all of them present a negative balance between levels of production and recycling, as well as the lack of inclusion within the current recycling system, which leads to a growing pollutant trend. The fractions of plastic waste chosen were: disposable coffee capsules (PPT), light plastic bags (LDPE), non-de- gradable wet wipes (PPF), cigarette butts (CA) and discarded fishing nets (PA6). Regarding the composition and properties offered by every plastic type, they were classified into two groups: crushed particles (PPT, LDPE) and reinforcing fibres (PPF, CA and PA6). In the first and second sections of this study, physico-me- chanical, thermal, and water-resistant properties of both composite groups were analysed. The maximum replacement levels used by groups were 10% for crushed plastics and 3.5% for composites reinforced with recycled plastic fibres, by weight of gypsum (wt%). After conducting a complete characterisation campaign, the optimal composite and dosage was selected by group. Attending to gypsum partially replaced by crushed plastic residues, gypsum composites with 7.5wt% PPF content showed the most significant improvements in compressive strength and water resistance when compared with the reference material. On the other hand, the use of a 2.5wt% replacement level of PA6 fibres also led to a wide-ranging enhancement of reference properties, particularly in terms of flexural and compressive strength. Furthermore, composites reinforced with 2.5wt% of PA6 fibres exhibited a more ductile post-fracture behaviour, preventing complete material disruption. During the third research phase, the performance of the optimal gypsum composites after being exposed to high 17 temperatures in service conditions (up to 300ºC) and fire (up to 1000ºC) was evaluated. As a results, it was found that the dehydration process of the gypsum matrix had a greater impact on the physical and mechanical degradation of the composites than the plastic content when gypsum specimens were exposed to high temperatures. Regarding fire reaction, new gypsum-based composites with plastic content (PPT and PA6) were classified as non-flammable materials, A2, according to standard. In the fourth stage of this work, two types of gypsum products for building applications using the optimal composites were proposed. One of them is an alveolar gypsum block prototype (AGB) that was developed using 7.5wt% PPT and 2.5wt% PA6 fibres, respectively. Encouraging results were obtained after comparing these new blocks with other similar commercial solutions. The use of PPT gave rise a product with similar properties to reference blocks but with a notable reduction in water permeability. Meanwhile, the addition of PA6 fibres led to considerable improvements in mechanical strength. Moreover, lower thermal conductivity values were registered in both cases. Leveraging the good performance of recycled PA6 fibres to improve the flexural strength of gypsum composites, a new application for PA6-containing gypsum was proposed. This involved creating false ceiling plates (FCP) with a replacement level of 2.5wt% PA6 fibres. By doing this, the plates met all standard requirements while also improving the flexural break load and impact resistance compared to the control plates. Additionally, the water absorption capacity was reduced. This suggests that it is possible to manufacture eco-friend- ly gypsum products containing 7.5wt% PPT and 2.5wt% PA6, which meet all standard requirements and improve physico-mechanical, thermal, water-resistant and environmental properties compared to reference gypsum products. In addition, a new recycling method is proposed, which promotes efficient management and elimination of highly pollutant plastic waste. 18 19 20 21 B L O Q U E A 22 23 INTRODUCCIÓN 1 SITUACIÓN DE LOS PLÁSTICOS La actual situación del planeta, en la que el cambio climático se torna realidad, requiere de soluciones inmediatas para poner freno a la devastadora crisis medioambiental que está afectando a la vida de ecosistemas marinos y terrestres. Tal y como se expone en la declaración medioambiental publicada en noviembre de 2019 (Riple et al., 2020), gran parte de la comunidad científica proclama el estado de emergencia climática y ambiental a nivel mundial como un hecho inminente. La decisión del Parlamento Europeo de apoyar dicha proclama, mediante la aprobación del Pacto Verde Europeo, convierte a Europa en el primer continente en dar el gran paso, a fin de limitar el calentamiento global, estipulando una serie de objetivos con los que conseguir reducir e incluso neutralizar la huella de carbono para 2050 (European Comission, 2019). Entre las principales acciones que suman a la contaminación climática y ambiental del planeta, la producción incontrolada de plástico se alza como una de las actividades más nocivas. La durabilidad, estabilidad y el bajo coste de los materiales poliméricos, los han convertido en productos de necesidad cotidiana. Sin embargo, la actual incapacidad de la sociedad para deshacerse de la desmesurada cantidad de residuos plásticos generados, haciendo especial mención a los de un único uso, conduce al planeta a una situación insostenible. Más de 8.300 millones de toneladas de plástico han sido fabricadas desde que comenzase su producción en masa en los años 50. De acuerdo con datos recientes, la producción mundial de plástico en 2021 ascendió a 390.7 megatoneladas (Mt), de los cuales un 15% fue producido por Europa (UE27+3). El Plastic Waste Makers Index 2023 (Minderoo Foundation, 2023) reveló que la población mundial consumió 139 Mt de plásticos de un solo uso en 2021, de los cuales un 98% fueron fabricados a partir de combustibles fósiles, representando por tanto la fracción de plásticos desechables, el 35.5% del total de la producción mundial. Según el estudio publicado por Sarah-Jeanne Royer et al. (2018), la contaminación química de los ecosistemas, no es la única consecuencia de la omnipresencia de los residuos plásticos a lo largo de todo el planeta. En el citado estudio demuestran, que la degradación de los plásticos debido a la exposición ultravioleta, el viento y las corrientes marinas, generan gases de efecto invernaderos, como metano y etileno. Por tanto, la acumulación plástica, consecuente de una producción incontrolada de plásticos de un solo uso y la inadecuada gestión para eliminar estos residuos, se alza como uno de los principales focos de gases contribuyentes al calentamiento global (450 Mt métricas de emisiones de CO₂ a nivel mundial en 2021). Dentro del ranking de los países europeos de mayor producción plástica, España se colocaría en el top cuatro, representando el 7.5% de la producción total. Entre los principales sectores consumidores de plásticos, la producción de envases, así como el amplio campo de la construcción se alzan como líderes, responsables del 39.1% y 21.3% del plástico consumido a nivel continental, respectivamente. Por otro lado, atendiendo al tipo de plástico consumido por los principales demandante de plástico europeos, según la resina polimérica, la demanda estaría protagonizada por el Polipropileno (PP) representando el 19.8% respecto del total de la demanda europea, seguido muy de cerca por el Polietileno de Baja y Alta Densidad (LDPE y HDPE) (16.8% y 12,6% respectivamente), así como el Policloruro de vinilo (PVC), 10.3% (Plastic Europe, 2022). Tras hacer una breve introducción de los principales datos de producción y consumo actual de plásticos, resulta de especial interés exponer cifras comparativas sobre su vida útil y como finaliza, ya que, a pesar de que el ciclo de vida de los plásticos puede ir desde 1 a 50 años o más, la mayoría de ellos se fabrican con la intención de ser de uso único y tras acabar este, se desechan. A pesar de la elevada tasa de producción europea de plásticos (52.7Mt en 2021), tan solo un 51% de los residuos posconsumo fue correctamente depositado para su recolección y posterior reciclado (35%) o conversión en energía (42%). Sin embargo, el 23% fue destinado a vertederos y posteriormente incinerados, con las consecuentes emisiones de gases contaminantes a la atmósfera (Plastic Europe, 2022). 30 Con la mejora de las prestaciones que ofrecen estos nuevos materiales se fomenta la apuesta por el acondicionamiento pasivo en los edificios, lo cual acentuará notablemente la eficiencia energética de los mismos. Por un lado, diseñando productos con prestaciones optimizadas, sobre todo en materia de aislamiento, que conlleven un menor consumo de energía al edificio, y por otro lado, igualando las mismas prestaciones que ofrece el producto original, pero con la reducción en materia prima que supone la reutilización de productos de desecho, lo cual mejorara el ciclo de vida del edificio, entre otros. Este trabajo de tesis parte de la base establecida por resultados precedentes del grupo de investigación TEP-205, quienes comienzan a trabajar en el estudio del uso de residuos procedentes de la construcción, para la creación de nuevos materiales. Siendo posteriormente, tras su participación en el proyecto de investigación I+D+I “Waste to resources (W2R)”, cuando comienzan a estudiar la incorporación de residuos plásticos a la construcción. Proyectos posteriores, como RAECE “Rehabilitación acústica eficiente de centros educativos”, avalan la implicación del grupo en el desarrollo de nuevos productos de reducido impacto ambiental, incorporando residuos plásticos, para mejorar el comportamiento y eficiencia de la edificación existente. Línea de trabajo en la cual quedaría enmarcada la tesis propuesta. Además, las relaciones establecidas con grupos internacionales punteros en el desarrollo de nuevos materiales de construcción ecoeficientes, como resultado de las estancias en CERIS, Instituto Superior Técnico de Lisboa, así como el apoyo de distintas empresas del sector del yeso, implicadas en las líneas de investigación en desarrollo por el grupo TEP-205, como PLACO-Saint Gobain, avalan la viabilidad y alcance de la investigación desarrollada en esta tesis. 31 INTRODUCTION 1 The current global situation, in which climate change is becoming a reality, requires immediate solutions to combat the devastating environmental crisis that affects the life of marine and terrestrial ecosystems. As set out in the environmental declaration published in November 2019 (Riple et al., 2020), much of the scientific community proclaims a global climate and environmental state of emergency as imminent. The decision of the European Parliament to support this proclamation by approving the European Green Pact makes Europe the first continent to take a closer step toward limiting global warming by stipulating a series of targets to reduce and even neutralise the carbon footprint by 2050 (European Commission, 2019). The uncontrolled production of plastics is one of the most harmful activities among the main actions that contribute to global climate and environmental pollution. The durability, stability, and low cost of polymeric products have made them a daily necessity. Be that as it may, society’s current inability to dispose of the disproportionate amount of plastic waste generated, especially single-use plastics, is leading the planet into an unsustainable situation. More than 8.3 billion tons of plastic have been manufactured since mass production began in the 1950s. According to recent data, global plastic production in 2021 was 390.7 megatonnes (Mt), 15% of which was produced by Europe (EU27+3).The Plastic Waste Makers Index 2023 (Minderoo Foundation, 2023) revealed that the world’s population consumed 139 Mt of single use plastics in 2021, 98% of which were made of fossil fuels. Thus, this fraction of disposable plastics represents 35.5% of total global production. According to the study published by Sarah-Jeanne Royer et al. (2018), chemical pollution of ecosystems is not the unique consequence of ubiquitous plastic waste across the planet. The study claims that plastics’ degradation (due to UV exposure, wind and ocean currents) generates greenhouse gases such as methane and ethylene. Therefore, plastic accumulation, resulting from the uncontrolled production of single-use plastics and the inadequate management to eliminate these residues, is deemed as the source of gases that contribute to global warming (450 metric Mt of CO2 emissions worldwide in 2021). Spain would rank in the top four within the classification of European countries with the highest plastic production, representing 7.5% of total good manufacturing. Among the main plastic consumption sectors, both the packaging and construction fields would be placed as leaders, 39.1% and 21.3%, respectively, at the continental level. Plastic consumption by the principal European plastic demanders draws full attention. Regarding the type of polymer resin, Polypropylene (PP) leads the ranking, representing 19.8% of total European demand, closely followed by Polyethylene, LDPE and HDPE (16.8% and 12.6% respectively), as well as Polyvinyl Chloride (PVC), 10.3% (Plastic Europe, 2022). After describing the principal data on current production and consumption of plastics, a comparative analysis of figures based on their usefulness and end-life management serves as a focal point. Although plastics’ life cycle can span from 1 to 50 years, the vast majority of these goods are manufactured as single-use products, which are discarded after concluding this purpose. In the specific context of Spain, notable progress has been recorded since 2006 in plastic waste recycling after consumption of up to +137%. In 2020, a total of 2.567kt of plastic was collected at correct classification points; 1.106kt of plastic was recycled, 923kt landfilled and 538kt destined for energy recovery.However, in this context, recycling does not progress as fast as it should, remaining an almost marginal activity for the plastics sector (Figure 1). Just looking around us is enough to see the abundance of single-use items that end up in nature instead of being properly recycled. Within this group of waste, those with the lowest recycling rate are plastic items from intimate hygiene such as wet wipes, which end up being flushed down the toilet, collapsing sewage networks due to their inability to degrade; waste derived from domestic habits, such as single-dose capsules; as well as waste from healthy harmful products with widespread demand worldwide, cigarette butts. It is important to acknowledge the widespread problem of plastic bag pollution, as despite efforts to reduce their production and use, plastic bags remain a necessary item in modern society. Therefore, alternative methods must be implemented to ensure their lifecycle is prolonged. 32 Improper waste management and irresponsible disposal of plastic bags have led to a major pollution problem in both terrestrial and marine ecosystems. Marine environments are particularly vulnerable, as they act as final sinks for waste that is carried by currents. Macroplastics (greater than 25mm) can cause entanglement, suffocation, and strangulation of marine wildlife, while microplastics (less than 5mm) can cause malnutrition due to ingestion (Greenpeace España, 2018). Shockingly, 200 kg of plastic waste is estimated to enter the ocean every second worldwide, resulting in a total of 12 million tonnes of plastic per year (Aquae, 2022). As 80% of marine litter is plastic waste (Acuña, 2017), this has become one of the most significant climate emergencies facing the planet, with the United Nations Environment Programme (UNEP) ranking it among the top six. During beach and ocean clean-up campaigns, plastic waste is the most prevalent residue found. It is estimated that 80% of this waste originates from land-based activities such as tourism and dumping, while 20% comes from marine activities such as fishing and maritime traffic (Greenpeace, 2018). According to studies conducted on different beaches in Europe, the most abundant waste includes cigarette butts, bags plastic fragments, plastic wrapping, caps and ropes (Dirección General de Sostenibilidad de la Costa y del Mar, 2022). However, debris from fishing nets and ghost fishing gear is most common in high seas (Richardson et al., 2019). More than 500000 to 1 million tons of fishing gear are discarded in seas and oceans annually, causing devastating consequences for marine ecosystems through sediment accumulation. Likewise, waste discharges from land-based activities are another major source of pollution of marine environments. Due to improper disposal of single-use sanitary articles (e.g. non-degradable wet wipes, sanitary towels, swabs, etc.) at the end of their useful life, many of these goods end up on the seabed . The composition of hygienic waste, non-woven polymeric textiles, makes their decomposition very difficult when they come into contact with water, generating serious problems of clogging in sewage networks when mixed with fats, which are finally discharged into the sea like microplastics (Ó Brian et al., 2020). The issue of plastics has become a major concern and is now classified as one of the top six environmental emergencies worldwide. This is reflected in the Sustainable Development Goal 14 “Life Below Water” of the United Nations Environment Programme (UN, 2017). The Mediterranean Sea is one of the seas most affected by pollution, particularly plastic waste. Studies have shown that despite only covering 1% of the world’s water, it accumulate 7% of the world’s microplastics. As a result, it is now the sixth largest source of marine litter worldwide. This serious problem has been extensively researched, with Boucher et al. (2020) providing valuable insights. It is estimated that there are approximately 1.25 million microplastic fragments per square kilometre floating in the Mediterranean Sea, in addition to the sediments that accumulate from sinking microplastics, which amount to around 10000 fragments per square kilometre. These plastic accumulations have a devastating impact on the flora and fauna residing in the various marine ecosystems. Every year, over 100000 marine species perish due to this issue, and the number of endangered organisms is rising, with 700 species currently at risk of extinction worldwide due to microplastic pollution (Alessi et al., 2018). POLICY ON ENGAGEMENT This section delves into the political strategies that encompass the current regulatory framework for balancing plastic production and its life cycle, while maximising its versatility and range of applications. European and national political entities have implemented regulatory measures to transform Europe into a continent that efficiently and sustainably uses its resources while generating employment and growth. Several directives have been introduced to promote a circular economy model. In 1994, the European Parliament approved Directive 94/62/EC, which aimed to prevent or minimise the impact of packaging and packaging waste on the environment. Spain incorporated this directive into its legal system through Law 11/1997(Head of State, 1997) and Royal Decree 782/1998 (Ministry of the Presidency, 1998). A new directive, 2015/720 (European Union, 2015) was approved at the European level in 2015 and implemented in Spain in 2018 through the Royal Decree 293/2018 of the Ministry of Agriculture and Fisheries, Food and Environment, as the previous directive did not address the issue of lightweight plastic bags (<50 µm), which are currently a major waste product. To tackle the negative impact of single-use plastic products on marine ecosystems, the European Union introduced Directive (EU) 2019/904, which aims to reduce the problems caused by these products. However, there is a regulatory deficiency that does not cover certain plastic waste despite the exponential increase in plastic consumption compared to recycling. New and efficient management solutions are required to solve this problem, specially focus 33 on plastic products consumed daily, such as single-dose coffee capsules made of both aluminium and plastic, which are not considered for differentiated recycling at present (Marinello et al., 2021). Currently, the waste sorting system is limited and only offers general recycling options such as paper, glass, plastics, packaging, and organic waste. To promote more inclusive recycling, it is essential to implement new recycling alternatives and expand the range of products that can be recycled. Educating the public about recycling and reducing its environmental impact is also crucial, as lack of consumer information contributes to the problem. Organisations such as ECOEMBES (2018) are leading the way in the implementation of these measures. It is important to reference the 14 Sustainable Development Goals set by the UN for every country to clarify the goals and scope of this research. This Ph.D. thesis primarily focusses on goal 12, which emphasises responsible consumption and production. (UN, 2017) CONTEXTUALISATION FOR THE PLASTIC WASTE CHOICE Although there are a variety of plastics available, the thesis developed specific waste selection criteria in order to hone in on the most polluting types. These plastic fractions were chosen from both land-based and maritime activities and were identified as highly polluting due to their low recycling rates, short useful life, unspecific composition, and lack of distinguished classification in the current waste management system. It should be noted that recycling rates for plastic waste can increase up to 13 times when they are collected separately (Plastic Europe, 2022). Therefore, it is crucial to encourage this dynamic, especially when it comes to single-use items whose valorisation should be considered essential. To achieve this, research from various sectors in the chain of production, consumption, and management of plastic waste is necessary to find alternative approaches that promote the extension of plastic products’ life cycle while balancing consumption and recycling. Currently, there is a lack of specific government regulations related to the consumption and recycling of products derived from maritime activities, such as fishing nets, and inefficient policies to reduce bag consumption. However, there are opportunities for the reuse of certain waste materials in the field of construction, particularly those made of polymeric composition such as wet wipes or coffee capsules (PP), cigarette butts (AC), plastic bags (LDPE) and fishing nets (PA6). The proposal to design eco-efficient building products from new recycled materials aligns with the new circular economy mode promoted by the European Union. This efficient recycling alternative encourages the sustainability of current and future buildings and responds to the objectives of the newly developing European policy to reduce environmental impact. ACTION STEPS DEVELOPED BY CONSTRUCTION SECTOR Use of plastic waste in the generation of new materials The construction industry is a significant contributor to plastic consumption worldwide, accounting for 18% of plastic production due to the beneficial properties of plastic products, particularly in terms of insulation (Plastic Europe, 2022). However, the recycling rate of plastic waste within the sector is insufficient, which poses a major challenge. Given the wide scope of the construction industry within the global economy, it has the potential to become one of the leading advocates for recycling, energy recovery, and reuse to achieve efficient plastic waste management and achieve the Sustainable Development Goals as set by the United Nations (UN, 2017), thus reducing global pollution. Although the potential of using polymeric residues in the production of new building materials is high, regulations related to this type of waste are limited and restrictive. However, the construction industry is increasingly focused on research and development initiatives that encourage the creation of new eco-friendly products by adding thermoplastic waste to them. This is because thermoplastic waste is widely available and easily recyclable. Furthermore, there is ongoing research on structural concrete performance, with current legislation focussing mainly on recycled materials in this area (European Commission, 2023). For example, there are interesting projects that use LDPE waste from plastic bags to develop lighter concretes (Jain et al., 2018), as well as those that use polypropylene fibres from carpets and brooms as recycled aggregate (Bendjillali et al., 2018; Mohammadhosseini et al., 2020), which improves the mechanical properties of concrete elements 34 Addition of plastic waste to binding matrices (gypsum/mortar cement) Researchers have delved into the realm of incorporating recycled plastic aggregates into the creation of mortars, based on previous structural concrete and derived novel products studies. The majority of research in this field has focused on two avenues. The first approach involves investigating means by which plastic can be employed to create ligther mortars. The second approach entails examining the addition of polymeric fibres to bolster the physical and/or mechanical attributes of the mortars. In the first research area, recent studies have focused on using readily available plastic waste, typically sourced from recycling plants. The main plastic types investigated include PP, PE and EPS, as explored by researchers such as Ghernouti et al. (2012), Da Silva et al. (2014), Jassim et al. (2017), and Laoubi et al. (2019). Notably, Ohemeng et al. (2019) proposed a strength prediction model for cement mortars that incorporate shredded LDPE waste as fine aggregate. Additionally, Rubio-de-Hita et al. (2018) developed novel lightened beams by partially replacing the aggregate with recycled mixed PP. In the concrete industry, it is common to add polymeric fibres to cement mortars to improve their mechanical properties and make them more resistant to cracking. This area of research is well developed, and commercial fibres are available for this purpose. Many studies have looked at innovative ways to achieve greater strength and better cracking behaviour in mortars, using materials like commercial glass or PP fibres (Záleská et al., 2016). Additionally, some researchers have proposed alternative recycled fibres, such as PP fibres from carpets and household brooms (Araya-Letelier et al., 2019), fishing nets (Ottosen et al., 2019), and cellulose (Hospodarova et al., 2017), for use in mortars. Focussing on recent research on studies conducted by San Antonio-Gonzáles et al. (2015) and Vidales-Barriguete et al. (2020) have explored the benefits of incorporating recycled construction plastics, specifically EPS and XPS, in composite materials as partial replacement for gypsum binder. Their findings indicate that this substitution leads to a notable weight reduction and enhances the thermal and acoustic properties of the resulting gypsum-based material. Nevertheless, the strength properties of both flexure and compression tend to decrease in most of those studies. Thus, knowing the great potential of fibres to address this issue with concretes and mortars, a new research line has been established. Researchers have been incorporating new and/or recycled fibres into the binder mix. Previous studies have explored the use of plastic fibres in two ways. Firstly, commercially available fibres made of polypropylene (PP) and other polyolefins have been used as a supplement in gypsum-based mixtures (Suárez et al., 2019). Secondly, lightweight materials like powdered graphite, diatomite, vermiculite, and crushed EPS have been used in place of gypsum, and these fibres have been added to the mix (Gencel et al., 2014, 2016; Alameda et al., 2016; Flores-Medina et al., 2017). At present, diligent researchers are focused on examining and discovering novel recycled fibres that have the potential to partially substitute gypsum. An exemplary study conducted by Cong Zhu et al. (2018) involved the development of gypsum composites with PP and PVA fibres, which effectively improved their mechanical properties and surface hardness. Furthermore, a noteworthy investigation by Gutiérrez-González et al. (2012) revealed that recycled polyamide can be used to manufacture lightweight composites with heightened thermal resistance. Gypsum-based prefabricated products Gypsum boasts an-array of valuable attributes, such as elasticity, fire resistance, and thermal insulation, which have cemented its place as a popular material for interior cladding and decorative purposes. However, in Spain, gypsum has found a new use in the form of laminated plasterboard (PYL) in recent years. The surge in PYL’s popularity can be attributed to two main factors: the need for faster construction times and the demand for partition walls that offer better thermal and acoustic insulation properties. The construction industry’s recent crisis has also played a role in the rise of PYL, as it is easier to assemble, takes up less space, and is more cost-effective than traditional brick partition walls. Despite its increasing use, PYL still provides excellent thermal and acoustic properties. As we look to the near future of the construction industry, it becomes clear that there are two key areas of focus: rehabilitation and non-residential building. PYL has established itself as a leading provider of partitioning and roofing elements throughout Spain and Europe thanks to its adaptable solutions, the use of prefabrication, and the superior qualities that they possess, as outlined previously. 35 The construction sector has experienced a research boom in the area of gypsum-based construction solutions over the past few decades. Previous studies aimed to enhance the properties of these solutions and highlighted the various applications of PYLs. Ferrández-Vega et al. (2019) conducted research focused on the development of mechanically, thermally, and acoustically improved gypsum-based products with polymeric additions. They patented the manufacturing process for obtaining laminated gypsum boards that are lightened with powdered polyamide waste, which are designed for use in partition walls. The final product showed improvements in basic properties, such as workability, finish, and thermal insulation. Vidales-Barriguete et al. (2021) explored the feasibility of using plastic waste from cables in the production of PYL and observed a significant improvement in water resistance properties, as well as acceptable fire stability. It is worth mentioning the impressive work of García-Santos (2009) in creating an environmentally friendly lighter gypsum composite by incorporating binary additions of EPS and PP fibres. This composite was utilised to produce large slabs and panels, leading to a registered patent. The composite displayed mechanical strength, porosity, and setting times comparable to those of the reference product. Despite this success, there have been limited studies on the use of gypsum-based materials in construction applications beyond PYL. An interesting alternative is the research conducted by Del Río et al. (1999), who evaluated precast gypsum partitions reinforced with glass fibre. These studies paved the way for a new partition wall construction system that can be utilised as wall cladding, interior partitions, or separations in residential buildings. Laminated plasterboard panels have been improved through the use of fibreglass during manufacturing. This has resulted in a lighter and more flexible final product, with consistent assembly due to the lateral tongue-and-groove joint. The panels can be directly assembled on the slab with an asphalt sheet, and come in dimensions of 50 x 190 cm with a thickness of either 7 or 9 cm. They offer a thermal insulation coefficient of 3.03 and 2.91 W/m² ºC respectively, as well as acoustic insulation of 34 and 35 dBA respectively (Del Rio-Merino et al., 2005). Following this line, a patented solution was developed and further research by Santa Cruz-Astorqui et al. (2007, 2019) focused on lightweight prefabricated gypsum partitions using natural materials like cork or recycled plastics, such as EPS. These products showed a significant decrease in density and improved thermal performance. Finite element simulation was applied to design these constructive elements (Del Rio-Merino et al., 2009). Further research has been conducted in the same line of gypsum block development for partition wall application, as an alternative to traditional ceramic solutions. On the one hand, several of those works partially replaced the raw material, i.e. gypsum, with waste from different origins, i.e. natural or plastic (Santamaría-Vicaerio et al., 2022; Singh et al., 2023). On the other hand, it should be highlighted that interesting works focus on saving raw material by optimising the block design, at the same time that the thermal and acoustic performance of these solutions is improved (Villoria-Sáez et al., 2020; Katman,202). CONCLUSIONS Upon careful analysis, it has become evident that limited research has been conducted on creating environmentally-friendly products utilising new materials that partially replace gypsum binder with plastic waste derived from daily use. These products have a short lifespan, are produced at high rates, and are rarely recycled due to a lack of classification in waste management systems. As a result, a new research initiative has been launched in the present work to explore plastic waste from domestic and everyday use, as well as marine activities that contribute significantly to pollution levels. Two studies have been initiated based on the waste’s composition - fibrous or non-fibrous. The objective is to leverage the different properties of each type of plastic waste to enhance new composites’ performance, with a specific focus on physical-mechanical, thermal, fire behaviour, and water resistance properties. The long-term goal is to develop a range of eco-friendly gypsum-based construction products. Innovative materials incorporated into architectural design can improve performance and promote natural conditioning, ultimately resulting in higher energy efficiency. An emphasis on product insulation can significantly reduce energy consumption in construction, while also promoting sustainability through waste reduction and material reuse. Such an approach can effectively optimise the life cycle of a building. The thesis has its roots in the research work of the TEP-205 research group, which focused on developing new materials using construction waste. They further explored the integration of plastic waste in construction, after participating in the I+D+I research project ‘Waste to resources (W2R)’. The participation of the group in subsequent projects, such as RAECE ‘Efficient acoustic rehabilitation of educational centres’, demonstrates its commitment to 36 develop new products with reduced environmental impact by incorporating plastic waste, to improve the performance and efficiency of existing buildings. The proposed thesis is aligned with this line of work. In addition, valuable collaborations with international research groups have been established through this thesis to advance the development of environmentally sustainable construction materials. These partnerships were formed during research visits to CERIS, Instituto Superior Técnico de Lisboa. Furthermore, several companies within the gypsum industry, including PLACO-Saint Gobain, have shown their support for the research conducted by the TEP- 205 group. These partnerships demonstrate the practicality and importance of the research presented in this thesis 37 HIPÓTESIS Y OBJETIVOS En base a las conclusiones obtenidas en las sección anterior, se enuncia como hipótesis de partirda la oportunidad para el desarrollo de nuevos materiales de construcción basados en residuos plásticos, como método alternativo para promover un modelo eficiente de gestión de residuos. En consecuencia, el presente trabajo tiene por objeto determinar la viabilidad del reciclaje de residuos plásticos potencialmente contaminantes sin clasificación en la dinámica actual de reciclaje, como reemplazo parcial de la matriz conglomerante en compuestos de yeso, para la producción de nuevos productos de construcción ecoeficientes. En línea con la hipótesis enunciada, el objetivo principal de esta investigación es analizar la influencia de distintos residuos plásticos clasificados en dos formatos, partículas o fibras, en el comportamiento físico-mecánico, térmico, de reacción al fuego e impacto medioambiental de los compuestos a base de yeso. Así mismo, se persigue el desarrollo de productos de yeso derivados de los compuestos óptimos para su incorporación como sistemas de tabiquería y revestimiento en edificación, en comparación con soluciones comercialies. En base al objetivo planteado, la Tesis Doctoral se estructura entorno a dos líneas prioritarias de investigación implantadas por la Comisión Europea para alcanzar los Objetivos de Desarrollo Sostenible (ODS), la gestión sostenible de residuos y la mejora de la eficiencia energética de los edificios. Se plantea, por tanto, una tesis capaz de generar una valorización de distintos tipos residuos plásticos para la generación de productos de construcción que mejoren la ecoeficiencia del edificio, integrandolos en los sistemas constructivos de estos. De forma trasnversal, se plantean los siguinetes objetivos específicos: Objetivo específico 1. Determinar y caracterizar aquellos residuos plásticos que puedan ser valorizados como materia prima de reemplazo en materiales conglomerantes a base de yeso para la fabricación de productos y sistemas constructivos. Objetivo específico 2. Evaluar la influencia de incorporar residuos plásticos triturados, como reemplazo parcial de la matriz de yeso. Caracterización de los compuestos de yeso resultantes mediante el análisis de su propiedades físico-mecánicas, térmicas y resistencia al agua. Objetivo específico 3. Analizar los beneficios de usar fibras plásticas recicladas como refuerzo en compuestos de yeso, reemplazando parcialmente la matriz conglomerante. Caracterización de los compuestos de yeso resultantes mediante el análisis de sus propiedades físico-mecánicas. Objetivo específico 4. Seleccionar los compuestos de yeso óptimos, teniendo encuenta como factores determinantes: máximo porcentage de reemplazo, mejora simultánea de propiedades y reducción del impacto medioambiental. Análisis del comportamiento fisico-mecánico de los compuestos óptimos seleccionados, tras exponerlos a temperaturas elevadas en condiciones de servicio y fuego. Objetivo específico 5. Analizar conjuntamente los valores obtenidos en los objetivos anteriores para proponer posibles productos y soluciones constructivas ecoeficientes utilizando los nuevos compuestos de yeso desarrollados. 2 38 39 METODOLOGÍA 3 En este apartado se describe el sistema metodológico seguido para desarrollar la investigación doctoral presentada. La estructura planteada se organiza en torno a tres grandes bloques: Recopilación bibliográfica; Campaña experimental; Análisis de resultados y obtención de conclusiones. Para el desarrollo de esta metodología, se sucederán las siguientes etapas, ligadas a cada uno de los objetivos perseguidos: ETAPA 0: Análisis documental y bibliográfico exhaustivo. En primer lugar, se llevó a cabo la recopilación de bibliografía existente sobre el tema de investigación. Como punto de partida, la búsqueda se centró en el campo de los materiales plásticos, con el fin de establecer un marco de conocimiento sobre el impacto de estos en el medioambiente, con especial afección a los ecosistemas marinos, así como las políticas en desarrollo para limitar la producción de plásticos y potenciar su reciclaje. Se prosiguió con la documentación sobre los distintos métodos alternativos de reciclaje de plásticos incorporados en el sector de la construcción. Este primer período documental, permitió determinar de forma fundamentada la elección de los distintos tipos de plástico que acotaran el ámbito de estudio. Por último, se analizaron las metodologías y resultados obtenidos en otras investigaciones y estudios de interés en los que se trabajan con adición polimérica a matrices conglomerantes, permitiendo establecer los antecedentes a la investigación presentada y establecer la hipótesis de partida. La información citada, se recoge en la introducción de este trabajo, así como en la sección introductoria de las diferentes publicaciones científicos que componen la tesis. ETAPA 1: Caracterización de los residuos plásticos, para su valorización como materia prima. Para proceder al desarrollo de esta etapa, se tomaron una serie de muestras de los distintos plásticos a estudiar, resultando necesario para ello: - Localizar plantas de tratamiento y/o vertederos, tanto a nivel local, como autonómico, identificando qué tipos de plásticos se reciclan y qué procesado de reciclado se realiza en este tipo de plantas (materia prima). - Detectar y localizar otros residuos abundantes en la actualidad cuya contaminación es excesiva debido a su escasa tasa de reciclaje, analizando las posibilidades de estos, así como el proceso de reciclado necesario para su posible reutilización en el ámbito de la construcción. - Establecer criterios para la toma de muestras y la valorización de los residuos plásticos a utilizar. - Seleccionar los distintos tipos de plástico y procesado de los mismos, en función a los criterios establecidos anteriormente. Tomadas las muestras de los residuos seleccionados para su posterior empleo como componente en mezclas de yeso, se procedió a la caracterización de estos mediante un análisis visual y físico-químico, catalogándolos en dos grandes grupos, en función de sus propiedades (Tabla 1). ETAPA 2: Caracterización de compuestos a base de yeso con residuos de plástico triturados como reemplazo parcial de la matriz conglomerante. De acuerdo con la normativa vigente, se llevó a cabo una primera batería de ensayos de caracterización para evaluar las diferentes propiedades esenciales de los compuestos elaborados, aplicando distintos niveles de reemplazo plástico. El número mínimo de muestras por ensayos se estableció de acuerdo con por las normas 46 Figura 3. Esquema metodológico de la vinculación de las etapas y contenidos de la tesis doctoral. Fuente: Elaboración propia 47 48 RESUMEN Y DISCUSIÓN DE RESULTADOS 5 (a) (b) Figura 5. Granulometría de los residuos plásticos empleados: (a)PP y (b)PEBD. Fuente: Elaboración propia (a) (b) (c) (d) Figura 4. Residuos plásticos (a) CCs deseachas, (b) PP triturado, (c) Bolsas de plástico ligeras, (d)LDPE triturado, usados en la investigación. Fuente: Elaboración propia 49 SECCIÓN 1: CARACTERIZACIÓN DE NUEVOS COMPUESTOS DE YESO PARCIALMENTE REEMPLAZADOS POR RESIDUOS PLÁSTICOS TRITURADOS. (Publicaciones 1-3) 1.1. Definición de los residuos plásticos empleados Los residuos plásticos empleados en formato triturado fueron polipropileno (PPT), procedente de cáspulas de café desechables (CCs), así como, polietileno de baja densidad (LDPE) reciclado a partir de bolsas de plástico ligeras (<50 μm), tal y como se muestran en la Figura 4. Para obtener las partículas plásticas con la graulometría adecuada para ser usado como reemplazao parcial de la matriz de yeso, ambos residuos se sometieron a un proceso de limpieza previo al triturado y tamizado final, descrito en las publicaciones asociadas (Romero-Gomez, et al., 2022a, 2022b, 2022c). De acuerdo con las granulometrías recogidas en la Figura 5 las partículas de PPT no superaron los 2mm, mientras que las piezas de LDPE empleadas estuvieron comprendidas entre 4-10 mm. La caracterización exhaustiva de ambos tipos de residuos, puede ser consultada en las publicaciones correspondientes (Romero-Go- mez, et al., 2022a, 2022b, 2022c). Figura 6. Valores de densidad aparante seca, obtenida para los compuestos G/PPT. Fuente: Elaboración propia. 50 1.2. Caracterización de los compuestos yeso-polipropileno triturado (G/PPT) A partir de los residuos de PPT mencionados, se elaboraron mezclas con diferentes porcentajes de reemplazo (2.5, 5, 7.5, 10 %) respecto al peso de yeso (wt%). Las dosificaciones empleadas para la preparación de las muestras puede consultarse en las publicaciones correspondientes. Así mismo, se elaboraron probetas de referencia, sin ningún tipo de adición plástica, para comparar los resultados obtenidos por lo nuevos compuestos. Tras completarse el proceso de curado de las muestras, estas fueron sujetas a un pormenorizado procedimiento de caracterización, basado en ensayos físico-mecánicos y evalucación del comportamiento frente al agua, tal y como se recoge en las Publicaciones 1 y 3 (Romero-Gomez, et al., 2022a, 2022b, 2022c). No obstante, en esta sección se hará referencia solo a las propiedades y resultados más destacados. Densidad aparente en estado seco En la Figura 6 se aprecian los resultados de densidad aparante seca obtenidos para los nuevos compuestos G/ PPT. A pesar de que se reemplazó parcialmente la matriz de yeso por un residuos de menor densidad (2650 kg/m3 vs. 1000 kg/m3, respectivamente), los valores de densidad fueron mayores en aquellos compuestos que contenían residuos plásticos. El valor máximo de densidad (aumento del 4% respecto al material de referencia) correspondió a compuestos con un 2.5 wt% de PPT, presentando una tendencia descendente a partir de este punto, aunque siempre con valores superiores al yeso de referencia. Por tanto, este fenómeno se atribuyó a un densificación del material debido a una mejora de la distribución de las partículas debido al tamaño de los gránulos de PPT inferiores a 2 mm, añadidos a las mezclas de yeso. Sin embargo, el descenso de densidad observado tras exceder 2.5 wt% de reemplazo, apuntó a que este es un contenido óptimo, a partir del cual, la compacidad del material comienza a debilitarse. Resistencia mecánica: flexión y compresión Respecto al comportamiento de resistencia a flexión de los nuevos compuestos, los resultados pueden observarse en la Figura 7a. Tal y como se esperaba, la adición progresiva de resiudos PPT a las mezclas de yeso, dió lugar a una disminución progresiva de la resistencia a flexión de hasta 35%, respecto al material de referencia, correspondiente a un 10 wt% de reemplazo plástico (G/PPT/10). Sin embargo, debe destacarse que todos los valores superaron el mínimo (1MPa) establecido por la normativa EN 13279-2 (2014). En relación a los valores de resistencia a compresión (Figura 7b), un inesperado comportamiento fue detectado tras sustitutir parcialmente la matriz de yeso por residuos de PPT, ya que la resistencia de los nuevos compuestos mejoró con la adición plástica. El valor más alto de resistencia a compresión se consiguió en mezclas con un porcentaje de reemplazo plástico de 2.5 wt% (9% de mejora, respecto al material de referencia). A partir de este punto, se observó una tendencia decreciente de los valores de resistencia a compresión. Por tanto, el comportamiento observado, tiene relación directa con la mejora de compacidad de los compuestos G/PPT, previamente detectada. Figura 8. Valores de absorción de agua por capilaridad obtenido para los compuestos G/PPT (a) (b) Figura 7. Resistencia mecánica: (a)Flexión y (b)Compresión de los compuestos G/PPT. 51 Propiedades de resistencia al agua: Absorción de agua por capilaridad Los resultados del ensayo de absorción de agua por capilaridad se presentan en la Figura 8. La capacidad de absorción de agua disminuyó alrededor de un 25, 26, 34 y 36 % para los compuestos con un contenido de PPT del 2.5, 5, 7.5 y 10 wt%, respectivamente, en comparación con la muestra de control. En cuanto a los valores promedio de absorción de agua por capilaridad, el valor más bajo correspondió al compuesto con mayor contenido plástico (G/PPT/10). Dado que la incorporación de partículas de PPT no generó una zona de transición interfacial (ITZ) porosa, como se puede observa en las microspías electróncias de barrido (SEM) recogidas en la Publicación 1 (Romero-Gómez et al., 2022a), la presencia creciente de PPT imparte una propiedad impermeable que conduce a un avance más lento del agua en los nuevos compuestos desarrollados. Figura 9. Valores de densidad aparante seca, obtenida para los compuestos G/LDPE. Fuente: Elaboración propia. 52 1.3. Caracterización de los compuestos yeso-polietileno de baja densidad triturado (G/LDPE) En base a la densidad de los residuos de LDPE seleccionados para esta investigación, se establecieron cuatro porcentajes de reemplazo (0.25, 0.5, 1, 5 wt%) respecto al peso de yeso. Las mezclas se prepararon de acuerdo a las dosificaciones presentadas en las publicaciones correspondientes (Romero-Gómez et al., 2022b). Nuevamente, se elaboraron muestras de referencia, sin ningún tipo de adición plástica. Concluido el proceso de curado de las muestras, se sometieron a un pormenorizado procedimiento de caracterización físico-mecánica, así como se analizo el comportamiento de los nuevos compuestos frente al agua, tal y como se recoge en la Publicación 2 (Romero-Gómez et al., 2022b). Sin embargo, en este apartado, solo se comentarán, a modo de resumen las propiedades más destacadas, descritas a continuación. Densidad aparente en estado seco En referencia a los valores presentados en la Figura 9, se detecta una progresiva disminución de la densidad aparente seca de los compuestos tras incorporar en la mezclas de yeso residuos de LDPE. El valor más bajo de densidad correspondió a compuestos con un porcentaje de reemplazo plástico del 5 wt% (2% de disminución comparado con el material de referencia). La sustitución parcial de la matriz conglomerante de yeso por residuos de LDPE de menor densidad (2650 kg/m3 frente a 920 kg/m3, respectivamente) podría explicar la tendencia decreciente observada en relación con la densidad aparente seca. Resistencia mecánica: flexión y compresión El comportamiento de resistencia mecánica obtenido estuvo en línea con lo esperado. Tal y como puede observarse en la Figura 10, tanto para la resistencia a flexión, como en el caso de la resisencia a compresión de los nuevos compuestos G/LDPE, se observó una tendencia decreciente a medida que aumentó el conenido plástico en las mezclas. La máxima reducción de resistencia a flexión (40% de disminución respecto al compuesto de referencia), correspondió a compuestos con un porcentaje de reemplazo del 5wt%. De la misma forma, el valor más bajo de resistencia a compresión (60% de reducción comparado con el material de refencia) se alcanzó con el máximo porcentage de reemplazo plástico. El comportamiento presentado por los nuevos compuestos G/LDPE, señaló que la cohesión en la ITZ entre las partículas de desecho de LDPE y la matriz de yeso no es tan fuerte como entre los productos de hidratación, pudiendo suponer un punto de falla. No obstante, tanto en los ensayos de resistencia a la flexión como a la compresión, todos los valores estuvieron dentro de los mínimos de 1MPa y 2Mpa, respectivamente, establecidos por la norma EN 13279-2 (2014). Figura 11. Valores de absorción de agua por capilaridad obtenido para los compuestos G/LDPE. Fuente: Elaboración propia Figura 10. Resistencia mecánica: (a)Flexión y (b)Compresión de los compuestos G/LDPE. Fuente: Elaboración propia. 53 Propiedades de resistencia al agua: Absorción de agua por capilaridad Los datos obtenidos a partir del ensayo de absorción de agua por capilaridad se recogen en la Figura 11. A partir de estos, se puede observar una reducción generalizada de la capacidad de absorción de agua para los nuevos compuestos que contienen LDPE. La disminución más significativa (hasta ~48 %) la alcanzaron los compuestos con un porcentaje de reemplazo del 5 wt%, en comparación con el yeso de referencia. En cuanto a la absorción de agua por capilaridad promedio, el valor más bajo correspondió al compuesto G/LDPE/5. Por lo tanto, se puede deducir que la presencia creciente de partículas de LDPE imparte una propiedad impermeable a la matriz de yeso, lo que ralentiza el avance del agua. 54 (a) (b) (c) (d) (e) (f) Figura 12. Residuos plásticos previo tratamiento reciclaje: (a)Toallitas húmedas desechadas, (b) Colillas de cigarrillo, (c)Redes de pesca descartadas; residuos procesados como fibras, listos para ser usado en las mezclas de yeso: (d)PPF, (e)AC y (f)PA6. Fuente: Elaboración propia 55 SECCIÓN 2: CARACTERIZACIÓN DE NUEVOS COMPUESTOS DE YESO REFORZADOS CON FIBRAS PLÁSTICAS RECICLADAS. (Publicaciones 4-8) 2.1. Definición de los residuos plásticos empleados Los residuos plásticos, que por su carácter fibroso se emplearon en esta investigación fueron: fibras de polipropileno (PPF) obtenidas a partir de toallitas húmedas no degradables; microfibras de acetato de celulosa (CA) recicladas a partir de filtros de cigarrillo usados y, por último, fibras de nylon (PA6) procedentes de redes de pesca desechadas. Los residuos fueron sometidos a un proceso de limpieza y desinfección previamente a la trituración de estos, para la obtención de las fibras. En el caso de la fibras de PPF y PA6 el corte fue manual, mientras que las microfibras de CA se trituraron usando un molino RETSCH SM 2000. El proceso previo para la obtención del formato de fibra final en el caso de cada residuo se explica pormenorizadamente en las correspondientes publicaciones asociadas a esta sección (Romero-Gómez et al., 2021, 2022d, 2022e, 2022f, 2023a). Respecto al tamaño de fibra usado en el caso de cada tipo de residuo, se seleccionó en función de las propiedades ofrecidas por cada tipo de plástico. Tal y como se observa en la Figura 12, en el caso de las fibras de PPF, se empleó una longitud variable entre 15-30 mm y ancho de 2–3 mm. Tras someter los filtros de cigarrillo a molienda, se obtuvieron microfibras de CA inferiores a ~1 mm. Por último, se compararon dos longitudes de fibras de PA6, PA6/I con 20-25mm de longitud y diámetro de 0.24mm y PA6/II, de igual diámetro, pero longitud entre 10-12.5mm. (c) (d) Figura 21. Gráfico tensión-deformación correspondiente a los compuestos a) G/PA6I, b)G/PA6II, c) G/EGF y d) G/ARGF. Fuente: elaboración propia. (a) (b) 62 (a) (b) Figura 22. Termogramas de: a)Residuos plásticos; b)Compuestos de yeso con residuos PPT/PA6. Fuente: Elaboración propia. 63 SECCIÓN 3: EVALUACIÓN EVALUACIÓN DEL COMPORTAMIENTO FÍSICO-MECÁNICO DE COMPUESTOS DE YESO CON RESIDUOS PLÁSTICOS, EXPUESTOS A ELEVADA TEMPERATURA EN CONDICIONES DE SERVICIO Y FUEGO. (Publicaciones 9 y 10) 3.1. Selección de compuestos de yeso óptimos. Tras evaluar la viabilidad de usar distintos tipos de residuos plásticos como reemplazo parcial de la matriz conglomerante de yeso, para el desarrollo de nuevos compuestos caracterizados en las secciones 1 y 2 de este trabajo, se procedió a la selección de los compuestos óptimos, uno por cada grupo: residuos trituraros y fibras. La selección de las mezclas óptima se hizo en función a dos factores clave. Por un lado, se seleccionó el mayor nivel de reemplazo que permitió conseguir mejoras simultáneas del comportamiento físico-mecánico, térmico y de resistencia al agua de los nuevos compuestos, respecto al material de referencia. Por otro lado, se tuvo en cuenta la simplicidad, viabilidad y eficiencia del pretratamiento necesario para obtener los residuos plásticos listos para ser usados en las mezclas de yeso. Con ello, se pretende llevar a cabo la caracterización completa de solo aquellos compuestos que serían más adecuados para ser desarrollados a nivel industrial.Como resultado, se seleccionaron como compuestos óptimos: G/PPT/7.5 en le caso de compuestos de yeso parcialmente reemplazados por residuos plásticos triturados y G/PA6/2.5 (tipo I, fibras largas) entre los compuestos reforzados con fibras plásticas recicladas. Por consiguiente, la presente sección en la que se evaluará el comportamiento de los compuestos tras la exposición a elevada temperatura en condiciones de servicio y fuego, estará enfocada exclusivamente en los compuestos óptimos mencionados. 3.2. Análisis de las propiedades termo-físicas de los compuestos. TGA análisis De acuerdo con las curvas TGA correspondiente a los residuos PPT y PA6 (Figura 22a), se observa una pérdida de peso total del 100% alcanzados los 500ºC, que podría atribuirse a la descomposición plástica, que se produce a partir de los 300ºC para PP y 350ºC para PA6. Por otro lado, las curvas TGA de los compuestos G/PPT/7.5 y G/ PA6/2.5, en comparación con el material de referencia (Figura 22b) mostraron una pérdida de peso total entre 30%- 27% alcanzada la máxima temperatura de exposición, 1100ºC. Durante el período de exposición, se registraron tres procesos de pérdida de peso para los materiales compuestos de PPT y PA6. El primer proceso corresponde a la deshidratación del yeso y ocurre en el rango de temperatura de 100-250ºC. El segundo rango entre 300-400ºC, donde la pérdida de peso se puede atribuir a la descomposición del PP/PA6, como se mencionó anteriormente. Finalmente, se observó una tercera pérdida de masa en el rango de 700-900ºC, debido a la descarbonatación del yeso. Así mismo, respecto al perfil de degradación térmica del compuesto de referencia, se observaron solo dos rangos principales de pérdida de peso (100-250ºC y 700-900ºC). Estos resultados fueron confirmados por análisis (e) (f) Figura 23. Microscopías SEM de: G/CM (a-800x) (b-3000x) antes de la exposición a elevada temperatura; G/CM (c-800x) (d-3000x) depues de la exposición a 150ºC y G/CM (e-800x) (f-3000x) tras esposición a 300ºC. Fuente: elaboración propia. (a) (b) (c) (d) 64 XRD complementarios, tal y como se especifica en la Publicación 9 (Romero-Gómez et al., 2023 c). 3.3. Análisis microestructural SEM análisis Con el fin de evaluar el efecto de la exposición a elevada temperatura en la microestructura de los compuestos de yeso con contenido plástico, se llevaron a cabo análisis SEM para los compuestos óptimos, en comparación con la muestra de referencia, tras exponerlos a temperaturas de servicio elevadas de 150ºC y 300ºC, así como, a temperatura ambiente (23±2ºC). En primer lugar, se hace referencia al comportamiento observado en las muestras de referencia, sin adición plástica (Figura 23). No se detectan cambios en la morfología de los cristales (20-30 μm de longitud) que componían la (a) (b) (c) (d) (e) (f) Figura 24. Microscopías SEM de: G/PPT/7.5 (a-800x) (b-3000x) antes de la exposición a elevada temperatura; G/PP/T7.5 (c-200x) (d-3000x) depues de la exposición a 150ºC y G/PPT/7.5 (e-200x) (f-3000x) tras esposición a 300ºC. Fuente: Elaboración propia. 65 matriz conglomerante, tras someterla a un proceso de calentamiento de hasta 300ºC. Sin embargo, pese a mantenerse la simetría de los cristales de yeso (2/m), el análisis XRD (Romero-Gómez et al., 2023 c) demostró que se sucedió una transformación del yeso en basanita a 150ºC y anhidrita a 300ºC. La exposición a altas temperaturas induce por tanto, un proceso acelerado de deshidratación que promueve la pseudomorfosis de los cristales. En la Figura 23f, se aprecia la pararición de grietas por la ruptura de los cristales después de exponer las muestras de referencia a 300ºC, debido a la frágil apariencia de los cristales deshidratados, ahora formados por anhidrita. La Figura 24 muestra la microestructura de la matriz de yeso reemplazada parcialmente por residuos PPT. En esta se observa que la morfología de la red de cristales fue modificada, respecto al yeso de referencia, debido a la adicción de partículas de PPT. Se detectaron cristales más delgados y pequeños (<15 µm), independientemente de la temperatura de exposición, que dieron lugar a la densificación de la matriz a temperatura ambiente. A pesar de que la rugosidad superficial de las partículas PPT, resultado del procedimiento de trituración, permitió la nucleación de los cristales de yeso, compensando la hidrofobicidad de PP, el menor tamaño de los nuevos cristales formados sobre los residuos PPT, así como la presencia de cristales sin forma (basanita inalterada), denota que (a) (b) (c) (d) (e) (f) Figura 25. Microscopías SEM de: G/PA6/2.5 (a-800x) (b-3000x) antes de la exposición a elevada temperatura; G/PA6/2.5 (c-200x) (d-3000x) depues de la exposición a 150ºC y G/PA6/2.5 (e-200x) (f-3000x) tras esposición a 300ºC. Fuente: Elaboración propia. 66 los residuos PPT tienen un efecto negativo en la hidratación del yeso, tras la exposición a elevada temperatura. Por consiguiente, se registró una débil cohesión en a ITZ entre la matriz de yeso y el PPT de la muestra sometida a 150 ºC, temperatura a la cual comenzó la fusión superficial del PPT, perjudicando la de los cristales. Como consecuencia de la microporosidad causada por los cristales de menor tamaño, en comparación con lamuestra de control, se aceleró la disrupción del material a medida que aumentó la temperatura de exposición. Tras superar los 300ºC, el PP aparece prácticamente fundido, dejando cavidades<4 mm, y restos poliméricos cubriendo la matriz. Añadir, que no se observaron cristales agrietados a 300ºC, lo cual se podría explicar por la compactación y la formación de una red entretejida de cristales alrededor de las partículas fusionadas de PPT, retrasando el deterioro de la microestructura observada en los cristales de yeso de referencia a 300ºC. Las microscopías SEM del compuesto de yeso reforzado con fibras PA6 recicladas, se muestran en la Figura 25. Se observó una morfología de cristales similar a la del material de referencia de yeso. Sin embargo, se detectó un aumento en el ancho del cristal. Además, se observó una cohesión débil en la ITZ entre el yeso y las fibras PA6, Figura 26. Densidad aparente seca de compuestos de yeso antes y después de la exposición a altas temperaturas. Fuente: Elaboración propia. 67 que puede atribuirse a la gran relación de aspecto de las fibras PA6, mucho mayor que las dimensiones de los cristales de yeso, así como, al insuficiente grado de rugosidad superficial de las fibras, no permitiendo una óptima precipitación de cristales. En consecuencia, se pueden visualizar cristales pequeños y desconectados sobre la superficie de la fibra, orientados paralelamente a la directriz de la fibra. Además, cabe señalar que el tamaño de poros observado en los compuestos reforzados con fibras PA6 a temperatura ambiente era más pequeños que en el yeso de referencia, indicando una densificación de la matriz. Sin embargo, el grado de adherencia fibra-matriz fue menor que el detectado en los compuestos que contenían PPT. Tras la exposición a 150ºC, se observó una pérdida de cohesión en la ITZ entre las fibras de PA6 y la matriz de yeso, así como un aumento de la porosidad, atribuido al cambio de volumen de las fibras PA6 después de alcanzar la temperatura de transición vítrea (55ºC). Tras ser sometido a 300ºC, se observó un fenómeno interesante en este tipo de compuestos, ya que las fibras PA6 se habían fundido, dejando una huella donde se detectaron agrupaciones de cristales con forma irregular y tamaños variables, cubiertos por una capa de partículas poliméricas. Una posible explicación a este este fenómeno sería la resolidificación de las fibras PA6 restantes después del enfriamiento, que condujo a la mejora de la cohesión de la matriz y retrasó la fragmentación de cristales observada en el yeso de referencia a 300ºC. 3.4. Ensayos físico-mecánicos tras exposición a elevada temperatura (condiciones de servicio) Los resultados correspondientes a esta sección se recogen detalladamente en la Publicación 9 (Romero-Gómez et al., 2023b), sin embargo, a continuación se enuncian brevemente los hallazgos más relevantes: Densidad en estado seco En relación a esta propiedad, se observó una disminución generalizada en todas las muestras después de haber sido expuestas a un rango de temperatura creciente (Figura 26). Respecto a la muestras de referencia ensayadas a temperatura ambiente (~23 ◦C), los compuestos G/PP/7.5 y G/PA6/2.5, que inicialmente mostraron valores de densidad ligeramente superiores, presentaron una mayor pérdida después de ser expuestos a temperaturas progresivamente más elevadas (hasta 300 ◦C). La mejora de la compacidad de la matriz observada en las muestras con contenido plástico, ensayadas a temperatura ambiente, fue revertida con la exposición de estas a elevada temperatura. Tras alcanzar ~150 ◦C, ambas muestras de yeso con residuos plásticos mostraron una pérdida de densidad ~12%, mientras que el material de referencia presentó una pérdida del 9,5%. Sin embargo, las diferencias entre la perdida de densidad de los citados compuestos se hizo más notable a partir de este punto. La mayor disminución de densidad observada en los compuestos con residuos PPT, respecto a los que contienen fibras de PA6, se debió al mayor nivel de reemplazo utilizado en el caso de partículas PPT, así como la consiguiente disposición homogénea de estas partículas en la matriz, lo que dio lugar a una mayor red de macroporos conectados tras de la fusión del PP. Además, dado que los coeficientes de expansión térmica de PP y PA6 aumen- (a) (b) Figura 27. Resistencia a flexión (a) y compresión (b) de los compuestos de yeso antes y después de la exposición a altas temperaturas. Fuente: Elaboración propia 68 reas a 250ºC, superaron el mínimo de 2 MPa establecido por la norma EN 13279–2 (2014). A diferencia de las muestras de referencia, que mostraron valores de resistencia a la compresión estabilizados entre 150-300 ºC, los compuestos reforzados con fibras PA6, registraron una mejora de hasta 37.5%, en relación con el valor más bajo de resistencia a la compresión (2.69 MPa), a partir de 150ºC de exposición. Una vez más, esto puede explicarse por una posible re-solidificación de las fibras PA6 a una forma similar a su original, permitiendo una mayor cohesión de la matriz severamente deshidratada. tan significativamente cuando alcanzan sus puntos de fusión, es probable que la expansión excesiva dentro de la matriz condujese a la destrucción parcial de esta y, por tanto, una mayor pérdida de masa. Por tanto, dado que el PP presenta un coeficiente de expansion térmica más alto que el PA6 y están presentes en mayor cantidad, se considera coherente la mayor pérdida de densidad registrada por los compuestos G/PP/7.5. Resistencia mecánica: flexión y compresión En relación al comportamiento mecánico de las muestras analizadas, los compuestos que contenían residuos plásticos mostraron una reducción generalizada de la resistencia a la flexión a medida que aumentó la temperatura de exposición (Figura 27a). En comparación con el yeso de control, el uso de PPT como reemplazo parcial de la matriz de yeso deterioró el comportamiento de resistencia a flexión de los compuestos.Sin embargo, la adición de fibras PA6 dió lugar a una ligera mejora de la resistencia a flexión de los compuestos de yeso después de haber sido expuesto a temperaturas elevadas. Una posible explicación a este hecho sería la alta relación de aspecto, así como, la alta resistencia a la tracción de las fibras PA6, que podrían haberse re-solidificado, mejorando la continuidad de la matriz de yeso debilitada a partir de ~150ºC debido a la deshidratación del yeso (CaSO4+2H2O). Asimismo, cabe señalar que los valores de resistencia a flexión de las muestras G/PP/7.5 no alcanzaron el mínimo de 1 MPa establecido por la norma EN 13279-2 (2014) después de superar los 100 ºC, mientras que los compuestos G/PA6/2.5 cumplieron con los requisitos estándar hasta superar los 200 ºC, con valores incluso ~20% superiores a los de las material de referencia sometidos a las mismas condiciones. Por otro lado, la adición de ambos tipos de residuos plásticos permitió mejorar ligeramente resistencia a la compresión de las muestras antes de su exposición a altas temperaturas, directamente relacionado con la mayor compacidad de la matriz previamente observada. Sin embargo, tal y como se puede observar en la Figura 27b, los compuestos con PPT y PA6 exhibieron una pérdida de resistencia a compresión ligeramente mayor después de la exposición a altas etemepraturas, probablemente debido a la destrucción de la matriz mencionada anteriormente debido al mayor coeficiente de expansión térmica de las partículas de plástico en paralelo con la deshidratación del CaSO4+2H2O. Además, cabe subrayar que todos los compuestos, a excepción del G/PP/7.5 expuesto a temepraturas superio- Figura 28. Comparación de la apariencia superficial de los compuestos G/CM, G/PP/7.5 y G/PA6/2.5 después de exposición a elevadas temperaturas (hasta 300 ◦C). Fuente: Elaboración propia. 69 Apariencia superficial De acuerdo a la información expuesta en la Figura 28, dos procedimientos químicos principales asociados a la exposición a altas temperaturas (calcinación y oxidación parciales) fueron detectados durante el proceso de calentamiento de los compeustos en cámara ventilada. No se realizaron modificaciones en el formato y el color de las muestras hasta superar los 150 ºC de exposición. A partir de esta temperatura, comenzó la deshidratación del yeso (de CaSO4+2H2O a CaSO4+1/2H2O), lo que dió lugar a la aparición de una capa externa grisácea/marrón de material afectado en las muestras, que redujo la conductividad térmica del material debido a la liberación de agua y la formación de poros, dificultando la transferencia de calor al interior de la pieza, a modo de retardante semiefectivo. A partir de 200ºC, se sucedió un cambio de color más intenso en los compuestos G/PPT/7.5, ya que tras alcanzar el PP su punto de fusión (~150 ºC), el proceso de degradación térmica conduce a su despolimerización y formación de hidrocarburos saturados e insaturados de bajo peso molecular, que pueden sufrir reacciones de oxidación y reticulación, provocando así la coloración pardusca, como se ve en la Figura 28. A pesar de los dife- 70 Exposición de la superficie y del borde inferior Código de la muestra G/CM G/PPT/7.5 G/PA6/2.5 Inflamabilidad No No No Llama 150 mm por encima del punto de aplicación de la llama (Fs > 150 mm) No No No Duración proceo de combustión 150mm --- Presencia de gotas/partículas inflamables No No No Inflamabilidad del papel No No No Código de la muestra PP PA6 G/CM G/PPT/7.5 G/PA6/2.5 Poder calorífico bruto (QPCS) (MJ/kg) 41.2 28.3 0.0 3.0 0.7 rentes productos de degradación térmica de cada tipo de polímero, los compuestos G/PA6/2.5 también exhibieron coloración marrón/grisacea por la liberación de hidrocarburos, aunque esta comenzó ~250 ºC, debido a la mayor estabilidad térmica del PA6 en comparación con del PP. Además, la degradación térmica de PP y PA6, condujo a un aumento del tamaño de los poros debido a la expansión térmica y liberación de gases, provocando la pérdida de la adhesión plástico/aglutinante. Por último, considerando la mayor conexión de macroporos observada en las muestras G/PP/7.5, estos exhibieron una mayor degradación a nivel interno y nivel externo (es decir, aparición de grietas superficiales, pérdida de volumen y fragilidad), en comparación con los compuestos G/PA6/2.5. 3.5. Ensayos de reacción al fuego, clasificación según normativa La clasificación de los nuevos compuestos con contenido plástico, G/PPT/7.5 y G/PA6/2.5, en función de su comportamiento frente al fuego, se llevó a cabo de acuerdo al Sistema de clasificación Europeo de Reacción al Fuego (Euroclases), establecido en la normativa EN 13501-1 (2019), en base a los siguientes ensayos: Inflamabilidad A partir del ensayo de inflamabilidad de los productos cuando se someten a la acción directa de la llama, cuyos resultados se exponen en la Tabla 4, se verificó que todos los compuestos presentaron una buena resistencia a la inflamabilidad, ya que no hubo propagación de la llama en la superficie o bordes de las muestras, ni liberación de humo durante el desarrollo del ensayo. Por tanto, los compuestos G/PPT/7.5 y G/PA6/2.5 pueden clasificarse como Clase E, de acuerdo con la normativa ISO 11925-2 (2020). Poder calorífico bruto Teniendo en cuenta los requisitos para la clasificación como material no inflamable, se realizó el ensayo de fuego de calor bruto de combustión. La Tabla 5 muestra los valores caloríficos obtenidos de esta prueba. Según las directrices establecidas por la norma ISO 1716 (2018), los compuestos de yeso con un 7.5 wt% de PPT y un 2.5 wt% de residuos de PA6 pueden clasificarse como productos no homogéneos. Por lo tanto, el poder calorífico final por compuesto se calculó considerando los valores de combustión de calor bruto individuales medidos por cada componente (es decir, PP y PA6, dado que el yeso no es inflamable (0,0 MJ/kg)). Finalmente, se concluye que tanto los compuestos G/PPT/7.5 como los G/PA6/2.5 pueden ser clasificados como material no inflamable clase A2. Dado que los compuestos con un contenido de fibras PA6 del 2.5wt% presentaron un valor de calor bruto de combustión inferior a 2.0 MJ/kg, se debe realizar un ensayo complementario de reacción Tabla 4. Resultados del ensayo de inflamabilidad desarrollado. Tabla 5. Poder calorífico bruto de las muestras ensayadas en esta investigación. 71 Código de la muestra TVOC (µg/m3) CO2 (ppm) CO (ppm) CH2O (ppm) PM10 (µg/m3) PM2.5 (µg/m3) G/CM 50 2900 -0.04 30 33 G/PPT/7.5 1640 7000 -0.09 27 47 G/PA6/2.5 280 1800 -0.05 22 34 Valor IDHL 500 40.000 1200 20 50* 25* *Valor límite diario (período 24h) al fuego denominado “Ensayo de Incombustibilidad”, según la norma ISO 1182 (2010), para establecer su clasificación como material no inflamable A1. 3.6. Ensayo experimental para determinar la toxicidad de los compuestos en caso de fuego Los datos obtenidos del ensayo experimental para determinar los gases potencialmente peligrosos y sustancias liberadas durante la combustión de los compuestos G/CM, G/PPT/7.5 y G/PA6/2.5, expuestos a elevadas temperaturas (hasta 1000 ºC) en un horno de mufla, se exponen en la Tabla 6. Los valores de materia orgánica volátil total (TVOC), dióxido de carbono (CO2), monóxido de carbono (CO) y formaldehído (CH2O), así como la emisión de partículas liberadas PM10 y PM2.5, fueron registrados y comparados con los valores de concentración en el aire “Inmediatamente peligrosos para la vida o la salud” (Ludwig et al, 1994) estipulados por el Instituto Nacional de Seguridad y Salud Ocupacional (NIOSH, 2019). En cuanto a la liberación de TVOC, las mediciones obtenidas para las muestras de yeso con contenido plástico, estuvieron muy por encima de las cifras registradas por el material de referencia. Sin embargo, sólo el compuesto G/PPT/7.5 excedió el valor IDLH límite. Asimismo, las muestras con partículas PPT presentaron los valores más altos de emisiones CO2 (7000 ppm) y CH2O (0,09 ppm), en comparación con el resto de compuestos, G/CM y G/PA6/2.5. No obstante, todos los valores Tabla 6. Emisión de sustancias potnecialmente peligrosas. (a) (b) (c) (d) Figura 37. Prototipos FCP sometidos a ensayo de resistencia a flexión: a) FCP/REF ; b)FCP/PA6/2.5. ; y dureza superficial (impacto): a) FCP/REF ; b)FCP/PA6/2.5. Fuente: elaboración propia. 78 Resistencia a flexión En cuanto a los resultados de los ensayos de resistencia a la flexión (Tabla 7), se verificó que las nuevas placas reforzadas con fibras PA6 cumplen los requisitos establecidos por la norma EN 14246 (2007), es decir, todas las placas soportaron una carga lineal uniforme de 6 kg (0,1 kN/m) aplicada en la zona central durante 30 min, sin mostrar signos aparentes de daño. Posteriormente, las placas fueron ensayadas a flexión hasta alcanzar la carga de rotura (Figura 37a-b). Como resultado, se obtuvieron valores más altos de resistencia a flexión (hasta un 11% de aumento) para las placas reforzadas con fibras PA6, en comparación con las placas de referencia. PLACA (FCP) Cumplimiento EN 14246 (6kg 30’) Carga de rotura (kN) Valor medio (kN) FCP/G/REF-1 OK 0.15 0.18FCP/G/REF-2 OK 0.18 FCP/G/REF-3 OK 0.22 FCP/G/PA6-1 OK 0.19 0.20FCP/G/PA6-2 OK 0.20 FCP/G/PA6-3 OK 0.21 Tabla 7. Resultados del ensayo de resistencia a flexión de las placas del falso techo. 79 PLACA (FCP) Impacto - Ø1 (mm) Impacto - Ø2 (mm) Impacto - Ø3 (mm) Valor medio - Ø (mm) FCP/G/REF-1 Roto - - Roto (15.3)FCP/G/REF-2 Roto - - FCP/G/REF-3 15 16 16 FCP/G/PA6-1 13 13 12 13.1FCP/G/PA6-2 13 14 12 FCP/G/PA6-3 13 14 14 PLACA (FCP) Masa (g) - Pre-ensayo Masa (g)-Post-ensayo Diferencia de masas (g) Absorción de agua FCP/G/REF-1 1631.8 2049.1 410.5 25.5 %FCP/G/REF-2 1642.7 2055.6 404.8 FCP/G/REF-3 1630.6 2048.7 411.6 FCP/G/PA6-1 1605.6 2007.2 393.9 24.7 %FCP/G/PA6-2 1670.9 2083.9 404.7 FCP/G/PA6-3 1620.3 2016.6 389.2 Dureza superficial Completada la caracterizarción de la dureza superficial de las nuevas placas de yeso, en comparación con el material de referencia, los resultados se muestran en la Tabla 8. A partir de estos datos se pudo observar que FCP/G/PA6 presentó un diámetro promedio de impacto alrededor de 13 mm. Es decir, el uso de2.5 wt% fibras PA6 como reemplazado parcial de la matriz de yeso dió lugar a una reducción de hasta el 14% del diámetro de impacto obtenido para el producto de referencia. Además, cabría añadir que sólo se pudo obtener una medida en el caso de las placas de referencia, ya que dos de las tres placas quedaron completamente rotas tras el primer impacto (Figura 37c-d). Absorción total de agua La prueba de absorción total de agua reveló una disminución (hasta un 3%) de la capacidad de absorción de agua de las placas de yeso debido a la adición de fibras PA6, en comparación con las placas de referencia (Tabla 9). Asimismo, las nuevas FCP/G/PA6, pueden categorizarse como H3 (capacidad de absorción de agua reducida), en base a la clasificación de absorción de agua marcada por la norma EN 520 + A1 (2005), dado que se obtubo un porcentaje de absorción inferior al 25%. Sin embargo, no puede hacerse una clasificación especial para el material de referencia. Tabla 9. Valores obtenidos a partir del ensayo de absorción total de agua para las distintas FCPs. Tabla 8. Datos obtenidos en el ensayo de dureza superficial para las placas de falso techo. (a) (b) Figura 38. Análisis visual comparativo de las propiedades físico-mecánicas, térmicas y ambientales de los a) AGB y b)FCP. Fuente: elaboración propia 80 4.3. Evaluación viabilidad aplicación produtos desarrollados. En primer lugar, dado que en el trascurso de esta tesis doctoral se desarrolla en mayor medida uno de los productos, bloque de yeso alveolar (AGB), se lleva a cabo un análisis comparativo detallado pormenorizadamente en la Publicación 11 (Romero-Gómez et al., 2023e) para evaluar el comportamiento de los nuevos AGBs en relación con otras soluciones similares disponibles en el mercado. A partir de esta comparativa, queda demostrada la viabilidad de fabricación y aplicación de los nuevos AGBS propuestos en este trabajo como un sistema de tabiquería interior, ya que ambas soluciones con contenido plástico reciclado AGB/PP/7.5 y AGB/PA6/2.5, ofrecen un rendimiento físico-mecánico, térmico y medioambiental similar o incluso mejorado a las soluciones comercializadas actualmente. En segundo lugar, a modo de resumen, en la Figura 39, se presenta una evaluación comparativa visual de las principales propiedades físico-mecánicas, térmicas y ambientales de ambos tipos de productos resultado de este trabajado de investigación, bloques de yeso alveolar-AGB (AGB/PP/7.5 y AGB/PA6/2.5), y placas de falso techo-FCP (FCP/PA6/2.5), en comparación con los productos de referencia sin adición de plástica. A partir de la Figura 38a, se puede afirmar que los AGBs reforzados con 2.5wt% PA6 presentaron el mejor comportamiento, en tanto en cuanto el cómputo general de las propiedades analizadas. Aunque se detectó un ligero aumento en la densidad en este producto, respecto al material de referencia, esto condujo a una mejora notable de la resistencia a flexión y compresión. Además, se consiguió una significativa reducción de la conductividad térmica. Si bien, los valores más bajos de permeabilidad al agua correspondió a los bloques AGB/PP/7.5, AGB/PA6/2.5 presentó una reducción de la capacidad de absorción de agua de un 48%, respecto a la referencia, ofreciendo solución a uno de los principales handicaps del uso de productos de yeso sin tratamiento hidrófugo. Por otro lado, la Figura 38b revela un mejor desempecho generalizado de las nuevas placas FCP/PA6/2.5, en tanto en cuanto, propiedades físico-mecánicas, térmicas y de resistencia al agua, en comparación con el producto de referencia. Dado su aplicación como componente de un sistema de revestimiento interior horizontal (falso techo), resulta especialmente destacable, la mejora de resistencia a flexión (hasta 11% de aumento), así como la reducida capacidad como conductor térmico. Finalmente, desde una perspectiva ambiental, para ambos aplicaciones, se lograron reducciones en los factores de impacto ambiental (es decir, EE y PGB), así como en los costos económicos, pese a que los porcentajes de disminución fueron moderados, como consecuencia de los bajos porcentaje de residuos plásticos utilizados. No obstante, debe destacarse, la posibilidad de conseguir una mayor reducción, mediante la optimización de las mejoras físico-mecánicas y beneficios económico-medioambientales para conseguir aumentar los niveles de sustitución de residuos plásticos. Abriéndose así, nuevas líneas de investigación futuras. CONCLUSIONES6 81 El trabajo de investigación desarrollado en el trasncurso de la tesis doctoral presentada, pretende aportar avances en la propuesta de nuevos productos ecoeficientes destinados a aplicaciones constructivas. Atendiendo en este caso a las diversas posibilidades de aplicación que ofrecen los materiales a base de yeso, así como su función no estructural, se ve una oportunidad para explorar la sustitución parcial de la matriz por materiales de desecho. En este caso, considerando la alarmante situación en la que la contaminación plástica está sumiendo al planeta, junto con la consecuente necesidad de buscar alternativas para promover un reciclaje eficiente y contrarrestar así la exponencial producción plástica, se dedice explorar la posibilidad de usar diferentes tipos de residuos plásticos, actuamente sin clasificación específica en el sistema de reciclaje existente, como reemplazao parcial de la matriz de yeso. Por tanto, a lo largo de este trabajo, se lleva a cabo la caracterización completa de diferentes compuestos de yeso con contenido plástico, atendiendo a la composición fibrosa o no de estos agregados. Tras analizar las propiedades físico-mecánicas, térmicas, resistencia al agua y comportamiento al fuego de los diferentes compuestos, se proponen dos tipos de elementos constructivos, bloques de yeso alveolar (AGB) y placas de yeso para falso yeso (FCP), a partir de los compuestos óptimos. A continuación, se enuncian las conclusiones parciales obtenidas para cada una de las secciones parciales que estructuran esta investigación. Para finalizar, se añade una reflexión general , así como la planificación de futuras líneas de investigación. SECCIÓN 1: Caracterización de nuevos compuestos de yeso parcialmente reemplazados por residuos plásticos triturados. Establecido como factor común a todas las mezclas una misma relación agua-yeso (A/G) de 0.55, se determinan los niveles máximo de reemplazo de dos tipos de residuos, PPT y LDPE, por la matriz de yeso, 10wt% y 5wt%, respectivamente, para conseguir mantener una correcta trabajabilidad de las mezclas de acuerdo a la normativa, sin aditivos. Tras analizar la influencia de ambos tipos de residuos en el comportamiento físico-mecánico y resistencia al agua de los compuestos de yeso, se concluye: - La adición de residuos PPT da lugar a una mejora generalizada de la resistencia a compresión (de hasta 9%, en comparación con el yeso de referencia), debido a la mejora de la compacidad de la matriz de yeso hasta superar 7.5wt% de reemplazo. Así mismo, debe destacarse el notable descendo de la capacidad de absorción de agua a medida que aumentó el nivel de reemplazo (hasta 36% de disminución, respecto al material de referencia), como consecuencia de la sustitución de la matriz de yeso por un material impermeable. - En relación al uso de residuos LDPE, pese a conseguirse una disminución de 2% de la densidad de los nuevos compuestos, en relación con el material de referencia, así como una notable disminución de la absorción al agua de los nuevos compuestos (hasta 48%, en comparación con el yeso de control), el comportamiento mecánico se vió negativamente afectado (resistencia a flexión y compresión reducida hasta un 40% y 60%, respectivamente), considerando los bajos niveles de reemplazo usados. SECCIÓN 2: Caracterización de nuevos compuestos de yeso reforzados con fibras plásticas recicladas. Siguiendo el mismo criterio que en la Sección 1, se fijo un valor de relación A/G común, 0.55 para todas las mezclas, lo cual determinó los niveles de reemplazo máximos para las distintos materiales estudiados: 3.5wt% en el caso de las fibras PPF y CA, y 3wt% para las fibras PA6. No obstante, debe señalarse, que para las fibras de CA resultó necesario aumentar la relación A/G a 0.60 para mantener una correcta trabajabilidad, debido a la elevada absorción de agua que presentó este material en formato microfibras. Caracterizados los tres tipos de compuestos, las conclusiones obtenidas fueron las siguientes: 82 - La adición de fibras plásticas recicladas, indiferentemente del tipo, contribuyó a la mejora de las propiedades mecánicas, especialmente, en referencia a la resistencia a flexión de los compuestos de yeso. En comparación con el material de referncia, el compuesto G/PPF/2.5 mostró los mayores incrementos de resistencia a flexión (19% de mejora), así como los valores más altos de resistencia a compresión (incremento del 7.5%) correspondieron al compuesto G/PA6/I/2. - Además, cabría destacar que, la adición de fibras PA6 recicladas consiguió mejorar la ductilidad de los compuesto ante la fractura, observándose un comportamiento similar al conseguido con las fibras de vidrio comerciales, normalmente aplicadas en porcentajes de adición más bajos. - En tanto en cuanto, a la adición de fibras CA, el mayor hallazgo radicó en la significativa reducción de la conductividad térmica detectada en los compuestos (16% reducción, respecto al yeso de referencia). Sin embargo, se hace notable la necesidad de ampliar la investigación en este campo, a fin de conseguir estandarizar y mejorar el pretratamiento del residuo, reduciendo el olor de los compuestos que lo - s incorporan y eliminando eficientemente sus compuestos tóxicos. SECCIÓN 3: Evaluación del comportamiento físico-mecánico de compuestos de yeso con residuos plásticos, expuestos a elevadas temperaturas en condiciones de servicio y fuego. Concluida la caracterización fisico-mecánica, térmica y evaluado el comportamiento frente al agua de los cinco tipo de compuestos analizados, se selecciona por cada grupo, residuos triturados o fibrosos, el compuesto y dosificación óptimo, en base a dos criterios: máximo porcetage de reemplazo para conseguir igualar o mejorar de forma simultánea las distintas propiedades estudiadas, respecto al yeso de referencia; así como, mayor eficiencia y simplicidad del proceso de pretratamiento para obtención de los residuos listos para ser usados en las mezclas. Atendiendo a ambos criterios, las mezclas consideradas óptimas fueron: G/PPT/7.5 y G/PA6/I/2.5. Estas muestras fueron sometidas a una serie de ensayos para determinar su comportamiento frente a condiciones de elevada temperatura (condicones de servicio hasta 300ºC) y fuego (hasta 1000ºC), deduciéndose las siguientes conclusiones: - Analizado su comportamiento a nivel microscópico tras ser somedidos ambos compuestos, junto con el material de referencia a condiciones de exposición de hasta 300ºC de temperatura, y comparándolas con el desempeño fisico-mecánico residual de las distintas muestras, se determinó que el detrimento de las propiedades de los compuesto de yeso se ve condicionada en mayor medida por los procesos de deshidratación de la matriz de yeso, que por la influencia del contenido plástico. - Sometidos los compuestos analizados a ensayos normalizados para obtener su clasificación en función de la reacción al fuego, se obtuvo que tanto los compuestos G/PPT/7.5, como G/PA6/2.5, pueden categorizarse mínimo como Euroclase A2, es decir, material no inflamable. - Respecto a la determinación del grado de toxicidad de las muestras tras ser expuestas a condiciones de fuego, se llevó a cabo un ensayo experimental, del cual se concluye que todas las emisiones analizadas (TVOC, CO2, CO, PM10), a excepción de las emisiones de partículas PM2.5 estuvieron muy por debajodel límite establecido como saludable, correspondiendo los valores más altos de PM2.5 al compuesto G/PP/7.5 (47 µg/m3). Sin embargo, realizada esta primera aproximación se determina la necesidad de ampliar el abanico de ensayos para determinar la toxicidad de los gases emitidos, basadas en el “método del calorímetro de cono” y el análisis de la dosis efectiva fraccionada (FED). SECCIÓN 4: Generación de productos con base de yeso y residuos plásticos. Tomando como base los compuestos seleccionados como óptimos, se desarrollan dos tipos de productos a base de yeso, como etapa de conclusión de este trabajo de ivestigación. Considerando las mejoras respecto a las propiedades del material de referencia alcanzadas por los dos tipos de compuestos propuestos, G/PPT/7.5 y G/ PA6/2.5, se analizan las aplicaciones constructivas más adecuada para cada uno de ellos. Como resutlado, se propone el desarrollo de un bloque de yeso alveolar (AGB), para el cual se aplicarán ambos compuestos, a fin de comparar los resultados teniendo en cuenta que, en ambos casos, se mejoraron las propiedades mecánicas, especialmente la resistencia a compresión, del yeso de referencia. Sin embargo, atendiendo a la notable mejora alcanzada respecto a los valores de resistencia flexión registrados por los compuestos que contenían fibras PA6, se decide desarrollar un segundo producto, en este caso, placas de falso techo registrables (FCP). Evaluado el comportamiento de ambos productos de acuerdo con las pertinentes normativas, se extraen las si- 83 guiente conclusiones: - Queda demostrada la viabilidad de fabricación y aplicación de los nuevos AGBs propuestos como un sistema de tabiquería interior, ya que ambas soluciones con contenido plástico reciclado AGB/PP/7.5 y AGB/ PA6/2.5, ofrecen un rendimiento físico-mecánico, térmico y medioambiental similar o incluso mejorado a las soluciones comercializadas actualmente. - Mientras que los AGBs reforzados con 2.5wt% PA6 presentaron el mejor comportamiento general, con una notable mejora de la resistencia a flexión y compresión en relación al producto de referencia (8% y 8.4%, respectivamente), además de una significativa reducción de la conductividad térmica (de hasta 18%), los valores más bajos de permeabilidad al agua correspondió a los bloques AGB/PP/7.5 (42% de reducción en comparación con el AGB de referencia). - Por otro lado, respecto a las nuevas placas FCP/PA6/2.5, se detectó una significativa mejora de las propiedades físico-mecánicas, térmicas y de resistencia al agua, en comparación con el producto de referencia. Especialmente destacable la mejora de resistencia a flexión (hasta 11% de aumento), así como la reducida capacidad como conductor térmico. - Desde un punto de vista económico-medioambiental, se lograron reducciones en los factores de impacto ambiental EE y PGB, así como en los costos económicos. Sin embargo, los porcentajes de disminución en ambas áreas fueron ligeros, dado que, los niveles de reemplazdo usados no superaron el 10wt%. Futuras línea de investigación Considerando los resultados obtenidos en esta tesis doctoral, se proponen las siguinetes líneas de investigación futuras: - Evaluación de porcentajes de reemplazo plástico mayores en aquellos compuestos considerados óptimos, aumentando la relación A/Y para mantener la trabajabilidad de las muestras, apostando por una mayor reducción del impacto ambiental de los productos, en detrimento de las propiedades físico-mecánicas (siempre asegurando los valores mínimos establacido por normativa). - Sustitución de la matriz conglomerante por yeso reciclado, a fin de conseguir un producto 100% reciclado, línea actualmente en desarrollo. Esta línea permitiría no solo reducir el impacto industrial de los productos de construcción resultantes, sino que ayudaría a reducir la tasa de contaminación deribada de la industria plástica, así como el sector de la construcción, ofrecería una solución eficiente para la gestión de residuos. - Aplicación de técnicas de optimización multiobjetivo, para explorar la combinación binaria de distintos tipos de residuos plásticos, triturados y fibras, con el proposito de obtener un producto con el mayor porcetaje de mejoras respecto al material de referencia o bien, enfocar las mejoras en una determinada propiedad, según la aplicación , maximizando así la competitividad de este producto a nivel industrial. 84 CONCLUTIONS6 85 During this Ph.D. dissertation research, an effort to developing new eco-friendly construction products with a focus on gypsum-based materials and their non-structural applications has been carried out. After exploring various avenues, an opportunity to evaluate the feasibility of using plastic waste as partial replacement of gypsum matrix, specifically plastic residues not currently classified in the recycling system, was found. To ensure the potential success of this approach, a thorough characterization of different plastic-containing gypsum composites, including those in crushed or fibre aggregate format, was conducted. These analyses encompassed the physico-mechanical, thermal, water-resistance properties, and fire reaction performance of the composites. On the basis of these results, two construction products were proposed: Alveolar Gypsum Blocks (AGB) and False Ceiling Plates (FCP) that utilised the optimal composites and dosages. To sum up, partial conclusions from each section, as well as a comprehensive reflection on the main findings are following drawn. Additionally, plans for future research lines to continue exploring the use of waste materials in construction products are provided. SECTION 1: Characterisation of new gypsum-based composites partially replaced by shredded plastic waste. In this study, new gypsum-based composites were examined with the addition of shredded plastic waste. A water/ gypsum ratio of 0.55 was used for all composites. This ratio determined the maximum replacement levels for two types of plastic waste, PPT and LDPE, at 10wt% and 5wt%, respectively, to ensure adequate workability without the use of additives. After evaluating the impact of plastic waste on the physico-mechanical and water-resistance performance of the composites, the following conclusions were reached: - PPT waste addition led to a significant improvement in compressive strength (up to 9% compared to the reference gypsum) until exceed 7.5wt% level of replacement, because of the increase of matrix compactness. PPT-containing gypsums also demonstrated a significant reduction in water absorption capacity (around 36% drop compared to the control material) due to the use of a waterproof material as a replacement for the hygroscopic gypsum matrix. - The use of LDPE resulted in a decrease in dry bulk density of around 2% compared to the control material. However, it also led to composites with reduced water absorption capacity (over 48% decreases at the highest replacement level). Unfortunately, the mechanical performance of these composites was negatively impacted (40% and 60% reduction in flexural and compressive strength, respectively) despite the low replacement levels used. SECTION 2: Characterisation of new gypsum-based composites reinforced with recycled plastic fibres. In the analysis of Section 2, novel gypsum-based composites reinforced with recycled plastic fibres were explored. To maintain uniformity, all mixtures adhered to a fixed W/G ratio of 0.55. This determined the maximum substitution levels for various plastic residues, such as 3.5wt% for PPF / CA fibres and 3wt% for PA6 fibres. However, it’s worth noting that mixtures containing CA fibres required a higher W/G ratio of 0.60 to achieve appropriate workability due to CA microfibers’ heightened water absorption. After characterizing the three composite types, several conclusions were drawn. Firstly, the use of plastic waste fibres as partial replacement of the binder matrix, regardless of their type, enhanced the mechanical characteristics of novel gypsum composites. The most notable improvement in flexural strength values was observed in G/PPF/2.5 (up to a 19% increase) compared to the reference gypsum, while the highest compressive strength values were noted in G/PA6/I/2 composite (around a 7.5% increase). Secondly, we found that composites containing PA6 fibres 86 showed a more ductile post-failure behaviour, similar to composites that contain commercially available glass fibres (frecuently used in lower percentages of addition). Lastly, the use of CA microfibers as partial replacement of the gypsum matrix significantly decreased thermal conductivity in all composites, with a greater decrease as the amount of waste fibres increased (up to a 16% reduction compared to control gypsum). However, further research is necessary to standardise the pre-treatment of CA waste (such as cleaning and disinfecting cigarette filters), mitigate the composites’ odour, and manage toxic effluent. SECTION 3: Evaluation of physico-mechanical behaviour of the new plastic waste-containing gypsum composites exposed to elevated temperature in service and fire conditions. Concluded the physico-mechanical, thermal and water-resistant characterisation of the five gypsum-based composites, the optimal composite and dosage by residue group (crushed or fibrous) were selected. The chosen composites, G/PPT/7.5 and G/PA6/I/2.5, met two key criteria: maximum replacement level to maintain or enhance simultaneously the different reference gypsum properties, as well as, the efficiency and simplicity of waste pre-treatment. Tests were carried out on optimal composites to assess their behaviour when exposed to elevated temperatures in service conditions (up to 300ºC) and fire (up to 1000ºC). From SEM microscopy analysis, it was concluded that the dehydration process of the gypsum matrix had a greater impact on the deterioration of physico-mechanical properties than the presence of plastic content after exposure to elevated temperatures. The Euroclass fire reaction classification of both composites (G/PP/7.5 and G/PA6/2.5) was determined to be A2, which indicates a nonflammable material. Toxicity of substances emitted during combustion procedure of samples with plastic content was also analysed, with emissions including TVOC, CO2, CO, PM10, and PM2.5. All values were below the healthy limit set by the standard, except for particle PM2.5 release, corresponding the highest values to G/PP/7.5 composite (47 µg/m3). Hovewer, further testing is needed to determine the toxicity of gases emitted by plastic waste-containing gypsum samples during combustion, based on the ‘cone calorimeter method’ and Fractional Effective Dose (FED) analysis. SECTION 4: Proposal of new eco-friendly gypsum-based products with plastic waste content. Based on selected optimal gypsum composites, two types of gypsum-based products were developed as the final stage of this research. Considering the improvements related to reference material properties, achieved by the two types of composites proposed, G/PPT/7.5 and G/PA6/2.5, the most suitable constructive application for every composite was evaluated. As a result, an alveolar gypsum block (AGB) prototype was proposed using both plastic waste-containing composites separately and compared with the reference products without waste addition because of the notable mechanical strength enhancements registered, specifically on compressive strength terms. On the other hand, attending to the remarkable improvements in flexural strength reached by composites reinforced with PA6 fibres, a second type of product was carried out, a false ceiling plate prototype (FCP). After evaluating the performance of both product types, it was found that: - Both proposed AGBs could be used as a partitioning system since they offer improved physico-mecha- nical, thermal, and waterresistant performance comparable to or better than commercialised solutions. - The AGB reinforced with 2.5wt% PA6 fibre demonstrated the best generalised behaviour with significant improvements in flexural and compressive strength (8% and 8.4% increase, respectively), as well as a notable decrease in thermal conductivity, in comparisson with reference product. However, ABG/PP/7.5 blocks showed the lowest water permeability values (up to 42% decrease). - New eco-friendly FCP/PA6/2.5 plates showed a significant improvement in physico-mechanical, thermal, and water-resistant properties, particularly in flexural strength (up to 11% increase) and reduced thermal conductivity, when compared with control product. - Partial replacements of the gypsum matrix with PP particles and PA6 fibres in the manufacture of new construction products could reduce environmental and economic costs. However, the dismiss percentages were light due to the low replacement levels used (10wt% maximum). 87 Future lines of research Evaluating the results obtained in this Ph.D. dissertation, further research focused on the following subjects could be proposed: - Study of higher percentages of plastic replacement in those compounds classifed as optimal, increasing the G/W ratio to maintain the workability of the samples. In this way, a greater reduction of products’ environmental impact is searched, despite the reduction of physical-mechanical properties (always meeting the minimum requirements established by standards). - Substitution of the raw binder matrix (i.e. commercial gypsum) with recycled gypsum sourced from construction waste, to achieve a 100% recycled product (currently developing research line). This line would not only reduce the industrial impact of the resulting construction products but would also help to reduce the pollution rate derived from the plastics industry, as well as the construction sector, offering an efficient solution for waste management. - Application of multi-objective optimisation techniques to explore the binary combination of different types of plastic waste, shredded and fibres, in order to obtain a product with the highest percentage of improvements with respect to the reference material by complementing the advantages of each type of residue or to focus the improvements on a specific property, depending on the application, thus maximising the competitiveness of this product at an industrial level. 94 95 PUBLICACIONES EN REVISTAS INDEXADAS 1 SECCIÓN 1 Publicación 1 Publicación en revista indexada JCR (2022): Journal of Cleaner Production, Q1(8/55). Factor de impacto 11.1. Segundo decil. Open Acess. Romero-Gómez, M.I., Vasco-Silva, R., M.F., Flores-Colen, I., De Brito, J. (2022). Influence of polypropylene residues on the physico-mechani- cal and water-resistance. Journal of Cleaner Production, 371, 133674, https://doi.org/10.1016/j.jclepro.2022.133674 96 Journal of Cleaner Production 371 (2022) 133674 Available online 20 August 2022 0959-6526/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Influence of polypropylene residues on the physico-mechanical and water-resistance properties of gypsum plasters M.I. Romero-G´ omez a , * , R.V. Silva b , ** , I. Flores-Colen b , J. de Brito b a Departamento de Construcciones Arquitect´ onicas 1, Escuela T´ ecnica Superior de Arquitectura, Universidad de Sevilla, Avenida Reina Mercedes, No. 2, 41012, Sevilla, Spain b CERIS, DECivil, Instituto Superior T´ ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal ARTICLE INFO Handling Editor: M.T. Moreira Keywords: Polypropylene waste Gypsum plaster Mechanical performance Water-resistance properties Eco-friendly material ABSTRACT The main objective of the present research is to determine the feasibility of using polypropylene (PP) waste to produce eco-friendly gypsum-based composites. Four different replacement levels of plastic waste were selected for analysis - 2.5%, 5%, 7.5% and 10% by weight of gypsum. The first stage of the experimental campaign consisted of analysing the resulting composites based on density, mechanical performance (flexural and compressive strength) and surface hardness (Shore C). In the second stage, samples were evaluated based on their water absorption by capillary action or exposing them to continuous moisture in a wet chamber, and water resistance by subjecting them to several wetting-drying cycles. After that, scanning electron microscopy (SEM) analyses were carried out to evaluate the quality of the cohesion at the interfacial transition zone (ITZ) between the PP waste particles and the gypsum matrix. The results showed equivalent compressive strength and even a slight improvement (up to ~9% when using 2.5% of PP waste when compared to the control material) most likely due to the greater compactness from the improved particle size distribution. In addition, a considerable reduction in water absorption and retention capacity was observed, due to the increasing incorporation of recycled PP into the gypsum matrix. The new PP-containing composites subjected to water absorption by capillary action and continuous moisture tests presented lower water absorption and adsorption capacity, wherein specimens with 10% of PP waste showed a decline of up to ~37% when compared to the reference material. It was generally observed that the waste material can be considered as a viable partial substitute to the current commercial gypsum for the production of gypsum plasterboards. 1. Introduction In recent years, the consumption of disposable plastic has been increased exponentially. According to recent findings put forward by the Minderoo Foundation (2019), 130 million tonnes of single-use plastics were discarded throughout the world in 2019. The uncontrolled production of this type of plastic is leading to a considerable widespread problem, due to its low recycling ratio, as almost all of it is burnt, accumulating in landfills or illegally discarded directly into the environment. Among the different sources of plastic waste that are causing this problem, disposable coffee capsules (CCs) are one of those presenting an alarming increasing consumption trend and, at the same time, with very limited solutions for their sustainable management (The Business Research Company, 2021). About 35 thousand million of CCs are thrown away each year worldwide, corresponding to approximately 576.000 t of global waste (Life-Plan4 Project Coffee, 2018; Marinello et al., 2021). Diversity of flavours, affordability and speed joined in a very simple system to use, are the main characteristics that promote the use of one-dose CCs instead of making coffee traditionally. According to the Spanish National Statistics Institute, 2.829.744 CC units were consumed in Spain in 2020. This corresponds to an average of 60.3 units per person (INE, 2021). The Directive 2019/904 has been established by the European Parliament to limit the use of some single-use plastic products starting from 2021 (European Union. Directive (EU) 2019/904), but disposable CCs are not included in the legislation, which increases the need for looking for new recycling alternatives for this type of waste. Indeed, this waste presents a discouraging recycling ratio; barely 10% of aluminium-containing CCs are recycled in Spain, whilst plastic-based do not have specific treatment methods within the * Corresponding author. ** Corresponding author. E-mail address: [email protected] (M.I. Romero-G´ omez). Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro https://doi.org/10.1016/j.jclepro.2022.133674 Received 1 December 2021; Received in revised form 8 August 2022; Accepted 14 August 2022 Journal of Cleaner Production 371 (2022) 133674 2 standard recycling system currently followed by municipal entities (´ Alvarez, 2019). The large majority of plastic-based CCs are composed of polypropylene (PP) and aluminium, apart from the coffee itself, which makes it difficult to carry out adequate mechanical recycling methods. For this reason, their destination is usually common landfills (Domingues et al., 2020). One of the possible solutions for this unrecyclable waste could be the availability of consumers to separate the different components of the CCs before discarding them. However, it should be the initiative of the original producers to manufacture a product that can be easily used and recycled at the end of its life, instead of relying on consumers to take on this task. Alternatively, some companies have launched a collecting campaign to recycle PP-CCs, by providing collection points, to produce new plastic products for the company or urban furniture (NESCAF´ E Dolce Gusto, 2021). Taking into account that PP is one major plastic that is used to produce single-use products and the high impact of disposable CCs, several research studies have been developed in order to analyse their recycling possibilities and evaluate their degradation during different processes of recycling (Campos de Bomfim et al., 2019). In the study of Domingues et al. (2020), a proposal was presented for a manual separation approach of all the components (body, lid, aluminium filter and plastic filter) of a disposable CC. Similarly, Campos de Bomfim et al. (2021) used a manual separation process, followed by cleaning of the plastic components of expresso CCs, to make a homemade compost product using the waste coffee grounds. However, a more industrialized recycling process should be developed to achieve a viable solution to the growing environmental problem produced by single-use plastics. Given the volume of materials within the construction sector’s supply chain, it could offer an adequate solution for the aforementioned waste, because of the fairly high versatility of many of the already commercialized products (Plastic Europe, 2020). Several researchers have focused their efforts on the development of new building materials incorporating different types of waste plastic (e.g. Polyethylene Terephthalate-PET; Polycarbonate-PC; Extruded Polystyrene-XPS; Expanded Polystyrene-EPS; Polyurethane Foam-PUR) as a partial replacement for raw material. Apart from offering a second life to this waste, they occasionally improve some of the properties of cementitious composites. Concerning this issue, Kim et al. (2021) quantitatively evaluated the economic and environmental effects in the replacement of natural sand with ocean-borne plastic flakes up to 20% by volume, to develop cement composites. Also, the use of PET waste as partial substitute for sand in concrete was examined by Almeshal et al. (2020), achieving a reduction in unit weight in spite of the mechanical strength and fire resistance of concrete decreasing. On the other hand, the incorpoation of PUR waste to gypsum mortar mixes was studied by Vicario et al. (2020), leading to porous and lighter mortars, with enhanced resistance to water-vapour permeability, but lower mechanical strength and higher levels of absorbency. Within existing research, PET, PE and PP are the main types of plastic waste used in such composites as: recycled reinforcement fibres to improve the mechanical properties of the composites (Araya-Letelier et al., 2019); an aggregate replacement to achieve lightweight materials or to improve their water-resistance, thermal or acoustic behaviour (Coppola et al., 2018). Some significant works in this line were published by Ohemeng and Ekolu (2019), who developed a strength prediction model for cement mortar with different type of plastic waste as fine aggregate. Likewise, other researchers proposed using chemically pre-treated plastic waste as aggregates in cement mortar mixes, getting a more porous matrix to the detriment of the mechanical properties, as the plastic dosage increased (Kaur and Pavia, 2021). Some studies on gypsum composites incorporating recycled plastic have been developed. Pedre˜ no-Rojas et al. (2019) studied the influence of crushed PC waste from shredded CDs and DVDs on the performance of gypsum composites. The authors assessed the use of PC with different particle sizes at different replacement levels up to 60% by weight of gypsum, wherein a ~15% density decrease was observed when compared to the control material. Despite the overall negative impact on the mechanical performance, improved behaviour in terms of flexural strength had been observed for mixes with up to 10% by weight of smaller PC particles (<4 mm). In addition, enhanced compressive strength was observed for mixes with up to 40% by weight of PC in comparison with the control specimens. Other interesting works have been published by the same authors, Pedre˜ no-Rojas et al. (2020), based on the mixture of two different types of waste, recycled gypsum and PC waste, to developed new eco-friendly gypsum-based materials, entirely recycled, wich improved flexural and compressive strength (up to ~15% and ~27% increase, respectively), as well as lower thermal conductivity (~43% decrease), related to the reference material. Romero-G´ omez et al. (2021) analysed the influence of adding PP-based reinforcement fibres, sourced from single-use wet-wipes, on the performance of gypsum composites. Level replacements up to 3.5% by weight of gypsum binder were used. The results showed that there was an adequate adhesion between the plaster and the recycled PP fibres resulting in a 19% increase in flexural strength for mixes with 2.5% of the waste by weight of gypsum. Interesting findings were also observed in the study of Vidales-Barriguete et al. (2020) regarding the water-resistance of plastic-containing gypsum composites. The addition of plastic cable waste (particle size >3 mm) up to 70% in weight of gypsum led to a significant decrease in water absorption and retention capacity, whilst keeping their mechanical properties within the required standardized limits. From a thermal behaviour point of view, some studies have been published, wherein those developed by and Guti´ errez-Gonz´ alez et al. (2012) and Villela et al. (2020) presenting encouraging findings. Both of them show a decrease of the thermal conductivity of gypsum composites containing recycled polyamide powder and crushed multilayer packaging, respectively. Concerning the use of PP waste as replacement in conventional mortars, Rubio-De Hita et al. (2018) analysed the influence of adding mixed PP plastic as partial substitute for sand. The composites with a content of 12.5% PP by weight of natural aggregate were selected as the optimal, as they exhibited the lowest density and maintained a mechanical performance within the standardized minimum requirements, with marginal changes to workability. Martínez-L´ opez et al. (2021) studied the impact of using PP and polyethylene (PE) from waste bottles on the mechanical properties of mortars. Different levels of sand replacement (1%, 2%, 3%, by weight) and particle sizes (0.7 mm; 1.4 mm; 2.38 mm) were tested. The results showed that the mixes with 1% of 0.71 mm-sized particles from both Abbreviations list CoV Coefficient of variation CC Coffee capsule E Modulus of elasticity EPS Expanded polystyrene EU European Union G Gypsum ITZ Interfacial Transition Zone PC Polycarbonate PE Polyethylene PET Polyethylene terephthalate PP Polypropylene PUR Polyurethane foam RH Relative humidity SEM Scanning electron microscopy W/G Water/gypsum XPS Extruded polystyrene M.I. Romero-G´ omez et al. Journal of Cleaner Production 371 (2022) 133674 3 types of waste led to an increase up to 27% in compressive strength when compared with the control specimens. Chen et al. (2019) studied an approach to reprocess PP waste (2 mm-sized pellets) from recycling plants as partial substitution of sand to make cement mortar composites. The authors observed decreasing density and improved toughness after replacing the sand up to 60% by volume. Nevertheless, the composite also showed worse mechanical performance as well as increased open porosity, water absorption and shrinkage, when compared with the control mortars. Even though some studies have been carried out on plasticcontaining gypsum/cement-based composites, none were found on the use of recycled waste PP granules from discarded organic mattercontaminated PP-based packaging waste as partial replacement of the binder to produce new eco-friendly construction materials. For this reason, this research presents innovative results of an experimental campaign analysing the behaviour of gypsum composites containing PP waste particles sourced from disposable CCs. Despite the relatively low amount of discarded plastic CCs in comparison with the large amount of undifferentiated plastic waste, this waste was chosen to be representative of a real waste material rather than using pure and uncontaminated PP. Additionally, since this product is showing a growing consumption trend over the last few years, its proportional environmental impact should not be neglected. Thus, the main purpose of this work is analysing the influence of PP waste granules on the physical (i.e. dry state density and surface hardness), mechanical (i.e. dynamic Young’s modulus, flexural and compressive strengths) and water-resistance properties of new gypsum composites according to standard EN 13279-2 (2014). The quality of the interfacial transition zone (ITZ) between the paste and PP particles was also evaluated. Concurrently, this study intended to carry out an approach to separately recycle the different components of CCs, focusing on the plastic fraction. 2. Materials and methods 2.1. Materials The gypsum used in this study was controlled setting plaster for construction (B1), according to standard EN 13279-1 (2009). Its properties are shown in Table 1. The water used for production was regular tap water in compliance with Council Directive 98/83/EC (European Union, 1998). The PP waste was sourced from used CCs, collected from different collection points around the Municipality of Almargen, Malaga, Spain. The composition of the CCs is shown in Fig. 1, as previously analysed by other researchers who used the same type of capsules in their research (Domingues et al., 2020). The mechanical performance and water absorption of these capsules are presented in Table 2. The real density of the PP components was found to be between 0.895 and 0.920 g/cm 3 , and 2.7 g/cm 3 for aluminium particles, by applying the pycnometer method. After the collection campaign, the residual organic matter (e.g. coffee, chocolate or tea powder) was manually removed from the capsules to simulate an eventual beneficiation step for this waste. In this cleaning step, the capsules were washed with water and air-dried. After that, the capsules were crushed in LAPIST lab of CERENA research unit by using a RETSCH SM 2000 cutting mill (Fig. 2), with an output of particles measuring up to ~4 mm (particle size distribution of the ground PP waste in Fig. 3). This material contained a mix of PP and aluminium. To adequately separate and recycle each of the materials, the mixed particles were submerged in water for 1 h to allow a density-based separation. After scrapping the PP granules from the surface, they were dried at 100 ◦C ±2 ◦C to constant mass. 2.2. Sample preparation The guidelines established in EN 13279-2 (2014) were followed for the preparation of samples. According to the standard, a water/gypsum (W/G) ratio of 0.55 was fixed for all samples. The mixes were produced with the following procedure: the gypsum powder and PP waste granules were dry-mixed by hand to obtain a homogenous mix and avoid any potential segregation during the mixing process; after that, water was added to the mix and the components were mixed manually for 2 min to a homogenous state. To keep the workability constant for all mixes without the need to adjust the W/G ratio or add plasticizers, the content of PP waste was limited to 10% by weight of gypsum. The resulting composites were placed in moulds producing 40 ×40 ×160 mm 3 prismatic specimens. The material proportions used for each of the mixes are summarized in Table 3. The mix code is the following: gypsum (G); type of waste used - polypropylene (PP); percentage of plastic waste addition (e.g. G/PP/2.5). After demoulding the specimens, they were placed in a dry chamber at a temperature at 24 ◦C and relative humidity (RH) of 50 ±1% to cure for seven days. Afterwards, they were placed in an oven at 40 ±2 ◦C, until constant mass and cooled to laboratory temperature in a desiccator. 2.3. Test methods 2.3.1. Fresh state properties - Setting time: according to procedure set by standard EN 13279-2 (2014) for pre-mixes gypsums, the influence of PP waste on gypsum mixes’ setting time was measured with Vicat needle method. Table 1 Properties and limits of B1 gypsum used as binder according to standard EN 13279-1 (2009) (Provided by manufacturer). Purity [%] Particle size [mm] Surface hardness [Shore C] Performance [kg/m 2 /cm thickness] Flexural strength [N/ mm 2 ] Compressive strength [N/ mm 2 ] Adherence [N/ mm 2 ] pH >75 0–1 ≥45 10–12 ≥2 ≥2 >0.1 >6 Fig. 1. Composition of coffee capsules (adapted from Domingues et al., 2020). Table 2 Mechanical properties of the plastic components of a disposable coffee capsule - Tensile strength ( σ ), Young’s Modulus (E), Yield point ( σ f ), ductility ( ε ), toughness (u t ) and resilience (u r ) (Domingues et al., 2020). Sample σ max [MPa] E [MPa] σ 1.max [MPa] ϵ [%] u t [J/ m 3 ] u r [J/ m 3 ] Body +plastic filter +lid 29.5 ± 1.4 655 ± 9 25.8 ± 1.5 120.3 ± 20.6 17.1 ±2.9 2.1 ±0.7 M.I. Romero-G´ omez et al. Journal of Cleaner Production 371 (2022) 133674 4 2.3.2. Physical and mechanical properties - Dry bulk density: after measuring the dry weight and volume of the test samples, as per standard EN 13279-2 (2014), the dry density of each specimen was then calculated as the relation between its dry mass and its volume. The final data of new composites density were obtained by arithmetic mean of the six measurements performed per replacement level. - Superficial hardness (Shore C): this property was determined for three samples per replacement level, prior to any destructive mechanical tests. Following the procedure in EN 13279-2 (2014), a durometer was used to measure the Shore C hardness in two longitudinal and parallel faces of each prismatic sample (Fig. 4a). A perpendicular force to each specimen was applied, choosing a minimum of five points placed in the central plane and separated at least 20 mm from the edges of the sample. Finally, the superficial hardness value of the compounds was expressed as the mean value of 30 measurements obtained per replacement level. - Flexural strength: based on the procedure explained in EN 13279-2 (2014), the pieces were supported in the Form and Test Seidner Strength testing machine-D7940, centred on rollers distanced by 100 mm (Fig. 4b). After that, a load was applied in the central plane of the sample with a speed of 10 N/s, until the specimen’s failure. For each of the specimens, the value of flexural strength was calculated by the arithmetic mean of the six measurements per replacement level. - Compressive strength: the 12 broken sections of samples obtained from the previous test were subjected to the compressive strength test, according to standard EN 13279-2 (2014). The section of the same testing equipment previously described, was used to apply a progressive centred load with a speed of 20 N/s to each of the 12 pieces per replacement level, after which the average value was calculated. - Dynamic Young’s modulus: this test was carried out according to the standard ASTM E1876-21 (2021), by using the GrindoSonic MK5 equipment to measure the fundamental resonant frequency of the test samples. In order to achieve this, the specimens were mechanically excited by a singular elastic strike using an impulse tool. The main value of the dynamic Young’s modulus of the new composites was calculated by using the appropriate fundamental resonant frequencies, dimensions and mass of the specimens. - Ultrasonic pulse velocity: carried out in accordance with the standard EN 12504-4 (2006), this non-destructive test was applied to check the relationship between the density and porosity of the compounds. By using the Proceq Pundit Lab +equipment connected to a pair of probes, an emitter and another receiver, with a frequency of 54 kHz, one longitudinal and three transverse measurements were carried out to six specimens per replacement level, whereby the average value then calculated (Fig. 4c). - Water absorption by capillary action: as determined by standard RILEM TC 25-PEM (1980), after the curing period, three prismatic specimens per replacement level were placed in a vertical position inside a container, with a distance of 10 ±1 mm of water from the bottom. Throughout a 10-min testing period, the water level in the specimens’ surface was measured every minute. The final value was expressed in mm/min (Fig. 5a). - Hygroscopicity range: this non-standardized test, designed by Del Rio-Merino (1999) in her thesis, consisted of subjecting prismatic specimens to constant moisture, by placing them in a wet chamber for 5 day at a temperature of 21 ±2 ◦C and a RH of 72 ±5% (high RH value over the healthy humidity range 40–60%, without achieving the dew point) (Fig. 5b). After that, the weight gain of the different samples was measured and then placed on a table in a Fig. 2. Selective recycling procedure: a) RETSCH SM 2000 mill; b) density-based separation of PP from aluminium; c) Recycled PP, crushed and ready to be used in mixes. Fig. 3. Particle size distribution of shredded PP waste from coffee capsules. Table 3 Mix proportions for six specimens of 40 ×40 ×160 mm 3 . Mix code Gypsum [g] Water [g] W/G ratio PP waste [g] G/CM 1800 990 0.55 – G/PP/2.5 1755 965 0.55 45 G/PP/5 1710 941 0.55 90 G/PP/7.5 1665 916 0.55 135 G/PP/10 1620 891 0.55 180 M.I. Romero-G´ omez et al. Journal of Cleaner Production 371 (2022) 133674 5 laboratory environment for 7 days. Finally, three specimens per replacement level underwent the surface hardness (Shore C), flexural and compressive strength tests with the purpose of comparing the results with those of the specimens with constant moisture. - Deterioration from water-oven cycles: water absorption and drying capacity, as well as the loss of mechanical strength due to the deterioration of compounds after being saturated and dried for a short period at standard environmental conditions, was evaluated based on the non-standardized procedure carried out by Del Rio-Merino (1999). Specimens were placed in a container, distanced from the bottom, and covered with water for 2 days (Fig. 5c). Afterwards, the water was removed, and the weight of the specimens measured. The specimens were then placed in an oven for 2 days at 40 ±5 ◦C. Three specimens per replacement level were evaluated based on their surface hardness (Shore C), flexural and compressive strengths after the first cycle, while the rest of the samples was subjected to two additional cycles of the aforementioned procedure. These tests were developed in the Construction Laboratory of CERIS (Civil Engineering Research and Innovation for Sustainability) at the Instituto Superior T´ ecnico of Lisbon, Portugal. 2.3.3. Other test methods - Scanning electron microscopy (SEM): this method was used to check the quality of the ITZ between the gypsum matrix and PP particles in order to clarify the behaviour of the materials developed from a microscopic level. Two different samples were analysed and compared: one of them without plastic addition (G/CM) and the other with a 7.5% PP waste content (G/PP/7.5). Due to the low conductivity of the samples studied, the specimens were coated with gold to improve the output’s quality. To carry out this analysis, the FEI TENEO field emission SEM belonging to the General Research Services area of the University of Seville, Spain, was employed. 3. Results and Discussion After evaluating the physico-mechanical properties of the proposed PP-containing composites in the fresh and hardened state, the results are presented and discussed in this section. Next, the water-resistance behaviour of the new materials after mixing a waterproofing waste (polypropylene) as partial replacement in the gypsum matrix, as well as how this affects its hygrothermal properties, are evaluated. 3.1. Setting time According to the data summarized in Table 4, the setting time of gypsum composites decreased as the PP waste content added to the mixes increased. Higher setting time reduction (�33.75%) compared to the reference material was presented by mixes with a 10% PP waste addition. This performance as previously analysed by other researchers (Vidales-Barriguete et al., 2018). Fig. 4. Physical-mechanical tests: a) superficial hardness (Shore C); b) flexural strength; c) ultrasonic pulse velocity (longitudinal measurement). Fig. 5. Water exposure tests: a) water absorption by capillary action; b) hygroscopicity range; c) deterioration from water-oven cycles. Table 4 Setting time of PP-containing composites (min). Specimens Time [min] Δ time compared to reference sample G/CM 20.00 – G/PP/2.5 16.00 �20.00% G/PP/5 15.25 �23.75% G/PP/7.5 14.00 �30.00% G/PP/10 13.25 �33.75% M.I. Romero-G´ omez et al. Journal of Cleaner Production 371 (2022) 133674 6 3.2. Dry bulk density The results of the specimens’ density and modulus of elasticity are presented in Table 5. Unexpected values of dry bulk density were obtained. Despite the PP’s lower density when compared to gypsum (around 1000 kg/m 3 vs. 2650 kg/m 3 , respectively), the new composites showed higher density values, peaking at 2.5% PP content (4% increase) and decreasing linearly after that. This behaviour could be attributed to the improved particle size distribution of the components leading to densification of the mix when recycled PP particles under 2 mm are incorporated. However, the declining linear trend observed after exceeding 2.5% of recycled PP, pointed out that this is an optimum content, after which the compactness started to deteriorate. Vidales-Barriguete, (2019, 2021) also observed decreasing density values with increasing plastic cable waste content to the binder matrix with replacement levels of up to 70% by weight of gypsum. In this study, it was observed that the reference sample presented the highest volume of porosity, which decreased with the increasing percentage of plastic cable waste. This suggests an inverse relationship between the porosity of the material and the bulk density was demonstrated, which implies a reduction of porosity and bulk density due to the waste’s impervious nature and lower density. This fact was also found in another previous research (Ahmed et al., 2011; Flores-Medina and Barbero-Barrera, 2017). Furthermore, a linear reduction of Young’s modulus was noticed in both, the work developed by Vidales-Barriguete (2019) and the present study, which pointed out a loss of the mixes’ stiffness of up to ~50% and ~19% when plastic cable waste and PP waste were added, respectively, compared to the reference composite. 3.3. Mechanical properties Concerning the flexural strength behaviour of the novel composites, the results are shown in Fig. 6. As expected, the incorporation of recycled PP particles had a negative influence on the gypsum matrix, leading to a decline in flexural strength of up to ~35% (G/PP/10) with respect to the control. However, it should be noted that, in all cases, the values were over the minimum 1 MPa set by EN 13279-2 (2014), thus showing the applicability of the waste despite the decline in performance. The previous findings of the present study are consistent with results in the literatures (Rubio-De Hita et al., 2018; Ohemeng and Ekolu, 2019), which showed major reductions in flexural strengths of mortars, as the incorporation of plastics waste to the mixes increased. Fig. 7 shows the cross-sections of two specimens with different PP contents (i.e. G/CM and G/PP/7.5) after the flexural strength test. It shows a random and homogenous arrangement of the PP particles throughout the gypsum matrix. The compressive strength showed an unexpected trend (Fig. 8); it increased when PP waste was incorporated. The highest compressive strength was achieved by mixes with a 2.5% PP content (i.e. 9.19 MPa), after which a slight downwards linear trend was observed for subsequent replacement levels (G/PP/2.5, G/PP/5 and G/PP/7.5 presented values of 8.42, 8.36 and 8.24 MPa, respectively). Moreover, considering that the minimum requirement is of 2 MPa for gypsum binders and plasters (EN 13279-2, 2014), all mixes were well within the limit. The Table 5 Summary table with the results obtained for density and modulus of elasticity tests developed in this work. Specimens Density [kg/m 3 ] (CoV [%]) E [MPa] (CoV [%]) G/CM 1204 (1.58) 6139 (0.45) G/PP/2.5 1251 (0.49) 5771 (3.56) G/PP/5 1240 (0.73) 5595 (2.50) G/PP/7.5 1236 (0.70) 5183 (3.89) G/PP/10 1230 (0.72) 4975 (3.20) Fig. 6. Flexural strength (MPa) of gypsum/polypropylene composites. Fig. 7. Cross-section of the (a) G/CM and the (b) G/PP/10. Fig. 8. Compressive strength (MPa) of gypsum/polypropylene composites. M.I. Romero-G´ omez et al. Journal of Cleaner Production 371 (2022) 133674 7 aforementioned behaviour presented by the new PP-containing composites is supported by published works. Martinez-L´ opez et al. (2021) noticed improvements of about 27% in compressive strength when plastic waste (i.e. PP and PE) replacement ratios up to 1% by weight of sand and a particle size lower than 0.71 mm were used. However, after exceeding this point, the values of compressive strength decreased gradually when increasing of either the size or concentration of the waste particles. Similarly, Pedre˜ no-Rojas et al. (2019) observed the effectiveness of using granulated PC waste with a particle size smaller than 4 mm to improve the compressive strength behaviour in samples up to 40% plastic waste added (~37% increase with 5% of PC waste addition). Moreover, after carrying out X-ray computed tomography test, these authors found that the compressive strength increase was due to the reduction of the studied mixes’ porosity when plastic waste particles smaller than 4 mm were used. 3.4. Superficial hardness (Shore C) The superficial hardness of gypsum materials is a property strongly associated with the compressive strength. The specimens showed improved superficial hardness up to 1.36% and all close to 95 (corresponding to mix G/PP/5) when compared with the control gypsum (Fig. 9). Thus, it can be inferred that, to improve the superficial hardness of gypsum-based composites, the optimal replacement level range of PP waste is of 2.5–7.5%, similar to that observed in the compressive strength test. 3.5. Scanning electron microscopy The SEM method was applied to a specimen with 7.5% PP content (G/PP/7.5) and to the control (Fig. 10). The image revealed a good involvement of PP waste particles throughout the gypsum matrix, thus explaining the positive results previously observed. The shredding process of waste CCs resulted in PP particles with reasonably rough surfaces that counteract the hydrophobic property of PP surface, thereby providing additional sites for gypsum crystal precipitation and growth. Nevertheless, despite the acceptable link between the gypsum matrix and the PP particles, the cohesion of the ITZ is not as strong as that in between the products of hydration and is likely to be a point of failure, especially given the different moduli of elasticity of both materials. According to the reference material, it can be confirmed that no alteration in the nature of the crystalline network of plaster was noticed after Fig. 9. Surface Hardness (Shore C) of gypsum/polypropylene composites. Fig. 10. Stereo microscopy image to G/PP/7.5 (a-20x); SEM analysis to G/PP/ 7.5 (b - 150 ×; c - 600 ×) and G/CM (d - 600 ×). M.I. Romero-G´ omez et al. 110 Journal of Building Engineering 34 (2021) 101874 Available online 11 October 2020 2352-7102/© 2020 Elsevier Ltd. All rights reserved. Characterization of gypsum composites with polypropylene fibers from non-degradable wet wipes M.I. Romero-G´ omez, M.A. Pedre˜ no-Rojas * , F. P´ erez-G´ alvez, P. Rubio-de-Hita Departamento de Construcciones Arquitect´ onicas 1, Escuela T´ ecnica Superior de Arquitectura, Universidad de Sevilla, Avenida Reina Mercedes, N_ 2, 41012, Sevilla, Spain ARTICLE INFO Keywords: Polypropylene waste Gypsum plaster Mechanical properties SEM Eco-friendly material ABSTRACT The research carried out studies how the addition of plastic polypropylene fibers from recycled non-degradable wet wipes can influence on the physical and mechanical behavior of composites with a gypsum binder matrix. Four different plaster blends were studied, corresponding to percentages of 2, 2.5, 3 and 3.5% by weight of recycled plastic fibers. To characterize the different test specimens, properties such as density and strength (flexural and compressive) were determined. Also, in order to find out about the internal structure of the different composites developed at a microscopic level, a Scanning Electron Microscopy (SEM) test was performed. The results show that more efficient materials can be obtained with a little decrease in their density compared to the reference material (gypsum). However, the improvement in the novel composites’ mechanical properties represents one of the most notable achievement of the study. A flexural strength rise of a 19% was achieved for mixtures with 2,5% of recycled PP fibers compared to the reference specimen. These results confirm the feasibility of using the gypsum composites produced for the design of new construction products and its application in building systems as wall and ceiling lining. 1. Introduction Nowadays the large production of plastic is presented as one of the most harmful activities that exacerbate the climate and environmental pollution suffered by the planet [1] with devastating consequences for terrestrial and marine ecosystems [2]. Due to characteristics like durability, stability and low-cost, polymeric products have become a daily necessity. However, the current inability of society to get rid of the high production of single-use plastics leads the planet to an unsustainable situation. The world plastic production peaked at 360 mT in 2018, 17% of which was produced by Europe (EU28 +NO/CH). This fact placed this continent as the third world’s greatest power related to plastic production, only surpassed by Asia (51%) and North America (18%). The polymer production is led by Polypropylene (PP) which represents the 19.3% of the total plastic manufactured in the EU28 +NO/CH [3]. Because of a large part of the plastic production is allocated to singleuse products, the annual balance between the production and recycling rate shows a negative result. At European level, 29.1 million tonnes of plastic waste were collected through official schemes for treatment in 2018, while the remainder ended up in illegal landfills or directly in marine and terrestrial environments. In addition, only 32.5% of the total waste collected was recycled and 42.6% used for energy recovery. Nevertheless, 24.9% still went to landfill, incinerated with the corresponding emission of polluting gases into the atmosphere that this entails. Currently, marine ecosystems are the most damaged by plastic pollution, because of the sea is the ultimate sink for the Earth’s waste. It is estimated that the amount of marine litter worldwide ranges from 6.4 to 8 mT per year, 80% of which corresponds to plastic waste [4]. This problem has gained so much relevance that the issue of plastics is already included in the six most serious environmental emergencies of the planet, according to the United Nations Environment Program [5]. A recent study developed in Spain found that 85% of the plastic waste which end up in the sea come from the beaches, caused by tourist activities, fishing works and wastewater discharges [6]. Recently, the dumping of plastic into the sea, sweeped along by wastewate currents is a problem that has gained considerable relevance. Wet wipes stand out among all residues which end up in the marine environment as a result of the misuse society makes of toilets. Unlike toilet paper which is composed of 100% cellulose fibers, wet wipes are classified as non-woven textiles and are composed of a high rate of polymeric fibers that makes them difficult to degrade in contact with * Corresponding author. E-mail addresses: [email protected], [email protected] (M.A. Pedre˜ no-Rojas). Contents lists available at ScienceDirect Journal of Building Engineering journal homepage: http://www.elsevier.com/locate/jobe https://doi.org/10.1016/j.jobe.2020.101874 Received 30 June 2020; Received in revised form 6 October 2020; Accepted 6 October 2020 Journal of Building Engineering 34 (2021) 101874 2 water [7]. Consequently, on the way to wastewater treatment plants, the union between wep wipes, greases and oils make congealed masses scientifically known as “fatberg”. As a result, devastating blockages in the sewerage network, with numerous consequences both economically and environmentally, are caused [8]. The high use of wet wipes with an average production of 8000 Kg/ day, in a city of 300,000 inhabitants. along with its lack of insertion into the current recycling chain make necessary to take measures beyond recommendations included in present regulations [9]. In addition to a correct certification and characterization of the product by manufacturers, the search for measures that allow the valuation of this type of waste is needed. This action means turning a waste into a resource, at the same time as its correct recycling is promoted. On the other hand, there is a growing desire to curb the problem of plastic pollution in the construction sector. That is why, many researchers have analysed the addition of plastic waste as partial replacement of raw material in the development of alternative building materials [10–14]. One of the plastics most used in these researches is the Polyethylene terephthalate (PET), as well as some type of plastic foam waste like Extruded Polystyrene (XPS), Expanded Polystyrene (EPS) and Polyurethane Foam (PUR). All of them have been used as aggregate in gypsum mixes to make new lightweight composites whit better thermal behavior [15–17]. Nevertheless, most of the studies mentioned show that the mechanical resistance of lightweight gypsum composites decreased as the residue percentage rose. For this reason, there is a new current of research that studies the incorporation of reinforcing fibers to gypsum mixtures, in order to make up for the plasters’ resistance decrease, keeping the environmental advantages of its use. Among the studies published by pioneering researchers in this area, the work developed by Rio-Merino [18] in her thesis about the addition of E glass fibers and other additives as reinforcement of lightweight gypsum panels, is remarkable. As a result, the toughness of the new composite was enhanced at the same time that an increase in Young’s modulus and an improvement in flexural strength were obtained. Afterwards, other researches focused on the study of structural behavior and the design methodology of Glass Fiber Reinforced Gypsum Panels (GFRG) were developed [19]. Because of the good behavior of glass fiber as reinforcing aggregate, an extensive use of it has been developed within the plasterboard production sector, where they are used together with the gypsum as main components of the plasterboard panels’ heart. As a consequence, a rise in the strength properties of this type of panels was achieved. Also, their behavior against fire was improved due to the stitching generated by the union between reinforcing fibers and the gypsum matrix that avoids the fragmentation of panels underwent the action of fire [20]. Some other remarkable studies were developed by Barakat y Flores- Medina & Barbero-Barrera [21,22]. These works show how the use of new PP fibers as reinforcing in blocks and plasterboard panels respectively, decreased the porosity and increased the mechanical resistance and durability of these products, while their cracking capacity was enhanced. According to this, the useful life of products studied was extended. Furthermore, the use of new fibers in gypsum blends at weight of up to 1% together with the partial replacement of the binding matrix by natural aggregates such as Diatomite and Vermiculite, achieved improvements of up to 200% regard to the flexural strength and 100% to compressive strength [23,24]. There are many other researchers who have looked for alternatives to polypropylene fiber. In this way, some comparative analyzes to prove its replacement for other types of polyolefins fibers were developed [25]. It is remarkable a study in which PP and PVA fibers were compared. In this work two different sizes of commercialized fibers were used to prepare a series of samples with different percentages up to a maximum of 1.2% by volume of gypsum. Finally, improvements in the novel composites’ mechanical and thermal behaviour were analyse and compared [26]. Interestingly, there is a line of studies focused on new lightweight gypsum composites development by adding various types of recycled plastic fibers. Recycled polymer fibers from textile sector, such as excess polyamide from the laser synthesizer industry, are one of the most used waste [27]. Despite several studies have been published on gypsum-based composites containing a variety of reinforcing fibers, there are not many researches relating to the use of recycled plastic fibers as reinforcing material in the gypsum matrix to produce new construction composites. In this sense, it is proposed to use PP fibers from recycled nondegradable wet wipes. It is a highly polluting waste which mainly affect deep-sea ecosystems. What’s more, Polypropylene is a plastic material with good mechanical properties, which makes it suitable for use in making these novel composites. With all these arguments, this work aims to study, in addition to develop a correct characterization of the waste, the physical (dry state density and thermal conductivity) and mechanical properties (flexural and compressive strength) of the new developed plasters. Moreover, techniques of Scanning Electron Microscopy (SEM) are applied to know the quality of the interface between polypropylene fibers and the plaster binder matrix. 2. Materials and methods 2.1. Materials The materials selected to make the new gypsum composites reinforced with PP fibers from non-degradables wet wipes are listed below: Abbreviations list CoV Coefficient of Variation CH Switzerland; E Elastic Modulus EPS Expanded Polystyrene EU European Union G Gypsum GFRG Glass Fiber Reinforced Gypsum Panels PP Polypropylene PET Polyethylene Terephthalate PUR Polyurethane Foam PVA Polyvinyl acetate NF Non-woven fabric NO Norway SEM Scanning Electron Microscopy W/G Water/Gypsum XPS Extruded polystyrene Table 1 Properties (limits) of B1 gypsum used as binder matrix according to standards UNE. Purity percentage [%] Granulometry [mm] Surface hardness [Shore C] Performance [kg/m 2 /cm thickness] Flexural Strength [N/mm 2 ] Compressive Strength [N/mm 2 ] Adherence [N/ mm 2 ] Ph >75 0–1 ≥45 10–12 ≥2 ≥2 >0.1 >6 M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 3 - Gypsum for construction: also known as B1 gypsum according to standard UNE-EN 13279–1 [28] was selected. It is a controlled setting gypsum whose properties are listened in Table 1: - Water. - Non-degradable wet wipes: The wet wipes used came from selective waste collection points at different locations around the University of Seville. Its composition has been provided by the manufacturer (Table 2). According to previous studies developed by Durukan & Karadagli [29] the mechanical properties of a non-degradable wet wipe are showed in Table 3. After the selective collection of wet wipes, these were subjected to an industrial process similar to that used for the recovery of textile fibers [30], in order to obtain the fibers waste which were added afterwards as reinforcement to the plaster blinder matrix. This pre-treatment consisted of three phases. A first washing stage to eliminate organic residues or wastes that they could contain, followed by a drying stage in an oven at 40◦for 24 h. Finally, a third phase, in which the wipes were transformed into fibers by using a mechanical shredding process for non-woven textiles. As a result, fibers of variable length between 15 and 30 mm and width 2–3 mm were obtained, as seen in Fig. 1. 2.2. Samples preparation Firstly the water/gypsum ratio (W/G) was determined, following the established procedure in UNE-EN 13279–2 [31]. Once a W/G ratio of 0.55 was obtained, the mixtures were prepared manually, starting with the reference samples (those gypsum mixtures without any type of fiber or aggregate). Next, a first experimental stage in relation to the samples preparation process was carried out, because, according to Durukan & Karadagli, non-degradable wet wipes made with non-woven fabric have a high absorption capacity (69%) [32], which can affect the setting time of the plaster. Therefore, a first series (I) of 6 specimens was prepared for a 2.5% percentage of recycled PP fibers by weight of gypsum and a G/W ratio of 0.55. Previously, an additional amount of water equal to PP fibers absorption capacity was added to these. The data obtained was compared with a second series (II) of 6 specimens for which the same G/W ratio was used. However, in this case, the amount of water previously added to the wet wipe fibers was deducted from the total water required for the mixture. In order to obtain conclusive results, in an incipient stage of the research, both series of specimens were characterized and tested for flexural and compressive strength (Table 4). As a consequence of excess water from the first series of specimens (I), a significant decrease in the dry density greater than that for the second series (II), was obtained. This reduction caused by the increase of gypsum porosity after the evaporation of excess water during the setting time of the plaster, Table 2 Wet wipe composition according to the manufacturer information. COMPONENTS MATERIAL NON-WOVEN FABRIC (NF) SPUNLACE - Polypropylene (20%) - Regenerated cellulose (80%) LOTION BASE - Purified water - Xanthan gum EMULSIFYING AGE NTS - Hydrogenated castor oil PEG-40 - Capric/caprylic triglycerides (coconut derivatives) - Dimethicone BIS-PEG/PPG-16/16 PEG/PPG 16/16 REGULATORS OF pH - Citric acid - Sodium citrate PRESERVATIVE SYSTEM - Sodium benzoate - Disodium EDTA - Sodium caprylate FRAGANCE PERFUME Table 3 Mechanical properties of a non-flushable wipe. Wet state (A) and Dry state (B). Sample Thickness [ μ m] F max (N) Tensile Strength [MPa] Basic weight [g/m 2 ] Tensile Index [Nm/g] Breaking Length [m] Elongation at Break [%] A 213–376 15 1023 162 7 715 28 B 178–531 15 1003 51 20.0 2044 23.4 Fig. 1. A) Recycled PP fibers, cutted and ready for use to make mixes. B) Fiber length. C) Fiber width. Table 4 Mechanical properties comparison of gypsum mixes type I and II. Sample Serie Density [kg/ m3] Flexural Strength [MPa] Compressive Strength [MPa] Reference 1205 3.73 8.45 G/PP/ 2,5 I 1096 3,69 3,15 G/PP/2′ 5 II 1194 4,35 4,75 M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 4 significantly affected the mechanical properties of the first samples. Thus, a better behaviour was obtained from series II specimens (those which the water previously added to the PP fibers was deducted from the total required for the preparation of the mixture according to the G/W ratio), so this same samples preparation process was followed for the entire experimental process. The percentage of added PP fibers was limited to 3.5% (based on the weight of gypsum used). This maximum was established according to the workability of the mixture, that is, the maximum amount of replacement aggregate which allow to reach a homogeneous mixture in 2 min without additives and with the W/G ratio constant at 0.55 [31]. According to these proportions, series of six prismatic specimens of 40 ×40 ×160 mm 3 [31], for each of the defined mixtures were produced. Table 5 provides the material proportions used in each gypsum mix as well as the gypsum mix identification (ID) code, where the first letter indicates the binder material, Gypsum (G); the second one refers to the kind of reinforcing fiber used, Polypropylene (PP) and the last number describes the percentage of fiber addition (e.g., G/PP/2). 2.3. Test methods After the samples were prepared, the specimens were cured at a temperature of 24 ◦C at a relative humidity of 50% ±1% for 7 days. Subsequently, they were dried in a stove at 40 ◦C ±2 ◦C, to constant mass and cooled to laboratory temperature. Once this procedure was completed, the physical and mechanical properties of the samples were studied, applying the following test methods: 2.3.1. Physical and mechanical properties - Density in dry state: according to the procedure in UNE-EN 13279–2 [31], the dry weight and volume of the different test specimens was obtained, in order to calculate the density, since it was defined as the ratio between the dry mass and the volume of the specimens. The dry density of the novel composites was defined as the arithmetic mean of the six measurements obtained per percentage. - Flexural Strength: after cooled the samples to room temperature, they were tested for flexural strength according to standard UNE-EN 13279–2 [31]. Each piece was supported on rollers positioned at 100 mm. Then, a centered load was apllied whit a speed of 10 N/s, until the specimen breaked. The flexutal value of the novel composites was expressed as the mean value of the six measurements performed per percentage. - Compressive Strength: following the standard UNE-EN 13279–2 [31], the 12 broken sections of specimens previously tested to flexural failure underwent compressive strenght testing. The compressive value of the novel composites was obtained as the mean value by applying a progressive breaking load with a speed of 20 N/s, to the center of each of twelve pieces per percentaje. Also, the stress-strain curves of the different composites were obtained from the compressive strength test. As a result, the elastic properties of the novel composites developed (E [MPa]) could be estudied. - Ultrasound velocity: the application of this type of non-destructive test according to the standard UNE-EN 12504–4 [33], seeks to confirm the relation between the density and the porosity of the new material. An electric-pulse-generating, a pair of probes, an emitter and another receiver, an amplifier and a timer were used to measure the time interval between the beginning of the impulse wave and the beginning of the wave upon arrival at the receiver probe. Following this method, three transversal and one longitudinal measurement were applied to each of the 6 samples per percentage. The test was performed before undergoing the samples to flexutal strenght testing. The average value of the transverse velocity of each composite was defined as the arithmetic mean of the three measurements taken for the total of the 6 samples. In the case of longitudinal velocity, its value was expressed directly as the mean value of the 6 measurements per percentage. The described tests were carried out in the Department of Architectural Technology I laboratory at the School of Architecture in Seville. To test flexural and compressive strength, an automatic multi-test press MCO-30 controlled by a computer module was used, with a 300 KN load capacity and an accuracy that varies according to the control type. Furthermore, the equipment used to test ultrasound pulse velocity was the PUNDILAB (NEURTEK) ultrasonic machine, with a precision of ±0.1 μ s and an emission frequency of 50 KHz. 2.3.2. Other test methods - Scanning Electron Microscopy (SEM): to know the quality of the interfacial transition zone between gypsum matrix and PP fibers, a comparative study between a reference sample and another mixture containing 2,5% recycled plastic fibers (G/PP/2,5) was performed. A scanning electron microscopy process was used to do it by applying the FEI TENEO field emission scanning electron microscope. Previously, the samples were coated with gold by a Sputtering process, to improve their conductivity. The analysis equipment used to SEM test belong to the General Research Services area of the University of Seville. 3. Results and discussion In this chapter, the results obtained in the different tests performed are presented and discussed in detail. Primarily, properties such as density and mechanical behaviour of the materials are commented since they are one of the more important data to characterize a new gypsum Table 5 Mix proportions of gypsum mixtures for six specimens of 40 ×40 ×160 mm 3 . Sample Series Serie Gypsum [kg] Water [kg] (total) Water [kg] (Gypsum mix) Water [kg] (Fiber mix) W/G Ratio PP waste fibers [kg] Reference III 1.800 0.990 – – 0.55 – G/PP/2 IV 1.764 0.970 0.867 0.103 0.55 0.036 (2,0%) G/PP/2,5 V 1.755 0.965 0.837 0.129 0.55 0.045 (2,5%) G/PP/3 VI 1.746 0.960 0.806 0.154 0.55 0.054 (3,0%) G/PP/3,5 VII 1.737 0.955 0.775 0.180 0.55 0.063 (3,5%) Table 6 Summary table with all the results obtained for the different tests developed in this work. Sample Series Density [kg/ m 3 ] (CoV [%]) Flexural Strength [Mpa] (CoV [%]) Compressive Strength [Mpa] (CoV [%]) E [Mpa] (CoV [%]) Reference 1205 (1.51) 3.73 (3.65) 8.45 (6.20) G/PP/2 1203 (0.92) 4.35 (8.09) 4.75 (8.35) 783.93 (8.05) G/PP/2,5 1202 (1.18) 4.45 (4.99) 4.13 (3.27) 315.63 (12.60) G/PP/3 1196 (0.49) 3.85 (8.09) 3.41 (9.08) 310.71 (29.26) G/PP/3,5 1194 (1.20) 3.42 (5.94) 3.15 (1.38) 391.13 (16.48) M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 5 composite. The compilation of all the numerical results obtained from the characterization tests carried out are presented in Table 6, with their coefficient of variation (Cov [%]). The observation of various cross-sectional images of the prismatic samples underwent flexural strength testing according to the different mixes’ percentages analysed, allowed to know the arrangement and orientation of the recycled fibers in the gypsum matrix. Therefore, the summary of images collected in Fig. 2, shows such the PP fibers are quite homogeneously distributed through the matrix in all the samples, independent of the percentage of addition. However, the orientation becomes more random as the plastic addition percentage rise. 3.1. Density The dry density values of produced gypsum composites are showed in Fig. 3, in which can be observed a variation between 1205 and 1194 kg/m 3 by depending on the weight percentage of addition. Although the reduction in density was not pronounced enough since the maximum fall in density was 1% respect to the reference mixture some interesting conclusions were got. The novel composites showed a linear declining trend regard to its dry density attributed to the rise in the percentage of recycled PP fibers, compared with the reference material. Support by those results provided by S. Guti´ errez-Gonz´ alez et al. [27], these densities values could be attributed to the replacement of plaster with a density of 2650 kg/m3 by recycled polypropylene fibers with a lower density of 1000 kg/m3. 3.2. Mechanical properties The mean values of flexural and compressive strength (measured in MPa) corresponding to the different composites analysed at 7 days, together with the variation in the resistance coefficients of the mixtures are graphically presented in Fig. 4 and Fig. 5. Unlike results obtained about new materials density behaviour, in relation to the flexural strength an increase of up to 19% with respect to the reference sample was achieved which correspond to the composite G/PP/2,5. According to the values presented for flexural strength Fig. 2. Cross-sectional image of the prismatic samples underwent flexural strength testing. Fig. 3. Density (Kg/m 3 ) of Gypsum/Poylpropylene fibers composites. M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 6 (Fig. 4), mixtures showed a linear upward trend regard to this property until peaked at 4.45 MPa, corresponding to an addition percentage of 2.5%. From this point of inflection, the trend was reversed. However, it was not until reach the dosage of 3.5% that the flexural strength value fell below the reference value. One reason for this is that the material increases its compactness as the content of PP plastic fibers rises. However, a maximum (G/PP/2,5) is reached from which the bond between the internal particles of the binder material and the PP fibers weakened. As a result, the porosity of the material increased and it became less compact, which affected its resistance capacity. It should be noted that in all cases, the flexural strength values obtained exceed the minimum 1 MPa standard requirement for gypsum binders and gypsum plasters [28]. This fact proves the feasibility of using any of the mixes studied in construction work. In contrast, the results obtained from the compressive strength test (Fig. 5) show a decrease in this property of up to 62.76% (G/PP/3.5) as the substitution rates increases, with respect to the reference value. In spite of the fact that the incorporation of recycled PP fibers weakened the compressive strength of the novel composites, all the values were over the minimum 2Mpa set by the relevant standard [28]. Based on the results discussed, a comparison between mechanical properties and density of the novel composites was carried out, in order to quantify the improvements in terms of physical and mechanical behavior. In the first place, the values of density and flexural strength obtained for the different composites are shown in Fig. 6. It can be observed that all the mixtures (G/PP/2; G/PP/2.5; G/PP/3; G/PP/3.5) are lighter than the reference material. Nevertheless, only three of them (G/PP/2; G/PP/ Fig. 4. Flexural Strength (MPa) of Gypsum/Poylpropylene fibers composites. Fig. 5. Compressive Strength (MPa) of Gypsum/Poylpropylene fibers composites. M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 7 2.5; G/PP/3) improved the value of reference flexural strength. Of these four composites G/PP/2.5 got the highest flexural strength value (4.45 MPa), while G/PP/3.5 showed the lowest density value (1194 kg/m 3 ). Secondly, the relation between compressive strength and density of the samples is shown in Fig. 7. It can be observed that despite achieving for all the novel composites (G/PP/2; G/PP/2.5; G/PP/3; G/PP/3.5) lower density, the compressive strength values are well below the reference value. In this case, the lowest density value corresponds to the compound G/PP/3.5 (1194 kg/m 3 ), while the highest compressive strength value (4.75 MPa) was achieved with an addition percentage of 2% (G/PP/2.5). On the other hand, the stress strain curves analysis was obtained from the compressive strength test (Fig. 8). In general, it shows that the modulus of elasticity of all the composites produced was reduced, respect to the reference sample, by increasing the percentage of polymeric fibers added to the plaster blend. In contrast, the mixtures with the highest rate of PP fibers had a greater plastic deformation due to they became more ductile. By observing the fracture lines, that is, quantity, shape and arrangement of these related to the different specimens after they were undergone flexural strength test (Fig. 9), it can be deduced that the addition of PP fibers cause a greater stress arrangement by increasing the rate of waste added. These results about the mechanical behaviour of novel composites developed, can be compared with other studies from the literature which are based on the use of gypsum matrix and diferent type of reinforcement fibers. They show that this work achieves the highest addition levels at the same time that good workability and streght values are kept within standards. The phenomenon detected for flexural streght is very similar to the work presented by Cong Zhu et al. [26]. Despite of using in their study a smaller size of fibers, the flexural strenght of the mixtures rises until reaching the peak corresponding to a medium percentage of PP fibers of addition between 0 and 1,2% by volume fraction. After that, the values significantly decreased. Related to compressive streght analysis, the values obtained are in accordance with those results reported by Cong Zhu et al. [26] for PP fibers. Based on their comparative study, it was notice the addition of comercial PP fibers decreased the compressive strenght up to 19.2%, Fig. 6. Flexural Strength vs. Density of the developed composites. Fig. 7. Compressive Strength vs. Density of the developed composites. M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 8 unlike the slightly resistance increase got by low addition percentages of PVA fibers. On the other hand, the best results for compressive strength were obtained in those researches where the synergic work of PP fibers and mineral waste were analised [21,23,24]. This fact could be explained, according to N. Flores-Medina et al. [21], with the nucleation of the gypsum matrix in contact with isoctatic graphite filler and the improvement in the quality of fiber/matrix bond when the mineral is added. 3.3. SEM (scanning electron microscope) In order to know the internal behavior and the extent of adhesion between the recycled PP fibers and the plaster, a sample with 2.5% plastic addition (G/PP/2.5) and another reference specimen urderwent scanning electron microscopy. Fig. 10 shows the plaster/PP fibers interface, in which gypsum crystals surround the fibers of polypropylene. Thus, SEM analysis has confirmed that there is a good cohesion between both materials interface. Nevertheless, compared to the reference specimen, an alteration in the nature of the crystalline network of the plaster was found as a result of the addition of this type of plastic waste. 3.4. Ultrasound velocity Ultrasound velocity’s propagation though a material depends on the density and elastic capacity of that. As Fig. 11 shows, ultrasound velocity decreases as the percentage of PP recycled fibers added to the gypsum binder increases. Also, the direct relation between both properties, density and ultrasound velocity’s propagation is confirmed in Fig. 12, since the porosity of the material increases as the amount of plastic waste added to the mixture rise, generating a reduction in the compactness of the material. 4. Conclusions The characterization of gypsum composites containing recycled PP fibers derived from non-degradable wet wipes developed throughout this work, offers the following conclusions: - The addition of recycled PP fibers showed no affections regarding the workability of the mixtures produced. It has been confirmed that using addition percentages of up to 3.5% is possible without causing any modification to the W/G ratio or adding additives. - The dry density of the novel composites studied was reduced due to the progressive addition of a growing percentage of PP fibers into the gypsum matrix by up to 1% (G/PP/3.5). Therefore, it has been demonstrated that there is a moderate linear declining trend of the density of the new material as the rate of plastic fibers used in the mixtures rises. Fig. 8. Stress-strain curves obtained from the compressive strength test for the Gypsum/PP fibers composites. Fig. 9. Types of cracks of the prismatic specimens subjected to the test for flexural strength. M.I. Romero-G´ omez et al. Journal of Building Engineering 34 (2021) 101874 9 Fig. 10. G/PP/2.5 SEM Analysis (A. 600 x zoom. B. 1200 x zoom) and Reference SEM Analysis (C. 600 x zoom. D. 1200 x zoom). Fig. 11. Ultrasound velocity of the developed recycled PP fibers-gypsum composites. M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 2 water. Having its nicotine fully released into water, the concentration may be up to fourteen times higher than the limit set by the European Union for hazardous tobacco waste, 7.1 mg/g vs 0.5 mg/g [5]. Besides nicotine release, thousands of dangerous chemicals are detached throughout the decomposition of CFs, affecting the health of terrestrial and marine ecosystems [16,17]. To reinforce worldwide government-made regulations to reduce cigarette consumption [18–20], several researchers have focused their efforts on the development of new recycling alternatives, especially on the production of secondary raw materials [21–22] or building products by promoting the recovery of CFs and their inclusion in the circular economy [10]. Their incorporation into construction materials [23], include asphalt concrete [24,25], lightweight fired bricks [26,27], sound-absorbing materials [28–30], thermal-improving ceramic material [31] and reinforced concrete [32], with variable, albeit manageable, physico-mechanical, thermal, and environmental performances. The incorporation of waste fibre reinforcement (synthetic or natural origin) to mortar-based composites is a well-known recycling technique, capable of improving their toughness, mechanical and post-crack performance. Choi et al. [33] investigated the effects of natural cellulose fibres sourced from hemp and jute on the setting of cement-based composites, with dosages up to 2 % by weight of cement. An increase in compressive strength was observed for specimens containing 0.25 % of fibres. In addition, it was observed that the setting time increased for higher fibres contents. Concerning the use of cellulose pulp as partial replacement in conventional gypsum, Nindiyasar et al. [34] analyzed the influence of adding natural cellulose as fibre reinforcement in gypsum-based composites, while Araújo-Carvalho et al. [35] used recycled cellulose pulp. Both evaluated the microstructure and mechanical properties, presenting encouraging findings related to Young’s modulus, compressive, and flexural strength. Also, the results showed that the use of natural or recycled cellulose pulp contributed to an increase in crystallization induction times of approximately 100 % when compared to the reference gypsum, due to the ability of cellulose fibres to absorb, retain, and provide water for gypsum crystallization. Recently, encouraging research results have been published on the influence of adding plastic fibres as reinforcement in a gypsum matrix [36]. Between them, it could be highlighted the use of recycled cellulose fibres from multi-layer packaging [37], PVA fibres obtained industrially [38], PET fibres from waste bottles [39], as well as PP fibres sourced from waste non-degradable wet-wipes [40]. Even though scarce studies about the incorporation of recycled CA in plasters were found, some research on natural cellulose fibre and recycled CA reinforced cement composites could be cited as precedent. On the one hand, Foˇ rt et al. [41] investigated the synergic effect of superabsorbent polymers (SAP) and cellulose fibres on the material properties of cement-lime plaster in terms of its mechanic, thermal, and hygric properties, obtaining encouraging results for humidity control accessed through the moisture buffer coefficient. Likewise, Stevulova et al. [42], studied the performance of sustainable cement-bonded mortars containing three types of cellulose fibres from waste paper recycling, based on workability, physico-mechanical and thermal properties. As a result, 2 wt% was stablised as the optimun fibre content in terms of workability, since values of a mean spill diameter similar to the standard plastic mortars was achieved. Regarding to the use of higher fibre contents, an increase of porosity was observed, obtaining lighter mortars with lower values of thermal conductivity and significantly higher values of water absorbency and a capillarity absorption coefficient, due to the increase of pores number and size. On the other hand, Tannous et al. [43], presented an innovative way to recycle industrial. pre-depolluted CA fibres sourced from CFs in cement mortars, as partial replacement of sand. Optimal results based on workability and mechanical strength performance were obtained at a 0.2 wt% replacement of sand by CA fibres without using superplasticizer. Furthermore, the quantity of water released by the CA fibres, as consequence of their high water absorption (about 853 %), led to cement hydration improvement. Only one study was found on the use of CFs on the performance of gypsum-based composites thus demonstrating the innovative character of the present work. Morales-Segura et al. [44] characterized gypsum composites incorporating CA fibres from CFs. The authors assessed the use of CF waste at different replacement levels up to 2.5 % by weight of gypsum, wherein an increase in superficial hardness, mechanical performance, and density of the new composites was observed when compared to the reference material. From a thermal performance perspective, De Oliveira et al. [45] studied the feasibility of lightweight recycled gypsum with improved thermal properties, by using EPS together with cellulose fibre waste as partial replacement. To produce the samples, the percentage of gypsum was maintained at 40 w/w% (20 % common gypsum and 20 % recycled gypsum), while the dosage of EPS and cellulose fibre varied (15–30- 45–60 w/w%). Samples with 60w/w% EPS, without cellulose waste, achieved the lowest conductivity value (0.18 W/(m.K)), which corresponded to a reduction of ~ 75 % when compared to the control gypsum. On the other hand, the increasing incorporation of cellulose fibres with subsequent decrease of EPS content led to worse thermal conductivity results obtained and the better the mechanical performance of gypsum composites related to similar previous studies. Interesting findings were also observed by Khorjitmale et al. [46] who carried out a mechanical characterization of gypsum boards reinforced with cotton dust fibres. Waste fibres 5, 10, 15, and 20 w/w% were mixed with gypsum plaster. As a result, a 90:10 gypsum/waste ratio was established as the appropriate proportion to obtain lighter gypsum boards (density reduction over 21 % when 20 w/w% fibres were added) with acceptable strength values according to the standardized requirements. Although many studies have been carried out on gypsum/cement mortars reinforced with waste from natural or plastic fibres, little research has focused on the use of recycled cellulose acetate fibres as partial replacement (up to 3.5 wt%) of gypsum to produce sustainable materials for construction applications. Therefore, this study aimed at analysing the influence of waste cellulose acetate fibres sourced from cigarettes filters on the physical (i.e. dry state density and surface hardness), mechanical (i.e. dynamic Young ’s modulus, flexural and compressive strengths) and thermal (i.e. conductivity) properties of new gypsum composites produced and tested according to standard EN 13279–2 [47]. Moreover, complementary stereomicroscopy and scanning electron microscopy (SEM) analyses were conducted to evaluate the fibres arrangement throughout the gypsum matrix as well as the quality of waste fibre-gypsum matrix interfacial transition zone (ITZ). 2. Materials and methods 2.1. Materials For the development of this study, a setting-controlled gypsum for construction (B1) was selected, according to standard EN 13279–1 [48]. Its main properties are described as follows: purity greater than 75 %, particle size range of 0–1 mm, surface hardness Shore C <45, flexural and compressive strength greater than 1 N/mm 2 -2 N/mm 2 , respectively and pH greater than 6. The water used for production was regular tap water in accordance with Council Directive 98/83/EC [49]. Waste CA fibres can be obtained from cigarettes and textile industry waste. In this study, the waste fibres were sourced from discarded CFs belonging to various brands collected from several ashtrays positioned throughout the Seville University School of Architecture, Spain. The composition of CFs is given in Table 1 according to previous analysis [7,26]. The average fibre diameter is estimated to be around 39 ±2 µm (measured by Maderuelo-Sanz et al. [29] using a 50 ×objective in a Sensofar Plu Neox laser scanning confocal microscope). Once gathered, the remaining tobacco was manually removed from M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 3 the CFs (Fig. 1a). Subsequently, based on an adaptation from previous works [44,50], the following procedure was developed: CFs were soaked in a 96 wt% ethyl alcohol solution at room temperature for 1 day (Fig. 1b). After that, they were subjected to a cleaning procedure with water. This step consisted of 5 cycles of water changes for 30 min (Fig. 1c), with the purpose of eliminating the toxic substances and odour present in the CA as much as possible. Later on, the remaining rolling and tipping paper was manually separated from the CA filters (Fig. 1d). After completing this treatment, the filters were dried at 80 ◦C to constant mass. Finally, short recycled CA fibres were obtained using a RETSCH SM 2000 cutting mill, with a fibre length output of up to ~ 1 mm (Fig. 2). 2.2. Sample preparation Specimens were prepared following the procedure set by standard EN 13279–2 [47]. First, a water/gypsum (w/g) ratio of 0.60 was fixed for all samples, according to the shaking table test. Three steps were followed to produce the mixtures: the waste CA fibres were manually separated to avoid any potential agglomerations. After that, the gypsum and the CA fibres were dry-mixed to obtain a homogeneous blend before the addition of water. Subsequently, all components (including water) were mixed for two minutes until complete homogeneity. The content of CA fibres was limited to 3.5 % based on the weight of gypsum in order to keep the workability constant for all mixtures without modifying the W/ G ratio or using water reducing admixtures. Mixes obtained were placed in prismatic 40 ×40 ×160 mm 3 and cylindric Ø90 ×20 mm 3 moulds, depending on the test to develop. Six Table 1 Composition of CFs (adapted from Marinello et al. [7] and Kurmus et al. [26]). TYPE OF WASTE COMPONENTS MATERIAL 1. Remaining tobacco a) Burned / Ash b) Unburnt tobacco DRIED TOBACCO BLEND (1-mm thick strip) ADDITIVES (to make smoking appealing): Propylene glycol glycerol, cocoa solids, liquorice, tobacco extracts, sugars. CHEMICAL SUBSTANCES: Acetaldehyde, Acetic acid, Acrolein, Butane, Toluidine, Nicotine, Naphthalene, Phenols, Pyrene, Toluene, Ammonia, Arsenic, Butane, Cadmium, Methanol, Methane, Bensopyrene, Carbon Monoxide, DDT, Lead, Mercury, Tar, Vinyl Chloride, etc. 2.Rolling paper CELLULOSE 3. Filter CELLULOSE ACETATE 4. Plug wrap paper POLYVINYL ACETATE EMULSION 5. Microperforated tipping paper CELLULOSE Fig. 1. CFs cleaning treatment: a) CFs waste without treatment; b) CFs soaked in ethyl alcohol solution; c) water cycles process; d) remaining paper removed from filters. M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 4 prismatic and three cylindrical specimens were produced per waste replacement level, including the reference gypsum without CA fibres. Table 2 summarizes the proportions of material used to prepare each of the mixes. The mix code is the following: control gypsum without fibres (G/CM); gypsum containing CA fibres with different contents (G/CA/x). Table 3. Thereafter, samples were removed from the moulds and were cured in a dry chamber at 24 ◦C and with a relative humidity of 50 ±1 % for seven days. After this period, the samples were placed in an oven at 40 ±2 ◦C, until constant mass and then cooled to ambient temperature in a desiccator to be subjected to characterization tests. 2.3. Test methods 2.3.1. Mesostructure and microstructure assessment - Stereo microscopy analysis: this technique was applied to recognize and analyse detailed features of the components and developed products from a mesostructure perspective, such as fibres size and arrangement throughout the gypsum matrix and other parameters (e.g. pores, ITZ). A Nikon SMZ645 stereomicroscope was used for this analysis (CERENA - Natural and Environmental Resource Center - DECivil at IST, UL, Portugal). - Scanning electron microscopy (SEM): the quality of the ITZ between the gypsum’s products of hydration and the CA fibres was evaluated at a microscopic level using this method. Two different samples were chosen for the analysis: the control without plastic (G/CM) and with 2.5 % and 3.5 % waste CA addition (G/CA/2.5 and G/CA/3.5). The FEI TENEO field emission scanning electron microscope belonging to the General Research Services area of the University of Seville, Spain, was the equipment used for this test. 2.3.2. Physical and mechanical properties - Dry bulk density: conducted in compliance with standard EN 13279- 2 [47], the dry density was defined as the relation between the dry weight and volume of every test sample. - Superficial hardness (Shore C): this property was measured in three different samples from each replacement level before subjecting them to destructive mechanical tests. According to the procedure set in EN 13279–2 [47], the Shore C hardness was measured in two longitudinal and parallel faces of each prismatic sample. Five points placed in the central plane and separated at least 20 mm from the edges of the sample were selected to apply a perpendicular force induced by a durometer (Fig. 3a). - Flexural strength: following the procedure explained in EN 13279–2 [47], the samples were placed in the Form and Test Seidner Strength testing machine-D7940, centred on rollers positioned at 100 mm (Fig. 3b). Afterwards, a progressive centred breaking load was applied to every sample with a speed of 10 N/s. - Compressive strength: using the twelve broken pieces per percentage obtained from the aforementioned flexural strength, the compressive strength test was developed in accordance with standard EN 13279–2 [47]. A load was applied in the central plane of the specimen with a speed of 20 N/s until the sample’s failure, by using the related section of the same testing equipment previously described. - Dynamic Young’s modulus: based on the standard ASTM E1876-21 [51], the specimens were mechanically excited by a singular elastic strike using a small hammer. As a result, the fundamental resonant frequency of test samples was measured by the GrindoSonic MK5 equipment (Fig. 3c). Finally, the WINEMOD software program was used to calculate the main value of dynamic Young’s modulus for the new compounds based on the dimensions, mass and natural frequency of the specimens. - Ultrasonic pulse velocity: in order to identify the relationship between density and porosity of the composites, this non-destructive test is Fig. 2. Shredding CFs waste procedure: a) RETSCH SM 2000 mill; b) disinfected CFs waste set to be shredded; c) recycled CA dibres, shredded and ready to be used in composite mixes. Table 2 Mix proportions for six specimens of 40 ×40 ×160 mm 3. Mix code Gypsum (g) Water (g) W/G ratio CA waste (g) G/CM 1800 1080 0.60 – G/CA/2 1764 1058 0.60 36 G/CA/2.5 1755 1053 0.60 45 G/CA/3 1746 1048 0.60 54 G/CA/3.5 1737 1042 0.60 63 Table 3 Dimensions of specimens prepared for each test. Series of samples Sample dimension Mix code Tests Serie I 40 ×40 ×160 mm 3 G/CM Dry densitySuperficial hardness (Shore C) Flexural strength Compressive strength Dynamic Young’s modulus Ultrasonic pulse velocity G/CA/ 2 G/CA/ 2.5 G/CA/ 3 G/CA/ 3.5 Serie II Ø60mm /e = 20 mm G/CM Thermal conductivity G/CA/ 2 G/CA/ 2.5 G/CA/ 3 G/CA/ 3.5 M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 5 defined in standard EN 12504–4 [52]. The procedure followed consisted of using the Proceq Pundit Lab +equipment connected to a pair of probes with a frequency of 54 kHz to measure three transverse and one longitudinal values of ultrasonic velocity for each of the six specimens per replacement level, whereby the average value then calculated (Fig. 3d). The final data of the composites’ density, superficial hardness (Shore C), dynamic elastic modulus, ultrasonic pulse velocity, flexural and compressive strength were expressed as the mean value of the measurements performed per replacement level. 2.3.3. Thermal performance - Thermal conductivity test: determined as per standard ASTM D5930- 17 [53], after the curing period, the thermal conductivity coefficient of three cylindrical samples per replacement level was measured by using the ISOMET 2114 equipment connected to a surface probe. It applied a dynamic measurement method spending ~ 30 min per measurement in an environment at around 20 ◦C and with a relative humidity (RH) of ~ 60 % (Fig. 4). 3. Results and discussion 3.1. Stereo microscopy analysis The stereo microscopy analysis was applied to a pair of specimens with different waste CA fibre content (i.e. G/CM and G/CA/3). The images collected in Fig. 5 revealed a random and somewhat homogeneous arrangement of CA fibres throughout the gypsum matrix. There were some agglomerations of CA fibres from the original clusters that failed to disperse during the mixing process. These groups were more common with increasing waste fibre content. On the other hand, comparing G/CA/3 with the control G/CM (Fig. 5a,b vs Fig. 5c, respectively), a clear increase of pore size and number of pores by cm 2 was observed when CA fibres were added to the gypsum mix. Previous studies developed by Tannous et al. [43], demonstrated that most important quantity of pores presented in CA-containing cement composites had a diameter between 10 nm and 1 µm. Moreover, it was observed that the number of pores with diameters higher than 1 µm tends to increase with the replacement rates, due to the formation of the transition zone between fibres and cement matrix, which could explain the phenomenon observed in this section. 3.2. Scanning electron microscopy Fig. 6 presents the SEM micrographs of specimens with 2.5 % and 3.5 % waste CA content (G/CA/2.5; G/CA/3.5) as well as the control (G/ CM). The images showed an acceptable cohesion at the ITZ between the recycled CA fibres and the gypsum matrix for levels replacement up to 2.5 % CA by weight of gypsum. Nevertheless, when CA content exceeded this replacement level, the arrangement of CA fibres became heterogeneous and led to the formation of clusters of loose fibres disconnected from the matrix (Fig. 6a). The lesser precipitation of gypsum products of Fig. 3. Physical-mechanical tests: a) superficial hardness (Shore C); b) Form and Test Seidner Strength testing machine-D7940 for flexural-compressive strength tests; c) dynamic young’s modulus; d) ultrasonic pulse velocity (transverse measurement). Fig. 4. Thermal conductivity test: a) ISOMET 2114 equipment; b) cylindrical samples (Ø90 ×20 mm 3 ) The tests described were carried out in the Construction Laboratory of CERIS (Civil Engineering Research and Innovation for Sustainability), DECivil (Department of Civil Engineering, Architecture and Georesources) at the Instituto Superior T´ ecnico (IST), University of Lisbon (UL), Portugal. M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 6 hydration around the fibres (i.e. at the ITZ) and thus lesser grip with the matrix will likely lead to a decline in the mechanical performance. Apart from the lack of dispersion of these fibres from their original clusters, the non-optimal link between the gypsum matrix and the CA fibres could be explained due their hydrophobicity and lack of surface roughness, both of which worsen the adhesion to the matrix for larger agglomerations. Also, the difference between dynamic moduli of elasticity of both materials could worsen this point of failure. Concerning the reference gypsum (Fig. 6d), good degree of crystallinity was observed and, as expected, mixes containing the fibres did not exhibit any phase modification after fibre addition. To support these results, previous research based on natural cellulose fibres added to cement-based composites published by Choi and Choi [33], observed the reason for mechanical improvements is considered to be the additional nucleation sites provided by the fibres for the hydration reaction products. Nevertheless, after exceeding the optimal point (0.25 w/w% hemp and jute fibres content) the compressive strength decreased due to the heterogeneous dispersion of fibres. Also, Zhang et al. [54] observed accumulations of fibres after exceeding the optimal level of addition, which provoked fibres were no able to cross the pores or defects that exist in the gypsum matrix, leading to mechanical performance reduction. 3.3. Dry bulk density The main values of the hardened density, mechanical strength and dynamic modulus of elasticity of the gypsum composites developed are given in Table 4. As expected, a decrease of dry bulk density (up to ~ 1 %) was observed with increasing CA fibre content since these present a lower density in comparison with that of the gypsum (around 1300 kg/ m 3 vs 2650 kg/m 3 , respectively). The lowest density was recorded for mixes with 2.5 % CA content and apparently increased linearly after that (Fig. 7). It is unclear if the larger presence of waste fibres led to a slight densification of the mix since both G/CA/3 and G/CA/3.5 exhibited relatively high standard deviations. Since larger pores and greater number of pores per cm 2 had been visually identified in the stereo microscopy analysis in Section 3.1, the discrepancy is likely due to experimental error. Previous studies developed by Morales-Segura et al. [44] verified that this variation on density, which slightly increased as the dosage of CBs fibre waste rose in their case, is not relevant, but must be considered when analysing mechanical properties. 3.4. Mechanical properties Fig. 8 and Fig. 9 show the results of the flexural and compressive strengths, respectively. An improvement in flexural strength was observed with the incorporation of recycled CA fibres; increases of up to ~ 9 % were attained by mixes with 2.5 % fibre content (i.e. G/CA/2.5) in comparison with the control sample. For CA fibre contents over 2.5 %, there was a linear decline in performance. These results fit the observations made in sections 3.1 and 3.2, wherein greater number of fibre clusters and number of pores were observed for content over 2.5 %. Nevertheless, all mixes were well within the minimum requirement of 1 MPa as per standard EN 13279–2 [47]. A similar trend was observed for the compressive strength (Fig. 9); the highest compressive strength value was achieved by composites with a 2.5 % waste CA fibre content (i.e. 7.29 MPa) and subsequent replacement levels (i.e. G/CA/3 and G/ CA/3.5) presented lower values (~3% decrease relative to the control). Additionally, in all cases, the values were over the minimum 2 MPa set by the standard for gypsum binders and plasters [47]. Because CoV calculated for 2.5 % CA replacement level is around 5 %, improved values should be considered significant. Also, these values are consistent with the results reported by Morales-Segura et al. [44], who used CA fibres from cigarette filters up to 2.5 % by weight of the gypsum and also with the results of Choi and Choi [33] for natural cellulose fibres of up to 2 %. A comparison between density and mechanical strength properties was carried out, the results of which are shown in Fig. 10. Concerning the flexural strength behaviour of the different compounds, it was noted that there were three samples (G/CA/2.5, G/CA/3 and G/CA/3.5) that exceeded the control value despite being lighter than the control specimens. Of these three composites, the specimens with 2.5 % CA fibres content achieved the largest decrease in bulk density (1182 kg/m 3 ), Fig. 5. Stereo microscopy images to G/CA/3 (a �20×; b �40 ×) and G/CM (c �20 ×). M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 7 whilst exhibiting the highest flexural strength value (2.56 MPa). On the other hand, G/CA/2.5 was the only composite that managed to show enhanced compressive strength (~1% increase) in relation with the control specimens even though it was the lightest compound. The enhanced performance of composites with 2.5 % waste CA fibres (optimal content) may be due to a marginal variation of pore size Fig. 6. SEM micrographs of: (a) G/CA/3.5 (100 ×); (b) G/CA/2.5 (400x); (c) G/CA/2.5 (1600x); and (d) G/CM (1600 ×). Table 4 Summary table with all the results obtained for the physico-mechanical tests carried out in this study. Specimens Density (kg/m 3 ) (CoV %) Flexural strength (MPa) (CoV %) Compressive strength (MPa) (CoV %) Ed (MPa) (CoV %) G/CM 1194 (0.54) 2.35 (6.53) 7.23 (5.83) 4945 (2.73) G/CA/2 1184 (0.75) 2.28 (13.7) 7.00 (3.89) 4897 (2.81) G/CA/2.5 1182 (0.44) 2.56 (5.87) 7.29 (2.39) 4844 (2.24) G/CA/3 1184 (1.15) 2.38 (7.46) 7.06 (5.96) 4685 (0.81) G/CA/3.5 1186 (1.13) 2.40 (5.60) 7.06 (4.08) 4470 (1.66) Fig. 7. Dry bulk density (kg/m 3 ) of G/CA composites. M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 8 distribution, adequate adhesion at the ITZ between the matrix and the CA fibres with minimum formation of clusters, all of which were hindered with higher CA fibre contents. 3.5. =Superficial hardness (Shore C) A strong relationship was observed between the superficial hardness and the compressive strength of the test samples. All registered values of the superficial hardness were close to 96, showing an increase close to 1 % (corresponding to mix G/CA/2.5) when compared with the G/CM sample (Fig. 11). Overall, these results suggest that the optimal replacement level of waste CA fibres to improve the superficial hardness of gypsum-based composites is 2.5 %, similar to that found in the mechanical strength tests (Section 3.4). Fig. 8. Flexural strength (MPa) of G/CA composites. Fig. 9. Compressive strength (MPa) of G/CA composites. Fig. 10. Bulk density vs (a) flexural strength / (b) compressive strength of the gypsum-based composited. Fig. 11. Surface hardness (Shore C) of G/CA composites. M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 9 3.6. Ultrasonic pulse velocity The ultrasonic pulse velocity test was applied to the composites to assess their compacity and internal cohesiveness as well as the relationship between their density and mechanical performance. In Fig. 12a, it is clear that the incorporation of CA fibres led to a linear decline in the ultrasonic pulse velocities (~8% decrease when 3.5 % CA content was used). These results are somewhat correlated with those observed for density. Also, there was a discrepancy for composite G/CA/2.5 (Fig. 12b)., which showed a slightly higher velocity that corresponds to the optimal CA fibres content (i.e. homogenous arrangement, minimum formation of fibre clusters, apparent marginal increase in pore size distribution). Fig. 13 shows the loss of composites’ compactness as the CA waste replacement level increases, due to pores size and distribution rising from 2.5 % CA content, according to the visual assessment presented in Section 3.1. 3.7. Thermal conductivity The results of the thermal conductivity test are shown in Fig. 14. A steady downwards linear trend can be observed with increasing waste CA fibres content. Therefore, the higher the waste replacement level, the lower the value of thermal conductivity coefficient achieved, due to the formation of large number of pores and their size increase. The use of 3.5 % CA fibres content led to a decrease of ~ 16 % when compared to the control gypsum. 4. Conclusions Concluded the complete physico-mechanical and thermal characterization of gypsum composites containing recycled CA fibres in accordance with the standard requirements established by EN 13279-2 [47], the conclusions obtained are as follows: – No changes were observed in terms of workability of the gypsum mixes after adding of up to 3.5 w/w% waste CA fibres, which allowed a constant 0.6 water/gypsum ratio without the use of water reducing admixtures; – Marginal decline hardened density of the gypsum-based composites containing CA fibres was observed when compared with the reference material (~1% decrease with 2.5 % CA content); – Mechanical strength properties were enhanced with the use of CA fibres. Maximum improvements in flexural and compressive strengths were achieved by composites with 2.5 % CA content (~9% and ~ 1 % increase, respectively, when compared to control sample). Despite a linear decline in flexural and compressive strengths for higher replacement levels, all values were well above the standardized minimum requirements; – Acceptable quality at the interfacial transition zone between the recycled CA fibres and the gypsum matrix was observed in the SEM analysis. However, for CA fibre contents over 2.5 %, the arrangement of the fibres became somewhat heterogeneous throughout the gypsum matrix with the formation of agglomerations; – The use of waste CA fibres allowed the production of specimens with lower thermal conductivity coefficient (~0.33 W/(m⋅K)) in comparison with that of the control gypsum (up to 16 % reduction when 3.5 % CA fibre content was used when compared to the control); The technical feasibility of the gypsum-based composites containing recycled CA fibres was demonstrated in this study and compared against relevant EU standardization. Its application for the manufacture of building products, such as gypsum plasterboards, it clearly demonstrated since the composites easily fulfil all the regulatory requirements. Additionally, not only did the gypsum-based composites exhibit lighter weight, enhanced surface hardness, mechanical and thermal performance with the use of recycled CA fibres as partial replacement of the gypsum, but, at the same time, a new recycling outlet is proposed to reduce the hazardous environmental pollution caused by nondegradable CFs. The incorporation of 3.5 wt% of CA fibres in gypsum mixtures could recycle around 93 kg of CA fibres in one cubic meter of gypsum. Future lines of research in this area are the standardization of the aforementioned pre-treatment (i.e. cleaning and disinfection of CFs) of CA fibres, the reduction of composites’ odour incorporating them, and toxic effluent management.. At the same time, further research on LCA study should be carried out to evaluate the environmental impact of this new recycling way proposed. CRediT authorship contribution statement M.I. Romero-G´ omez: Writing – original draft, Writing – review & editing. R.V. Silva: Writing – review & editing, Supervision. M.F. Costa-Pereira: Writing – review & editing, Supervision. I. Flores- Fig. 12. Ultrasonic pulse velocity of (a) composites with increasing CA fibres content and (b) correlation with bulk density. M.I. Romero-G´ omez et al. Construction and Building Materials 356 (2022) 129308 10 Colen: Writing – review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgements Romero-G´ omez, M.I. (author) wishes to acknowledge the financial support provided by the FPU Program of Spanish Ministry of of Science, Research and Universities (FPU 18/02405) and (EST21/00483) and also to the FCT - Foundation for Science and Technology, through the research projects PTDC/ECI-CON/29196/2017 (RInoPolyCrete) and EXPL/ECIEGC/0288/2021 (ECO 2 Alkrete). The authors also acknowledge the support of CERIS and CERENA research units, from DECivil, IST, University of Lisbon, Portugal References [1] Statista, Global consumption of cigarettes 1880-2017. https://www.statista. com/statistics/279577/global-consumption-of-cigarettes-since-1880/, 2021 (accessed 14 October 2021). [2] T.E. Novotny, K. Lum, E. Smith, V. Wang, R. Barnes, Cigarettes butts and the case for an environmental policy on hazardous cigarette waste, Int. J. Environ. Res. Public Health. 6 (5) (2009) 1691–1705, https://doi.org/10.3390/ijerph6051691. 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Sukpancharoen, Gypsum boards reinforced with cotton dust fibre. IOP Conf. Ser. Earth, Environ. Sci. 692 (3) (2021), https://doi.org/10.1088/1755-1315/692/3/032021. [47] EN 13279-2:2014. Gypsum binders and gypsum plasters-Part 2: Test methods. [48] EN 13279-1: 2009. Gypsum binders and gypsum plasters-Part 1: Definitions and requirements. [49] European Union. Council Diretive (EU) 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. OJ L. 330, 5.12.1998, 32–54. [50] C. Rojas-Hern´ andez, P. Guía, L. Lares-L´ opez, Filterm: Revestimiento de interior a partir de colillas de cigarro, Universidad de Chile, Facultad de Arquitectura y Urbanismo, 2019. [51] ASTM E1876-21, Standard Test Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio by Impulse Excitation of Vibration. [52] EN 12504-4: 2006. Testing concrete. Determination of ultrasonic pulse velocity. [53] ASTM D5930-17, Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line- Source Technique. [54] B. Zhang, Y. Haibin, X. Tao, T. Waiching, C. Hongzhi, Mechanical and Thermo- Physical Performance of Gypsum-Based PCM Composite Materials Reinforced with Carbon Fiber, Appl. Sci. 11 (2) (2021) 468, https://doi.org/10.3390/ app11020468. M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 4 flexural strength test and, subsequently, the resulting twelve broken prismatic sections were used in compressive strength tests. The ultrasonic pulse velocity non-destructive test was carried out according to the standard EN 12504–4 [41], by using the Proceq Pundit Lab +equipment to verify the relationship between the density and the porosity of the composites. 2.3.2. Deformability tests The deformability of the gypsum composites was analysed by considering the modulus of elasticity and the stress/strain curves obtained from the flexural and compressive strength tests. The modulus of elasticity was defined according to standard ASTM E1876-15 [42], by applying the GrindoSonic MK5 equipment to measure the fundamental resonant frequency of the test samples. The aforementioned tests were developed in the Construction Laboratory of CERIS (Civil Engineering Research and Innovation for Sustainability) at the Instituto Superior T´ ecnico of Lisbon, Portugal. 2.3.3. Other test methods Scanning electron microscopy (SEM) was used to assess the ITZ between the gypsum matrix and the different types of fibre reinforcement added and understand their differences from a microscopic level. Four samples were analysed and compared; one of them without plastic addition (G/CM) and the other three with 2% PA6, E-glass fibre and AR- glass fibre content, respectively (G/PA6/I/2, G/EGF/2 and G/ARGF/2). Prior to the test, the specimens were coated with gold to improve the output’s quality, due to their low conductivity. The FEI TENEO field emission scanning electron microscope from the General Research Services area of the University of Seville, Spain, was employed. 3. Results and discussion 3.1. Density Higher dry density values were achieved when nylon and glass reinforcement fibres were added (4.6% and 5.2%, respectively) when compared to the control gypsum specimens. This behaviour could be attributed to a decrease in porosity as a consequence of the densification of the blend when fibres were added. The addition of short PA6/I fibres resulted in slightly higher density values than those obtained with long PA6/II fibres. However, both peaked at 2 wt% waste nylon fibre content and decreased linearly after that. Similarly, an increasing linear trend was observed when AR-glass fibres were incorporated into the mix up to 2 wt% (Fig. 2). Concerning E-glass fibre-reinforced composites, the highest density was recorded for 1 wt% of fibre content. This had been observed in previous studies developed by Del Rio [43]. The declining linear trend observed in recycled nylon fibre-reinforced gypsum specimens after exceeding the density peak value inferred that 2 wt% of waste PA6I/II fibres is an optimum content comparable with AR-glass fibres addition and higher than that achieved with E-glass fibres. After that, the compactness started to deteriorate due to weak binder matrix-fibre interlock. 3.2. Mechanical properties Encouraging results were obtained from the flexural strength test, as shown in Fig. 3. As expected, the partial replacement of gypsum by commercial glass fibres had a positive influence on the performance of gypsum mixes, leading to a rise in flexural strength of up to ~ 75% (G/ EGF/2) with respect to the control material. Although the flexural strength values of waste nylon fibre-reinforced gypsum specimens were lower than those achieved by commercial fibre-reinforced composites, it should be noted that the long PA6/I fibres led to enhanced flexural strength (i.e. up to ~ 7.5% increase for a 2 wt% content - optimal flexural strength for this type of fibre). The opposite was observed for the short PA6/II fibres, which cause a decline in performance. Nevertheless, all values of these specimens were over the minimum 1 MPa set by EN 13279–2 [37], which means that the mixtures perform well relative to the standard requirement. Fig. 4 shows the cross-sections of four specimens with 2 wt% content of the different types of fibres (i.e. G/EGF/2, G/ARGF/2, G/PA6/I/2 and G/PA6/II/2) after being subjected to a flexural strength test. It shows a homogenous and multi-directional arrangement of the fibres throughout the gypsum matrix. Many small holes and fibres sticking out without fractures can be observed in the specimens containing PA6 fibres, which shows that fibres were pulled out of the gypsum matrix instead of breaking, after the flexural strength test. This suggests that the ITZ between the matrix and fibres is relatively weak. The results of the compressive strength test (Fig. 5) suggest an overall improvement with the addition of reinforcement fibres to the gypsum matrix in comparison with the reference material. The incorporation of AR glass fibres of 2 wt% led to the highest compressive strength (i.e. 11.28 MPa, corresponding to blend G/ARGF/2). The highest of mixes Fig. 2. Dry density (kg/m 3 ) of gypsum /fibres composites. Fig. 3. Flexural strength (MPa) of gypsum/fibres composites. M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 5 with E-glass fibres was at 1 wt% content, which has also been witnessed by others [43]. The use of waste nylon fibres led to relatively lower values, though better than the control specimens. In this case, the length of the waste nylon fibre had a great influence over the compressive strength performance of the gypsum composites. The addition of 2 wt% of long PA6/I fibres led to an enhancement of up to ~ 17% (G/PA6/I/2) with respect to the control. On the other hand, a content of 2.0–2.5 wt% of short PA6/ II fibres led only to a slight improvement (i.e. up to ~ 3% increase, corresponding to G/PA6/II/2.5). Of the fibre-reinforced mixes, only two presented compressive strength below the control - G/PA6/II/1 and G/ PA6/II/3, with 7.48 MPa and 7.45 MPa, respectively. Nevertheless, considering that the minimum requirement is of 2 MPa for gypsum binders and plasters [37], all mixes were well within the limit, which makes the use of any of the blends evaluated viable in building applications. The analysis of variance (ANOVA) of the mechanical properties of both types of waste nylon fibre-reinforced composites (i.e. G/PA6/I vs. G/PA6/II; see Table 4), pointed out that the p-values associated with the content factor in the flexural strength values, and the fibre length factor in compressive strength results, are lower than the level of significance ( α =0.05). Therefore, the null hypothesis of the two factors, which says that the mean is the same for all groups, can be ruled out with 95% confidence. It can also be observed that the p-value of the fibre length factor in flexural strength, the percentage of addition factor in compressive strength and the interaction between both factors in both cases are not significant, since their values are higher than the 0.05 level of significance. Thus, the influence of the fibre length in flexural strength, the level of addition in compressive strength, as well as the interaction effect between both factors studied in composites’ mechanical strength performance can be rejected. 3.3. Superficial hardness (Shore C) As a consequence of the enhanced compressive strength by waste nylon fibre-reinforced composites, the superficial hardness (Shore C) improved up to 3.35% (highest values presented by G/PA6/II/3 mixes) when compared with the control mixes (Table 5). The addition of commercial glass fibres led to even higher superficial hardness (up to ~ 3.4% increase, corresponding to G/ARGF/3). Therefore, it can be inferred that long waste nylon fibres could be used as a replacement for commercial glass fibres because the results obtained are comparable with those achieved by commercial glass fibre commonly used for this purpose. Fig. 4. Cross-section of the composite with 2% of: (a) E-glass fibre (G/EGF/2); (b) AR-glass fibre (G/ARGF/2); (c) PA6/I fibre (G/PA6/I/2); (d) PA6/II fibre (G/PA6/ II/2). Fig. 5. Compressive strength (MPa) of gypsum/fibres composites. Table 4 Results of ANOVA for flexural and compressive strength test results of PA6- gypsum composites: influence of PA6 fibre length, percentage of waste added and the interaction of both factors. Control factor Sum of Square (SS A ) Degrees of freedom (f) Mean Square F AQ P- value Flexural Strength Fibre length [mm] 0.77 1 0.77 3.01 0.09 PA8 Fibre Waste Addition [%] 3.33 4 0.83 3.27 0.02 Interaction 0.62 4 0.15 0.61 0.66 TOTAL 17.48 59 – – – Compressive Strength Fibre length [mm] 7.60 1 7.60 8.29 0.01 PA8 Fibre Waste Addition [%] 5.17 4 1.29 1.41 0.24 Interaction 1.65 4 0.41 0.45 0.77 TOTAL 60.30 59 – – – Table 5 Results obtained from the superficial hardness (Shore C) test carried out in this research. Fibre content (%) 0 1 2 2.5 3 CM 94.5 (1.72) G/EGF 97.4 (1.77) 97.5 (1.20) G/ARGF 97.6 (2.16) 97.7 (2.12) G/PA6/I 95.7 (2.06) 96.7 (1.63) 97.3 (0.86) 97.6 (1.05) G/PA6/II 95.4 (0.79) 95.2 (1.07) 95.7 (1.15) 95.8 (1.06) M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 6 3.4. Ultrasonic pulse velocity The ultrasonic pulse velocity test was carried out to further understand the internal layout of the new composites, since the propagation of ultrasonic pulses depends on the density and modulus of elasticity of the material. Fig. 6 shows an increase of ultrasonic pulse velocity with the increasing addition of waste nylon fibre, peaking at 2 wt% waste fibre content. Similar values were observed for both types of nylon fibre length (PA6/I and PA6/II). This fact pointed out that there is an optimal relationship between the recycled PA6 fibre content, arrangement, and pore distribution. The polynomial trends of the ultrasonic pulse velocity suggest that the porosity of the fibre-reinforced gypsum composites decreased when using up to certain fibre content (~1.0–2.5 wt% deemed as optimum depending on the type of fibre) thus inferring an overall improvement of compactness. After that, the decrease in velocity, when compared to the control material, suggests increased porosity. Comparing the performance of both commercial glass fibres, a higher increase in density was presented by AR glass fibre-reinforced composites, leading to a greater densification of the material that could explain the better compressive strength performance presented by G/ARGF compounds. 3.5. Scanning electron microscopy analysis Four specimens with 2 wt% fibre content were subjected to SEM analysis (i.e. G/EGF/2, G/ARGF/2, G/PA6/I/2, G/PA6/II/2) and compared to the control (G/CM). The morphology of the gypsum matrix shown in Fig. 7 is typically presented by hydrated calcium sulphate. As expected, no alteration in the nature of the crystalline network was detected with the incorporation of fibres regardless of their type given their inert nature. It shows an acceptable connection at the ITZ between the fibres and the gypsum matrix, which explains the improvement of mechanical properties. The adhesion appears sounder for commercial glass fibres than for waste nylon fibres. This was to be expected from fibres with a higher aspect ratio; for a given length and fibre content, as the diameter decreases, this implies the existence of a higher surface area and thus more sites for gypsum crystal precipitation promoting adhesion between the matrix and the fibre. It is possible that, with the ensuing degradation of fishing nets through use and UV exposure, the fibres gain reasonable rough surfaces, thereby allowing improved grip. Also, it should be noted that the difference in moduli of elasticity of the fibres and that of the gypsum matrix could be a failure point that weakens the cohesion of the ITZ between both materials. 3.6. Deformability behaviour The influence of the addition of waste nylon and commercial glass fibres on the deformability of gypsum composites was analysed by the modulus of elasticity (Young’s modulus). From the results presented in Table 6, it was noticed that the incorporation of reinforcement fibres led to an overall decrease in the specimens’ modulus of elasticity when compared to the control material. As expected, the enhanced mechanical performance when using commercial glass fibres was related to the higher values of Young’s modulus in comparison with gypsum composites reinforced with waste nylon fibres. According to the results observed in section 3.2, the highest values of Young’s modulus corresponded to composites with optimum fibre content that rendered maximized compressive strength values (G/EGF/1, G/ARGF/2, G/PA6/ I/2 and G/PA6/II/2.5), thus confirming the enhancement of the gypsum’s compactness. Specimens made with larger waste nylon fibres showed lower overall moduli of elasticity when compared to composites with short fibres, except for G/PA6/I/2 composites. The reinforced gypsum composites’ pre-failure and post-failure performance were also evaluated through their stress/strain curves of the flexural and compressive strength tests. The representative stress/ strain curves in the compressive strength of each compound are presented in Fig. 8. It was observed that fibre-reinforced gypsum composites presented a more ductile failure when compared to the control gypsum. Although the new composites showed a greater slope of the prefailure line, the decay of the load after the peak was lower. Therefore, the gypsum composites reinforced with fibres are still able to support some load and deflection after failure, unlike the control specimen that presented a brittle failure (given by an abrupt downward slope of the tangent to the curves after failure). Specimens reinforced with the longer PA6/II fibres presented a higher maximum load, as well as lower decay post-peak, thus developing tougher gypsum composites. Concerning the influence of fibres’ nature on the post-peak behaviour, the specimen with the greatest ductility was that of composites with 2 wt% E-glass fibre content. Furthermore, it should be noted that the failure performance developed by composites with 1 wt% E-glass fibre content is comparable with those of specimens with 1 wt% recycled PA6 fibre content, regardless of the fibre length. Similar stress–strain curves acquired from a compressive strength test had been reported by other authors focused on the development of fibre-reinforced composites [18,44]. Fig. 9 illustrates the stress–strain curves obtained from the flexural strength test. The incorporation of fibres resulted in a more ductile behaviour in comparison with the control gypsum. Although the control presented a brittle failure due to its inability to withstand any more load after the peak, the fibre-reinforced mixes were still able to support a smaller load and deflected for a brief period post-peak. Regarding the type of fibre used, composites with commercial E-glass fibres presented the highest tenacity, regardless of fibre content, because of the optimal balance between higher values of strength and deformation before failure. Increasing the AR-glass fibre content from 1 wt% to 2 wt% considerably improved the pre-failure behaviour of G/ARGF compounds. Some influence was noted concerning the different lengths of the recycled fibres. In both composites (i.e. G/PA6/I and G/PA6/II), there was an increase of ductility using a fibre content of ≥2 wt%. Nevertheless, when using long fibres PA6/II, composites with a 1 wt% fibre content modified their behaviour pre-failure towards a more ductile model. Also, composites with longer PA6/I fibres presented lower decay of the load after the peak than that showed by G/PA6/II specimens up to the failure point. Comparable stress–strain curves obtained in flexural strength tests had been reported by other authors [16,22,25]. Finally, by observing the fracture model presented by samples subjected to the flexural strength test, it should be highlighted that the Fig. 6. Ultrasonic pulse velocity of composites with increasing fibre content. M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 7 addition of reinforcing fibres led to a greater stress arrangement with increasing fibre content (Fig. 10). Moreover, the addition of long nylon fibres (PA6/I) avoided the brittle destruction of gypsum composites post-failure. Therefore, it can be inferred that a higher pull-out resistance was developed by long nylon waste fibres (PA6/I), due to their greater anchorage length. 4. Conclusions The effectiveness of using waste fishing nets as recycled nylon fibres to reinforce gypsum composites (up to 3 wt% with two different fibre lengths) was evaluated in this study. The influence of these waste fibres on the composites’ physico-mechanical properties was analysed and discussed in comparison with two types of commercial glass fibres. The main findings are as follows: - The inclusion of up to 3 wt% of waste nylon fibres did not affect the gypsum composites’ workability, thus allowing maintaining a constant 0.55 W/G ratio without the use of additives; - Fibres led to widespread densification of the gypsum composites - increase of up to ~ 5% with 2 wt% fibre content, regardless of fibre type, when compared with the control mix. As consequence, the compactness of the reinforced composites increased, peaking at 2 wt % content of PA6/I, PA6/II and ARGF fibres and declining after that due to weakened matrix-fibre interlock; - The mechanical strength of the PA6 fibre-reinforced gypsum composites improved in comparison with the reference material. By comparing the two types of PA6 fibre lengths analysed, the greatest mechanical improvements correspond to 2 wt% long PA6/I fibre content (G/PA6/I/2), which increased flexural strength by ~ 7.5% Fig. 7. SEM analysis of G/CM (a �400 ×), G/EGF/2 (b �400 ×), G/ARGF/2 (c �400×; d �800 ×) and G/PA6/I/2 (e �400×; f �800 ×). Table 6 Modulus of elasticity - E [MPa] (CoV [%]). Fibre content (%) 0 1 2 2.5 3 CM 1063 (11.7) – – – – G/EGF – 1014 (4.7) 921 (2.0) – – G/ARGF – 1019 (7.8) 1048 (0.4) – – G/PA6/I – 882 (11.3) 1038 (2.8) 968 (0.2) 925 (2.4) G/PA6/II – 938 (0.6) 979 (2.5) 1000 (0.1) 971 (6.3) M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 8 Fig. 8. Strain/stress curves of fibre-reinforced gypsum composites from the compressive strength test: a) G/PA6/I; b) G/PA6/II; c) G/EGF; d) G/ARGF. Fig. 9. Strain/stress curves of gypsum composites from the flexural strength test: a) G/PA6/I; b) G/PA6/II; c) G/EGF; d) G/ARGF. M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 9 and compressive strength by ~ 17 %. Poorer mechanical behaviour was observed for shorter PA6/II fibres and fibre contents above 2 wt %. The use of commercial glass fibres resulted in the best performance enhancement when compared to recycled nylon fibres - the use 2 wt% ARGF fibres led to flexural and compressive strength increases of ~ 75% and ~ 46%, respectively, when compared to the control; - The SEM analysis showed acceptable adhesion between the gypsum matrix and the recycled nylon fibres. PA6 fibres used in this study presented slightly embossed surfaces, when compared with the smooth surface of glass fibres, due to the former material’s outer degradation from environmental exposure, enhancing the adhesion at the ITZ. Nevertheless, the higher aspect ratio of commercial glass fibres, countered the surface’s lack of roughness, which meant a more effective adhesion of the gypsum matrix to a larger fibre surface area for the same content, thereby significantly improving the postpeak load carrying capacity; - Lower moduli of elasticity and better pre-failure performance were observed when recycled nylon and commercial glass fibres were incorporated into the gypsum mixes. The highest values offered by glass fibre-reinforced gypsum composites, in comparison with those showed by gypsums reinforced with waste PA6 fibres pointed out a clear relation with compactness improvements, peaking at 2 wt% ARGF, PA6/I and PA6/II fibres content. At the same time, a more ductile behaviour post-peak in compressive strength was noticed as the fibre content increased. Also, a similar pre-failure deformation model was presented by the fibre-containing composites, with 1 wt% EGF, PA6/I and PA6/II fibre content presenting the greatest ductility; Fig. 10. Types of cracks of the samples subjected to flexural strength test: a) G/PA6/I; b) G/PA6/II; c) G/EGF; d) G/ARGF. M.I. Romero-G´ omez et al. Construction and Building Materials 387 (2023) 131675 10 - The inclusion of fibres in gypsum composites subjected to flexural strength transformed their brittle fracture into a more ductile one. The addition of commercial glass fibres led to a significantly greater improvement in the composites’ tenacity when compared with that obtained from incorporating recycled nylon fibres. However, long PA6/I fibres led to a lower decay of load post-peak when compared to composites with short PA6/II fibres. In addition, as evidenced by the type of fracture, higher pull-out resistance of PA6/I fibres was detected as a result of potentially better stress transfer from the matrix to the fibres. Thus, the brittle destruction of gypsum composites post-failure was avoided; - This study proved the feasibility of using end-of-life fishing nets as fibres to produce reinforced gypsum-based products according to standard requirements. The new gypsum composites clearly showed enhanced performance in terms of mechanical strength, surface hardness and pre-failure behaviour, when compared to the plain control gypsum. Additionally, from an environmental perspective, the widespread implementation of the polluting plastic waste fibres (i.e. nylon-based fishing nets) as the partial replacement of a raw material (i.e. gypsum) will lead to a decrease in the demand for natural resources. Moreover, the use of waste fishing nets as reinforcement in gypsum matrixes, rather than commercial polymeric fibres, implies a decrease of landfilled waste, as well as a smaller energy consumption to industrially produce and process in comparison with that practiced for commercially available fibres; - Further research is still necessary in terms of the standardization of the cutting procedure needed to obtain regular fibre length from fishing nets, since the manual process used at the experimental scale is time-consuming and unsuitable for real manufacturing. Moreover, the post-failure behaviour of fibre-reinforced gypsums is yet to be investigated. CRediT authorship contribution statement M.I. Romero-G´ omez: Writing – original draft, Writing – review & editing. R.V. Silva: Writing – review & editing, Supervision. I. Flores- Colen: Writing – review & editing, Supervision. P. Rubio-de-Hita: Writing – review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgements The author Romero-G´ omez, M.I. would like to acknowledge the FPU Program of Spanish Ministry of Science, Research and Universities (FPU 18/02405) and (EST21/00483) for the financial support. The authors also want to acknowledge CERIS, from IST, for the research staying and the support provided by Manuel Francisco Costa Pereira of CERENA (IST, University of Lisbon) with optical microscopy analysis. This research was also funded by FCT - Foundation for Science and Technology, through the research project EXPL/ECI-EGC/0288/2021 (ECO 2 Alkrete). 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Mater. 303 (2021), https://doi.org/10.1016/j.conbuildmat.2021.124424. M.I. Romero-G´ omez et al. 150 151 SECCIÓN 3 Publicación 9 Publicación en revista indexada JCR (2023): Journal of Building Engineering, Q1(9/68). Factor de impacto 6.4. Segundo decil. Romero-Gómez, M.I., Costa-Pereira, M.F., Soares-Dias, A.P., Flores-Colen, I. (2023). Influence of high-temperature exposure on the properties of gypsum-plastic waste composites: thermophysical and microstructural analysis. Journal of Building Engineering, 79, 107862 https://doi.org/10.1016/j.jobe.2023.107862 TABLE 1. Mix composition for six prismatic samples 40x40x160mm Sample Series Gypsum [g] Water [g] (total) Water [g] (Gypsum mix) Water [g] (Fibre mix) W / G ratio PPF [g] PPWF [g] G/CM 1800 990 - - 0.55 - - G/PPF/2 1764 970 - - 0.55 36 - G/PPF/2.5 1755 965 - - 0.55 45 - G/PPF/3 1746 960 - - 0.55 54 - G/PPF/3.5 1737 955 - - 0.55 63 - G/PPWF/2 1764 970 867 103 0.55 - 36 G/PPWF/2.5 1755 965 837 129 0.55 - 45 G/PPWF/3 1746 960 806 154 0.55 - 54 G/PPWF/3.5 1737 955 775 180 0.55 - 63 A total of 24 prismatic samples of 40x40x160 mm3 were prepared for each type of fibre evaluated (i.e. six prismatic specimens for each series of mixture), as well as the control specimen (G/CM). Those specimens were designed to test the physico-mechanical properties of the new gypsum composites. Test Methods Concluded the specimen preparation, the resulting samples were cured and tested according to the standard EN 13279-2 [28]. After being stored for seven days in a dry chamber at a temperature of 24ºC and a relative humidity of 50 ± 1%, the samples were placed in an oven at 40 ± 2 ◦C to constant mass and cooled to laboratory temperature in a desiccator. Then, the dry density of the new composites was calculated by measuring the mass and volume of the test samples and expressed as the main value of the six measures per series. Flexural strength was determined by subjecting six samples for mixture to three-point bending test, as well as the resulting 12 test sections were undergone compressive strength test. The automatic multi-test press MCO-30 equipment, with a 300 KN load capacity was used to develop flexural and compressive strength. A progressive centered load with a speed of 10 N/s and 20 N/s, respectively, was applied until sample failure. Finally, the deformation behaviour was evaluated using the elasticity modulus and the stress-strain curves obtained from the compressive strength test. RESULTS AND DISCUSSIONS The values obtained for the different test methods applied to characterize the new reinforced gypsum composites are summarized in Table 2. TABLE 2. Summary table with the results obtained for the physico-mechanical tests. Sample Series Density [Kg/cm 3 ] Flexural Strength [MPa] Compressive Strength [MPa] E [MPa] G/CM 1205 3.73 8.45 859 G/PPF/2 1199 6.88 8.82 963 G/PPF/2.5 1195 8.28 8.73 853 G/PPF/3 1183 8.20 7.65 647 G/PPF/3.5 1182 7.86 7.27 489 G/PPWF/2 1203 4.35 4.75 784 G/PPWF/2.5 1202 4.45 4.13 391 G/PPWF/3 1196 3.85 3.41 316 G/PPWF/3.5 1194 3.42 3.15 311 080015-4 08 October 2023 19:16:21 Dry density With respect to the data observed in Figure 2, the density values of the gypsum composites, in all cases, decreased slightly as the percentage of fibre addition increased. Compared to the control material, a more pronounced density reduction was observed in samples with PPF content (~2% decrease when a percentage of 3.5 wt% PPF was added). Regarding to PPWF-containing gypsums, the density reduction was ~1% (G/PPWF/3.5), compared to the reference sample. The declining trend observed in both cases could be attributed to the partial replacement of gypsum matrix by PPF/PPWF fibres with lower density (around 2650 kg/m3 vs. 910/1000 kg/m3, respectively). FIGURE 2. Dry Density comparative results Flexural Strength Encouraging results were obtained for the flexural strength test, as can be observed in Figure 3. In all cases, the addition of fibres led to an increase in flexural strength. Although composites reinforced with PPF presented higher flexural strength values (up to ~121% increase), the incorporation of PPWF into gypsum mixtures improved the flexural strength behaviour by up to ~19.5%, compared to the control material. In both cases, the highest improvement in flexural strength (8.28MPa for G/PPF/2.5 and 4.45MPa for G/PPWF/2.5) corresponds to composites with a 2.5wt% fibre content. So, it can be inferred that the 2.5wt% PPF/PPWF replacement level is a peak, from which a decreasing trend was observed as the percentage of fibre addition rose. Also, it should be noted that all values were well within the 1MPa limit as per standard EN 13279-2. FIGURE 3. Comparison of flexural strength results 080015-5 08 October 2023 19:16:21 Compressive Strength Figure 4 shows the results of the compressive strength test of the composites evaluated. A notable difference in compressive performance was detected related to the type of fibre used. For PPF-containing gypsum, an improvement of up to ~4.5% was achieved using 2wt% fibre content, compared to reference gypsum. From this point on, the compressive strength showed a downward linear trend with values below the control material data after exceeding the 3 wt% PPF replacement level. On the other hand, the higher the percentage of PPWF addition was added to the blends, the lower the compressive strength values were achieved in relation to the reference material. The maximum fall of 62.7% was reached by G/PPWF/3.5 composites. Despite the declining linear trend showed by PPWF-containing composites, all the resistance values were higher than the minimum 2MPa requirement set by standard EN 13279-2 [28]. The relation between compressive strength and density of the new composites can be observed in Figure 5. The widespread reduction of density pointed out an increase of gypsum matrix void. This meant the compactness of PPWF- containing composites deteriorated as the percentage of fibre rose, because of a weak fibre-matrix bond. However, the incorporation of PPF up to 2.5wt% replacement level led to the development of lighter composites with enhanced compressive strength performance. So, it can be inferred that the good cohesion between PPF-matrix observed until reach this optimal content, got worse when higher levels of PPF addition were used. FIGURE 4. Comparative results of compressive strength FIGURE 5. Compressive Strength vs. Density of the composites evaluated 080015-6 08 October 2023 19:16:21 Deformation Behavior Regarding the influence of PP fibre addition on the deformation behaviour of gypsum composites, it can be observed that the modulus of elasticity (Table 2) of PPWF reinforced gypsum composites was reduced by increasing the level of fibre addition, compared to the reference value. However, PPF-containing composites presented a higher modulus of elasticity when percentages of addition lower than 2.5wt% were used; after that content, the values were above the reference. This fact indicates that G/PPF/2 and G/PPF/2.5 composites are stiffer than the control material, supporting the improvement of composites compactness up to reach the optimal content (2wt%). On the other hand, after analysing the pre-failure performance of new composites by the stress-strain curves obtained from the compressive strength test (Figure 6), it could be deduced a more ductile failure of fibre-reinforced composites, benhmarked to the brittle failure shown by the reference material. When comparing the influence of both types of fibres, PPF-reinforced composites showed the greatest tenacity, due to the optimal balance between higher values of compressive strength and deformation pre-failure. However, more plastic deformation was performed by PPWF-reinforced composites when 3.5 wt% fibre content was added. (a) (b) FIGURE 6. Stress-Strain curves from the compressive strength test: a) G/PPF composites; b) G/PPWF composites. CONCLUSION In this work, the effectiveness of using new and recycled polypropylene (PP) fibres as reinforcement of gypsumbased composites, partially replacing the raw material, was evaluated. After analyzing the physico, mechanical and deformation performance of the new gypsum composites, the following conclusions were drawn: -The incorporation of PPF and PPWF into gypsum mixtures leads to a slight decrease in the density of composites in the dry state as the percentage of fibre content increases, compared to the reference material. The lowest density values were achieved with PPF-containing composites. -The flexural strength of fibre-reinforced composites was significantly enhanced, peaking at 2.5wt% fibre content. Higher values of flexural strength were reached by PPF-containing composites. Also, the addition of up to 2.5wt% PPF shows an enhancement of compressive strength. However, the use of PPWF leads to a decrease in compressive resistance, which is greater in mixtures with a higher level of fibre addition. -Gypsum-based composites reinforced with PPF and PPWF exhibit a more ductile behavior before failure, compared to the control gypsum, which shows a brittle failure. Finally, it can be inferred the feasibility of using PPF as partial replacement of the binder matrix to improved mechanical performance of gypsum composites. Likewise, encouraging results on the substitution of these commercial fibres by PPWF from waste wet wipes were demonstrated. As a result, new lighter, environmentally friendly PPWF- containing gypsums with improved flexural strength performance and acceptable compressive strength values within 080015-7 08 October 2023 19:16:21 the minimum standard requirement were developed. Thus, a more sustainable material as substitute for commercial gypsum in buildings, included in a circular economy model was proposed, by decreasing the use of raw material (i.e. gypsum) and offering a new recycling way to an pollutant waste (i.e. wet wipes). ACKNOWLEDGMENTS Romero-Gómez, M.I. (author) would like to acknowledge the financial support provided by the FPU Program of the Spanish Ministry of Science, Research and Universities (FPU 18/02405). REFERENCES 1. Plastics Europe, Plastics the Facts 2021. An Analysis of European Plastics Production, Demand and Waste Data (2021) 2. The Minderoo Foundation, Plastic Waste Makers Index (2021). https://www.minderoo.org/plastic-waste- makers-index/ 3. OCU, Las toallitas húmedas, nunca al WC (2018). https://www.ocu.org/consumofamilia/bebes/noticias/toallitas-humedas/pasos-laboratorio 4. Y. Xu, G. Xing, J. Zhao, and Y. 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Gypsum Binders and Gypsum Plasters - Part 2: Test Methods 080015-9 08 October 2023 19:16:21 260 261 SECCIÓN 4 Publicación12 Comunicación en congreso internacional. Publicación en libro de actas del congreso. Romero-Gómez, M.I., Rubio-de-Hita, P., Pedreño-Rojas (2023).Recovery of plastics marine debris as fibre reinforcement to develope new eco-friendly gypsum products. In Proceedings of CEES 2023 - International Conference on Construction, Energy, Environment and Sustainability. Itecons. ISBN: 978-989-54499-3-4. 262 CEES 2023 | 2nd International Conference on Construction, Energy, Environment & Sustainability 27-30 June 2023, Funchal - Portugal 1 RECOVERY OF PLASTICS MARINE DEBRIS AS FIBRE REINFORCEMENT TO DEVELOPE NEW ECO-FRIENDLY GYPSUM PRODUCTS M. Isabel Romero-Gómez1 Paloma R. Hita 1 M. Alejandro Pedreño-Rojas 2 1 Departamento de Construcciones Arquitectónicas I, Escuela Técnica Superior de Arquitectura, Universidad de Sevilla | Spain 2 Departamento de Construcciones Arquitectónicas y su Control. Escuela Técnica Superior de Edificación, Universidad Politécnica de Madrid | Madrid Corresponding author: [email protected] Keywords Polypropylene waste; Nylon waste; Fibre reinforcement; Mechanical properties; Eco-friendly product Abstract According to recent data, more than 200 kg of plastics are throw into the ocean every second around the world, producing disturbing consequences to the marine ecosystem. It is estimated that oceans accumulate up to 150 million tons of plastic, coming mainly from fishing activities and wastewater discharges. In numerical terms, more than 640.000 tons of fishing gear are worldwide lost and discarded into the ocean every year (20% of marine litter). At this problematic, it should be added the dumping of single-used plastics into the sea coming from terrestrial activities (80% of marine pollution). Among this type of plastic waste, personal hygiene disposable products (e.g., non-degradable wet wipes) dragged away by wastewater currents, should be specially considered. Nylon (PA6) and polypropylene (PP) were found as the main plastic raw material of fishing nets and non-degradable wet wipes debris, respectively. Taking advantage of durability and strength properties of these plastics, the present work proposes an alternative recycling solution within the construction sector for PP and PA6 waste sourced from disposable non-woven wet wipes and discarded fishing nets, as reinforcement fibres. To this purpose, two types of waste fibres, PP (15-30 mm length) and PA6 fibres (20-25 mm length), were studied as partial replacement of gypsum matrix. After comparatively evaluating the influence of waste fibres on physico-mechanical properties (density, E modulus, flexural and compressive strength and deformability) of gypsum-based composites, notable improvements on flexural (up to ~19% when using 2.5%wt PP fibres) and compressive strength (up to ~17% when using 2%wt PA6 fibres) related to the control material, were observed. Furthermore, the pre-failure deformability was significatively enhanced after adding both types of fibres to the gypsum-mixes. Finally, the design of a new construction product i.e. false ceiling plate was proposed by using the optimal proportion mix and evaluated according to the standard requirements. 1. INTRODUCTION Every day more 200 kg of plastic waste are thrown into the ocean, so more than 12 million tons of plastics flood and damage the ocean’ ecosystems annually. Because of this discharge, plastics represent almost the 80% of the total marine litter [1]. Among the main activities that promote marine pollution, fishing works and wastewater discharges should be highlighted [2-3]. After analysing the most representative residues coming from both type of activities, fishing nets and non-degradable wet wipes were found as the most abundant discarded products. Since the primary components of these types of waste are non-biodegradable high-quality plastics, such as nylon (PA-6) and polypropylene (PP), respectively, their complete degradation could be issued for up to 600 years, turning into harmful marine residue with serious long-lasting environmental consequence [4-5]. In recent years, researchers from construction sector have become aware of the lead paper of the sector in the world economy. Hence, considering the high level of plastic consumption and waste generation derived from construction activities, there is a responsibility to offer alternative recycling solutions that diminish the damage derived from worldwide plastic pollution. Many studies have been focused on the recycling of plastic waste as partial replacement of binder matrix, such as mortar and gypsum, in the development of new eco-friendly buildings materials [6–8]. Although lighter materials, with enhanced thermal, acoustic and water resistance properties were obtained, mechanical properties generally decreased as the amount of waste content rose. To solve mechanical strength reduction, commercial reinforcing fibres have been used in numerous works [9]. 8 Composite Lightened with End-of-Life Tyres’ Recycled Materials for False Ceiling Plates,” Materials 15(16), 5660. [10] Brazão-Farinha, C.; De Brito, J.; Veiga, R., 2021. “Incorporation of high contents of textile, acrylic and glass waste fibres in cement-based mortars. Influence on mortars’ fresh, mechanical and deformability behaviour,” Construction and Building Materials 303. [11] Ming, Y.; Chen, P.; Li, L.; Gan, G.; Pan, G., 2021. “A comprehensive review on the utilization of recycled waste fibers in cement-based composites,” Materials 14 (13). [12] Araya-Letelier, G.; Maturana, P.; Carrasco, M.; Antico, F. C.; Gómez, M. S., 2019. “Mechanical-Damage Behavior of Mortars Reinforced with Recycled Polypropylene Fibers,” Sustainability 11(8), 2200. [13] Xuan, W.; Chen, X.; Yang, G.; Dai, F.; Chen, Y., 2018. “Impact behavior and microstructure of cement mortar incorporating waste carpet fibers after exposure to high temperatures,” Journal of Cleaner Production 187, 222–236. [14] Romero-Gómez, M. I.; Silva, R. V.; Costa-Pereira, M. F; Flores-Colen, I., 2022. “Thermal and mechanical performance of gypsum composites with waste cellulose acetate fibres,” Construction and Building Materials 356, 129308. [15] Ottosen, L. M.; Svensson, S. J.; Bertelsen, M. G., 2019. “Discarded nylon fishing nets as fibre reinforcement in cement mortar,” WIT Transactions on Ecology and the Environment 231, 245–256. [16] Park, J. K.; Kim, D. J.; Kim, M. O., 2021. “Mechanical behavior of waste fishing net fiber-reinforced cementitious composites subjected to direct tension,” Journal of Building Engineering, 33. [17] Spadea, S.; Farina, I.; Carrafiello, A.; Fraternali, F., 2015. “Recycled nylon fibers as cement mortar reinforcement,” Construction and Building Materials 80, 200–209. [18] Orasutthikul, S.; Unno, D.; Yokota, H., 2017. “Effectiveness of recycled nylon fiber from waste fishing net with respect to fiber reinforced mortar,” Construction and Building Materials 146, 594–602. [19] Bertelsen, I. M. G. and Ottosen, L. M., 2022. “Recycling of Waste Polyethylene Fishing Nets as Fibre Reinforcement in Gypsum-based Materials,” Fibers and Polymers volume 23, 164–174. [20] Vidales-Barriguete, A.; Piña-Ramírez, C.; Serrano-Somolinos, R.; Del Río-Merino, M; Atanes-Sánchez, E., 2021. “Behavior resulting from fire in plasterboard with plastic cable waste aggregates,” Journal of Building Engineering, 40. [21] Pedreño-Rojas, M. A.; Morales-Conde, M. J.; Pérez-Gálvez, F.; Rubio-de-Hita, P., 2020. “Reuse of CD and DVD Wastes as Reinforcement in Gypsum Plaster Plates,” Matererials 13(4), 989. [22] Alameda, L.; Calderón, V.; Junco, C.; Rodríguez, A.; Gadea, J.; Gutiérrez-González, S., 2016. “Characterization of gypsum plasterboard with polyurethane foam waste reinforced with polypropylene fibers,” Materiales de Construcción, 66, (324). [23] EN 13279-1: 2009. Gypsum Binders and Gypsum Plasters-Part 1: Definitions and Requirements. [24] European Union. Council Diretive (EU) 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. OJ L. 330, 5.12.1998, 32-54. [25] Romero-Gómez, M. I.; Pedreño-Rojas, M. A.; Pérez-Gálvez, F.; Rubio-de-Hita, P. “Characterization of gypsum composites with polypropylene fibers from non-degradable wet wipes,” Journal of Building Engineering 34, 101874. [26] EN 13279-2:2014 Gypsum Binders and Gypsum Plasters-Part 2: Test Methods. [27] Romero Gómez, Vasco Silva, R.; Flores Colen, I; Rubio de Hita, P., 2022. “Mechanical Performance of Waste Fishing Net Fibre-Reinforced Gypsum Composites,” SSRN Electronic Journal. [28] EN 14246: 2007. Gypsum elements for suspended ceilings - Definitions, requirements and test methods. [29] EN 520:2005+A1:2010. Gypsum plasterboards - Definitions, requirements and test methods. 271 272 273 SECCIÓN 2 Publicación 8 Póster en congreso internacional. Romero-Gómez, M.I., Rubio-de-Hita, P., Pedreño-Rojas, M.A., Morales-Conde, M.J., Pérez-Gál- vez, F. (2022). Sustainable solution as alternative to polypropylene reinforcement fibres used in gypsum composites. In 13th International Conference on Energy Efficiency and Sustainability in Architecture and Urbanism (EESAP 13). San Sebastián, España: Escuela Superior de Arquitectura de San Sebastián, Universidad del País Vasco. PÓSTERS PRESENTADOS EN CONGRESOS 5 274 (a) (b) (a) (b) (a) (b) Mª Isabel Romero-Gómez; Paloma Rubio-de-Hita; Manuel Alejandro Pedreño-Rojas; Mª Jesús Morales-Conde; Filomena Pérez-Gálvez España | Escuela Técnica Superior de Arquitectura, Universidad de Sevilla | 954556592 | [email protected] | 41012 En los últimos años, el acelerado desarrollo de los países emergentes ha provocado un notable aumento de los índices de contaminación ambiental. Dentro de esta problemática, la propuesta de estrategias de economía circular para la gestión eficiente de residuos plásticos se alza como principal reto. En esta línea, los residuos plásticos derivados de textiles no tejidos están provocando un alarmante daño medioambiental. Debido a su composición, pricipalemente polipropileno (PP), este tipo de residuo no es biodegradable. Por tanto, su correcta disposición tras concluir su vida útil, es crucial para promover un reciclaje de calidad y reducir el vertido de fibras plásticas al mar. Por ello, considerando el alcance mundial del sector de la construcción y comprometidos con los Objetivos de Desarrollo Sostenible (ONU), el presente estudio analiza la viabilidad de reciclar fibras residuales de PP procedentes de textiles no tejidos (i.e. toallitas húmedas desechables), como alternativa a las fibras de refuerzo de PP comerciales, para producir compuestos de yeso ecoeficientes mejorados fisicomecánicamente. MATERIALES Y MÉTODOS MATERIALES Y PREPARACIÓN DE MUESTRAS: - Yeso para la construcción (B1), de fraguado controlado (EN 13279-2) - Agua (Directiva municipal 98/83/EC) - Fibras de polipropileno comerciales (PPF) : SikaFibers M-12 (12mm x Ø31µm) - Fibras de polipropileno recicladas (PPWF): obtenidas a partir de toallitas húmedas no degradables desechadas. (15-30mm x 2-3 mm) Las muestras analizadas fueron preparadas de acuerdo con las directrices establecidas en la norma EN 13279-2. Se fijó una relación agua/yeso de 0.55 para todas las mezclas, lo que limitó el porcentage máximo de adición de fibra a 3.5% en peso de yeso, para manterer una correcta trabajabilidad de las mezclas sin usar aditivos. Por tanto, los niveles de adición de fibra usados fueron: 2, 2.5, 3 y 3.5% en peso de yeso (wt%). MÉTODOS DE ENSAYO: - Densidad en estado seco - Resistencia a flexión y compresión - Deformación Realizados según la normativa EN 13279-2, para yesos de construcción y conglomerantes a base de yeso para la construcción Figura 1. PPF (a) y PPWF (b) fibras incorporadas a las mezclas de yeso Figura 2. (a)Densidad; (b)Resistencia compresión vs. Densidad de nuevos compuestos Figura 3. Resultados comparativos ensayos de resistencia a (a)Flexión y (b)Compresión Figura 4. Curvas tensión-deformación de los nuevos compuestos: a) G/PPF; b) G/PPWF Tabla 1. Proporción de las mezclas para seis muestras de 40x40x160 mm (a) (b) RESULTADOS Y DISCUSIÓN CONCLUSIONES - Ligera disminución de la densidad de los compuestos a medida que aumenta el contenido de fibra, en comparación con el material de referencia. (~1-2% reducción). - Mejora notable de resistencia a flexión de compuestos reforzados con ambos tipos de fibra, alcanzando un máximo con 2,5wt%. (G/PPWF: ~19.5% aumento). - Mejora la resistencia a compresión (~4.5) incorporando hasta un 2,5wt% PPF. Sin embargo, el uso de PPWF conduce a una disminución de la resistencia a la compresión de hasta ~62.7%, correspondiente con nivel de adición de fibra de 3.5wt%. - Los compuestos reforzados con PPF y PPWF presentan un comportamiento más dúctil antes de la ruptura, comparado con el yeso de control, con una ruptura frágil. Se mostraron, por tanto, resultados alentadores en la sustitución de fibras de PPF por PPWF a partir de toallitas húmedas de desecho. Como resultado, se desarrollaron nuevos compuestos de yeso que contienen PPWF, más ligeros, con un comportamiento de resistencia a flexión mejorado y valores aceptables de resistencia a la compresión, según normativa. 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