Characterization and life cycle assessment of alkali treated abaca fibers: the effect of reusing sodium hydroxide

Characterization and life cycle assessment of alkali treated abaca fibers: the effect of reusing sodium hydroxide Stefany Alcivar-Bastidas a,b , Daniel M. Petroche c , Angel D. Ramirez c , M.J. Martinez-Echevarria a,* a Department of Construction Engineering and Projects of Engineering, University of Granada, Severo Ochoa s/n, Granada 18071, Spain b Facultad de Ingeniería, Universidad Cat´ olica de Santiago de Guayaquil, Av. Carlos Julio Arosemena Km 1 1/2, P.O. Box, Guayaquil 10369, Ecuador c Facultad de Ingeniería en Mec´ anica y Ciencias de la Producci´ on, Escuela Superior Polit´ ecnica del Litoral, ESPOL, Campus Gustavo Galindo, Km. 30.5 Vía Perimetral, Guayaquil 090902, Ecuador ARTICLE INFO Keywords: Abaca Natural fiber Life cycle assessment LCA Carbon footprint Alkaline treatment Circular economy ABSTRACT The increasing demand for natural fibers, driven by their advantageous attributes such as low density, sustainability, and high specific strength, has promoted the adoption of sustainable alternatives in composites. Although alkali treatments are known to improve fiber properties, they entail challenges regarding NaOH consumption and environmental impact, making it necessary to explore cleaner production strategies. This study evaluated the effects of implementing a circular economy approach through the recirculation of an NaOH solution on the treatment of abaca fibers. The fiber properties were assessed using thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and tensile strength testing, along with an evaluation of the carbon footprint through a life cycle assessment. New life cycle inventories were developed to reflect the NaOH recirculation process. Comparative analyses were conducted using polypropylene fibers. The findings indicate that the recirculation of the NaOH solution remains effective for up to eight cycles, producing consistent TGA, SEM, and tensile strength results while achieving a 25 % reduction in the carbon footprint compared to conventional treatment. Additionally, this study highlights the environmental advantages of abaca over synthetic fibers, with increased tensile strength (8–46 %) and carbon footprint reduction (55–86 %) compared to polypropylene fibers. These results highlight the potential of abaca fibers to contribute to the circular economy, enhance resource efficiency, and mitigate climate change. 1. Introduction The application of natural fibers has brought benefits to various industries, and their use has been growing [1]. Natural fibers possess key properties such as low density, sustainability, high availability in nature, cost-effectiveness, reduced dependence on non-renewable energy/material sources, and relatively high specific strength [2]. Natural fibers are essential for the production of comfortable and breathable garments in the textile industry [3]. Natural fibers are used in the automotive and aerospace industries to develop lighter and biodegradable components [4]. They are used in the fabrication of composites and reinforcement of construction materials [5–8], offering the additional benefit of potentially reducing the carbon footprint of cement composites [9]. These examples reflect the growing interest in the use of natural fibers as sustainable alternatives in various industrial applications. Natural fibers are compared to polymer fibers because both address key challenges regarding traditional cementitious matrices, considering that concrete has low tensile strength and poor energy dissipation capacity, which makes it prone to cracking [10]; among polymer fibers, polypropylene (PP) is particularly valued in the concrete industry owing to its ease of processing and cost-effectiveness [11]. Natural fiber treatments significantly contribute to improving their intrinsic properties and expanding their versatility in various industrial applications. These treatments primarily aim to improve the adhesion and compatibility between natural fibers and polymeric matrices in composite materials [12,13]. Despite these advantages, many challenges arise concerning the use of natural fibers in applications, such as poor dispersion in the matrix and, most critically, the low durability of fibers in the matrix [14]. Hence, two strategies have been developed regarding whether to modify the matrix [15] or fiber by applying different treatments [14,16]. * Corresponding author. E-mail address: [email protected] (M.J. Martinez-Echevarria). Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat https://doi.org/10.1016/j.conbuildmat.2024.138522 Received 11 June 2024; Received in revised form 24 September 2024; Accepted 25 September 2024 Construction and Building Materials 449 (2024) 138522 Available online 3 October 2024 0950-0618/© 2024 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). Among the treatments intended for composite applications, alkaline treatment is notable, in which the fibers are subjected to a sodium hydroxide (NaOH) solution to modify their surface and enhance adhesion with polymeric matrices [17,18]. Additionally, treatments with silane coupling agents aim to establish chemical bonds between the fibers and resins to optimize the load transfer and improve the mechanical properties of the composite [15,19]. Other treatments, such as thermal, enzymatic, and peroxide treatments, have also been used to adjust the characteristics and compatibility of natural fibers with matrices. These treatments enable the production of natural fiber-reinforced composites with specific properties tailored to diverse applications [20]. Notation NaOH Sodium hydroxide. TGA Thermogravimetric analysis. DTG Derivate thermogravimetric analysis. SEM Scanning electron microscope. LCA Life cycle assessment. LCI Life cycle inventory. LCIA Life cycle impact assessment. GWP Global warming potential. kg CO 2 eq. Kilograms of carbon dioxide equivalents. T0 No treatment cycle performed. T1 Single-use of NaOH solution T2-T10 Reuse of NaOH solution, from second use to tenth use. CT Conventional treatment with a single use of NaOH solution. RT Reuse of NaOH solution per 8 cycles of recirculation NAF Natural abaca fiber. TAF-CT Treated abaca fiber - conventional treatment. TAF-RT Treated abaca fiber - reuse treatment. PPF Polypropylene fiber. ANOVA Analysis of variance. n number of samples per treatment. P P-value. ISO International Standards Organization. ASTM American Society for Testing and Materials. PCR Product category rules. ESPAC Ecuadorian survey of surface and continuous agricultural production. INAMHI Ecuadorian Institute of Meteorology and Hydrology CH 4 Methane. N 2 O Nitrous oxide. IPCC Intergovernmental Panel on Climate Change. Frac LEACH Partitioning factor for the fraction of fertiliser and manure nitrogen applied to soil that is lost through leaching and runoff. CO Colombia. EC Ecuador. IN India. PE Peru. US United States. GLO Global. RoW Rest of world. Table 1 Sodium hydroxide consumption per 1 kg of natural fiber in the reviewed literature. Fiber Fiber: Solution ratio a Alkaline treatment (%) Ref. 1 2 3 4 5 6 7 8 9 10 12 15 20 50 Sodium hydroxide (kg) Abaca 1:28 - - 0.83 - - - - - - - - - - - [30] Abaca 1:33 - 0.65 - - - - - - - - - - - - [28] Abaca 1:30 0.30 - - - 1.43 - - - - - - - - - [22] Abaca 1:15 - - - - 0.71 - - - - 1.36 - 1.96 2.50 - [36] Alfa 1:15 0.15 - 0.44 - 0.71 - 0.98 - - - - - - - [37] Bagasse 1:20 0.20 - 0.58 - 0.95 - - - - - - - - - [33] Bamboo 1:20 - 0.39 - - - 1.13 - - - 1.82 - - - - [24] Bamboo 1:15 - - - - - - - - - 1.36 - 1.96 2.50 5.00 [23] Banana 1:15 0.15 - - - - - - - - - - - - - [38] Coir 1:23 - - - - 1.11 - - - - - - - - - [26] Mauritius hemp (Furcraea foetida) 1:20 - - 0.58 - - 1.13 - - 1.65 - 2.14 2.61 - - [29] Hemp 1:6 - - - - - - - - - 0.55 - - - - [25] Hemp 1:15 - - - - 0.71 - - - - - - - - - [39] Hemp 1:55 - - - - - - - - - 5.00 - - - - [40] Hemp 1:20 - - - - 0.95 - - - - - - - - - [41] Hemp, Flax 1:40 - - 1.17 - 1.90 - - 2.96 - 3.64 4.29 5.22 6.67 - [42] Jute 1:15 - - - - 0.71 - - - - - - - - - [34] Jute 1:15 - - - - 0.71 - - - - - - - - - [43, 44] Kenaf 1:20 - - - - 0.95 - - - - - - - - - [35] Palm (Phoenix sp.) 1:20 - - - - 0.95 - - - - 1.82 - 2.61 3.33 - [31, 45] Doum palm 1:20 0.20 0.39 0.58 0.77 - 1.13 - 1.48 - - - - - - [46] Royal palm 1:25 - - - - 1.19 - - - - - - - - - [47] Pineapple, ramie, sansevieria 1:100 - 1.96 - - - - - - - - - - - - [48] Sisal 1:25 - 0.49 - - - - - - - - - - - - [49] Sisal 1:20 - - - - - - - - - 1.82 - - - - [50] a By mass No reported: - S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 2 1.1. NaOH consumption in the alkali treatment of natural fibers Alkaline treatment or mercerization is one of the most widely used chemical treatments for natural fibers [21]. This treatment has been shown to be effective for fibers from coconut, sisal, jute, banana, bagasse, flax, oil palm, and abaca [17]. This process usually involves immersing the fibers in alkaline solutions, typically NaOH, at solution concentrations ranging from 1–50 % [22,23]; the treatment can be conducted at room temperature or elevated temperatures [24,25], with exposure times varying from 30 min to 72 h [26,27]. Subsequently, the mercerized fibers are washed with water to remove any residual alkaline content and then neutralized to prevent further degradation [28,29]. Finally, the fibers are dried at room temperature [30,31] or at temperatures ranging from 50 to 105◦C [18,32], for varying periods of time, typically between 6 and 72 h [33,34]. However, this procedure can lead to a significant consumption of NaOH, which not only affects production costs but also raises environmental concerns [35]. The quantity of NaOH required per kilogram of natural fiber depends on the solution concentration and fiber type. For example, low-concentration NaOH solutions (1 %) consume 0.15–0.20 kg of NaOH per kg of natural fiber. At common concentrations (3–5 %), NaOH consumption ranges between 0.44 and 1.90 kg of NaOH per kg of natural fiber, while at high concentrations of NaOH (10–50 %), up to 6.67 kg of NaOH per kg of natural fiber may be required. This consumption also depends on the type of natural fiber and fiber solution ratio, as shown in Table 1. Consequently, the main environmental challenge of alkaline treatment of natural fibers is the production of sodium hydroxide (NaOH) through electrolysis, which is an energy-intensive process [35]. This has a significant environmental impact on the treatment processes. Additionally, when fibers are removed from the alkaline bath, the generation of wastewater adds environmental concerns in terms of the treatment itself and the disposal of effluents [51]. Together, these factors create the need for more sustainable practices in the alkaline treatment of natural fibers. Several studies have focused on the reuse of NaOH solution from the natural fiber surface treatment process as part of a circular economic strategy. This approach aims to reduce both NaOH consumption and wastewater generation [50,52–54]. A 40 % NaOH solution effectively removed lignin from empty palm fruit bunch fibers, and the solution was reusable for multiple lignin extractions [52]. A recent study explored the reuse of NaOH solution in the treatment of Spanish broom fibers with 15 % NaOH. They concluded that the maceration effect reduced the lignin content in the fibers, and the same alkali solution could be used for at least five preparations [53]. Reusing NaOH solutions in different applications has proven to be effective in enhancing material properties and offering some environmental benefits. In one approach, sisal fibers treated with 10 % NaOH and incorporated into alkali-activated slag-based composites demonstrated improved flexural and compressive strengths with reduced electrical conductivity and density using the residual NaOH solution as an activator [50]. Another method involves residual bagasse fibers for use in the production of fiber-reinforced biocomposite pellets, where a NaOH recovery process allows for the recycling of up to 61 % of the NaOH used, although the reduction in carbon footprint is 1 % [54]. For these reasons, circularity measures, such as reducing chemical consumption and wastewater generation, can be implemented to mitigate the environmental impacts associated with the production of treated natural fibers. Several researchers have proposed the use of life cycle assessments (LCA) to assess the effectiveness of cleaner production methods and circular economy alternatives. [55–57]. According to the FAO [58], abaca fiber ranks as the sixth most produced natural fiber worldwide, with an annual production of 107 kt in 2022, ahead of jute, flax, hemp, kenaf, and sisal. Abaca plays a crucial role in the production of paper, monetary paper, textiles, ropes, packaging, and in the automotive industry because of its strength and durability [59,60]. It shows promising results in construction applications in the reinforcement of cement composites, where it enhances tensile strength, crack resistance, and durability [61,62]. Despite their significant industrial importance, the environmental impact of abaca fibers remains underexplored compared to that of other fibers such as jute, flax, hemp, and kenaf [63–67]. Therefore, it is essential to generate a detailed life cycle inventory and environmental profile for abaca to better understand and quantify its environmental implications across various applications. The objectives of this research are (i) to study the effect of recirculation and reuse of NaOH solution for the treatment of natural fibers and analyze its influence on the properties of abaca fibers, (ii) to generate life cycle inventories for the treatment process of natural abaca fibers with NaOH solution, (iii) to quantify the carbon footprint of the NaOH treatment process for abaca fibers and the proposed recirculation and reuse of residual NaOH solution as a circular economy scenario using a life cycle perspective, and (iv) to evaluate the carbon footprint results in the context of polypropylene fibers as a conventional synthetic alternative. 2. Materials 2.1. Abaca fibers Abaca natural fibers obtained from the Manila hemp plant (Musa textilis) are known for their strength, durability, and versatility [68]. Their water resistance and ability to withstand biological degradation make them particularly suitable for applications that require robustness and longevity [61,69]. In the construction industry, when applied to cement-based composites, they act as a reinforcement, improving the tensile and flexural strength [70,71] and potentially serving as a replacement for synthetic fibers. The abaca fibers used in this study were cultivated in Ecuador, the second-largest exporter of abaca worldwide [58]. 2.2. Abaca NaOH treated fiber Alkaline treatment of the natural abaca fibers was performed as described by Alcivar-Bastidas et al. [30]. This involved preparing a 3 % NaOH solution in distilled water and immersing the natural fibers in a fiber:solution ratio of 1:28 by mass for 4 h. Subsequently, the fibers were removed from the solution and washed with tap water until the rinse water became clear. Finally, the fibers were dried in an oven at 85 ±1 ◦C for 24 h, after which they were stored and packaged at room temperature. The materials and quantities used are listed in Table 2. Natural abaca fibers and abaca fibers treated with 3 % NaOH have a length of 30 mm, which is the optimal size for application in masonry mortar cement composites [30]. 2.3. Polypropylene fiber Polypropylene fibers are synthetic fibers manufactured by the extrusion of polypropylene. These fibers act as reinforcements to improve the mechanical properties of cement-based composites [72,73]. Table 2 Material quantities for abaca fiber alkali treatment with 3 % NaOH solution. Material Weight (g) Input  Deionized water (solution) 1000 NaOH 30 Abaca natural fiber 36.1 Tap water (wash water) 4326 Output  Abaca treated fiber 28.7 S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 3 In this study, the polypropylene fibers were made of 100 % virgin material. The fibers were made according to the ASTM C1116 standard [74], for which the specifications were as follows: 19 mm length, 0.03–0.05 mm diameter, tensile strength between 300–350 MPa, 910 kg/m 3 density, and high resistance to alkalinity [75]. 3. Methods 3.1. Reuse of NaOH solution in alkali treatment of abaca fiber on a laboratory scale As part of the circular economy initiative, a scheme for the recirculation and reuse of the NaOH solution is proposed, as shown in Fig. 1, where the solution is reused after fiber immersion. An experimental testing framework was defined, and treatment 1 (T1) was initiated, as described in Section 2.2. Subsequently, the residual NaOH solution became the initial solution for the second cycle of treatment (T2), maintaining the material proportions shown in Table 2, a fiber:solution ratio of 1:28, and a fiber:wash water ratio of 1:120 by mass. This procedure was repeated until the maximum number of treatments with the same solution was achieved. 3.1.1. Titration Method for measuring NaOH solution concentration The standard Titration Method described in the Standard Methods for the Examination of Water and Wastewater was used to determine the NaOH solution concentration in all samples, as described in Section 2320 B. Titration Method [76]. The method involves titrating the sample with standard sulfuric acid (H₂SO₄) and chemical indicators (phenolphthalein). The volume of acid used was recorded, and the alkalinity was calculated using a formula that accounts for the volume of acid, its normality, and sample size. The result was expressed in milligrams per liter as calcium carbonate (CaCO₃), providing a reliable measure of the water’s buffering capacity. Finally, a known solution of NaOH was measured and compared to the CaCO₃, and a conversion factor was used to determine the concentration of NaOH in percentage (%). 3.2. Fiber properties tests 3.2.1. Thermogravimetry/derivative thermogravimetry analysis (TGA/ DTG) Thermogravimetric analysis was conducted on untreated and treated abaca fibers. This was performed by comparing the rate of weight loss and decomposition temperature to observe changes in the fiber mass. This test indicated the thermal decomposition, moisture content, and thermal stability of the fibers. The analysis was developed with the use of SDT Q600 Simultaneous Thermal Analyzer equipment, under a ramp rate of 15◦C/minute from room temperature to 104℃ and 50℃/minute from 104℃ to 1000℃. This test was performed as described in the studies by Alcivar-Bastidas et al. [30], Shahril et al. [29], Fu et al. [77], and Kathirselvam et al. [78]. The effectiveness of thermal treatments on natural fibers, such as the removal of lignin and hemicellulose, has been observed [79–81]. 3.2.2. Scanning electron microscope (SEM) To analyze the morphology of the fibers, an SEM test was performed using a FEI Inspect® scanning electron microscope, and the recommendations from similar studies by Wei et al., [82,83] were followed. This test visually provides the surface roughness, porosity, and fiber wall structure, allowing an examination of the removal of substances such as impurities, wax, hemicellulose, lignin, and fatty acids due to alkaline treatment [29,38]. 3.2.3. Tensile test The tensile test used to determine the tensile strength of untreated abaca fibers and alkaline-treated fibers with different treatment cycles was performed according to Cai et al., [84,85] and Alcivar-Bastidas et al. [30]. This involved placing a fiber in a sample holder, which was secured with epoxy at the bottom. It was then subjected to tension at a velocity of 1 mm/min using a universal testing machine Shimadzu AGS-X with a load cell of 500 N. The tensile strength at breaking point was obtained from the testing machine, and the diameter was obtained by the SEM test described above using Motic Images Plus 3.0. Also, an analysis of variance (ANOVA) was performed, with α =0.05 and 30 samples (n=30) for each scenario. This analysis was performed to determine if there were significant differences in tensile strength between the treatments [86]. It is considered essential to statistically evaluate the behavior of the natural abaca fiber (NAF), single-use of NaOH solution (T1) samples and the solution reuse treatments, with the main purpose of determining the extent to which the use and recycling processes remained effective. Fig. 1. Flow chart of alkaline treatment of abaca fiber. (a) Conventional treatment. (b) Proposal for recirculation and reuse of NaOH solution. S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 4 3.3. Life cycle assessment Life Cycle Assessment (LCA) is a methodology used to evaluate the environmental impacts associated with a product’s life cycle, from raw material extraction through production, use, disposal, and recycling [87]. By conducting LCA, it is possible to identify areas where improvements can be made to reduce the environmental footprint of products or processes. ISO 14040 and ISO 14044 are LCA standards that provide the principles and framework for conducting LCA studies [88, 89]. This framework includes (i) defining the goal and scope and establishing objectives and boundaries; (ii) conducting a life cycle inventory (LCI) and collecting data on inputs and outputs; (iii) performing a life cycle impact assessment (LCIA) and evaluating environmental impacts; and (iv) interpretation and drawing conclusions for decision-making and improvement. 3.3.1. Goal and scope definition The natural abaca fiber and alkali-treated abaca fiber within this study are considered for use in cement-based composites, in accordance with ASTM C1116 [74], as an alternative to polypropylene fibers; therefore, they are defined as construction products. A cradle-to-gate scope is defined according to Product Category Rules (PCR) for construction products in EN 15804:2012 [90]. These include A1, raw material supply, A2, transport, and A3, manufacturing. Abaca is produced globally, with the Philippines being the leading producer, accounting for 63.56 % of abaca production, followed by Ecuador at 34.50 %, and other countries at 1.94 % [58]. In this study, the processes of abaca cultivation, abaca fiber production, and alkaline-treated abaca fiber production were associated with the geographic scope of Ecuador. The polypropylene fiber was associated with the geographical scope of the US, which is the origin of the fibers according to the manufacturer. 3.3.1.1. System boundaries. Four systems are studied: (i) Natural abaca fiber (NAF) production (Fig. 2. a): The system covers two phases: abaca and fiber production. Abaca production involves seedling production, growth until the seedlings are transplanted into the crop, and forming groups of stems that grow from the base (Fig. 3. a). The abaca plant matures within a period of 18–24 months and can be harvested thrice a year [91]. Harvesting involves cutting the stems approximately 10 cm from the ground with an outward beveled cut to prevent the plant from rotting [60]. After cutting, the leaves are removed and the harvested pseudostems are grouped. Tuxiying then begins, which involves cutting the pseudostem until it reaches the central zone (tuxy) containing the biomass to be transformed into a fiber [92]. (ii) In the second phase of fiber production, the tuxies are stripped using a mechanism consisting of a diesel engine, pulleys, and blades (Fig. 3.b). Finally, the fibers are dried in the sun for 1–3 days (Fig. 3.c). (iii) Treated abaca fiber – conventional treatment (TAF-CT) production (Fig. 2.b): This system covers natural abaca fiber production plus conventional 3 % alkali treatment as described in Section 2.2. (iv) Treated abaca fiber – reuse treatment (TAF-RT) production (Fig. 2.b): This system is a variant of TAF-CT. This includes NAF production plus 3 % alkaline treatment with recirculation of the NaOH solution, as proposed in Section 3.1. (v) Polypropylene fiber (PPF) production (Fig. 2.c): This process begins with the input of polypropylene granulates, which are produced through the polymerization of propylene monomers [93]. This product serves as the primary raw material for the fiber manufacturing process. Electricity is crucial for powering extrusion, spinning, and other machinery involved in converting polypropylene into fibers [94]. The functional unit was 1 kg of fiber, with four different scenarios depending on the system boundaries: 1 kg of natural abaca fiber, 1 kg of abaca fiber treated with a 3 % NaOH solution, 1 kg of abaca fiber treated with a reused 3 % NaOH solution, and 1 kg of polypropylene fiber (Table A.1 in Appendix A). 3.3.2. Life Cycle Inventory 3.3.2.1. Natural abaca fiber production. The Ecoinvent LCI calculation tool for crop production [95] was used to model the life cycle inventory (LCI) of abaca production. It employs various models to calculate emissions from field activities, land transformation and occupation, irrigation, and carbon uptake by plants [96]. This tool requires agricultural and meteorological inputs from the plantation sites. The agricultural input was obtained from the Ecuadorian Survey of Surface and Continuous Agricultural Production (ESPAC 2022) [97]. This survey used the multiple frame sampling methodology, and for abaca production, a sample of farms that represented 24.35 % of the total abaca fiber produced was considered [98]. An expansion factor was then applied to obtain the total results for all items surveyed for all abaca fibers produced in Ecuador [99]. The information surveyed included planted and harvested areas, harvested tons, use of irrigation, and the quantity and type of fertilizers and pesticides [97]. According to the ESPAC survey, there are seven types of abaca plantations that vary according to the use of fertilizers and pesticides, depending on the organic or inorganic origin of these substances. The predominant plantation does not involve the use of fertilizers or pesticides, representing 54.93 % of the Ecuadorian abaca harvest [98]. Owing to its representativeness, the current study was limited to modeling this type of plantation. Meteorological data were obtained from the Ecuadorian National Institute of Meteorology and Hydrology [100]. The selected meteorological station is located in the province of Santo Domingo, which accounts for 90 % of Ecuadorian abaca production [98]. To generate inventories of biomass and organic waste, the average biomass distribution in the abaca plant (Fig. 4. a) was obtained from Cortez et al. [101]: leaves (16.36 %), pseudo-stem waste (63.79 %), and tuxy (19.85 %). Seven farms in Santo Domingo-Ecuador were visited, from which it was evident that organic residues such as leaves, pseudo-stem waste, and stripping waste were left in the plantation. According to farmers, this is a common practice in Ecuador. The biomass serves as a nutrient source for new plants [102,103]. Owing to the absence of anaerobic conditions, CH 4 emissions were not generated [104]. In this study, the waste biomass was considered to remain within the system; this approach has been used in studies of other plantations under similar conditions [101,103,104]. To model the inventories of energy in biomass and carbon dioxide uptake by biomass for abaca production, the Ecoinvent LCI calculation tool for crop production requires a reference crop. In this study, banana (Musa spp.) was selected as the base crop because of its close botanical relationship with abaca (Musa textilis), as both belong to the Musaceae family [107,108]. Additionally, they are cultivated in the same region in Ecuador [105] and exhibit similar biomass distribution patterns (Fig. 4); in the case of banana, the main product is the fruit, while for abaca is the tuxy. As the biomass distribution is a critical input for the Ecoinvent LCI calculation tool [96], this similarity in biomass distribution makes banana an appropriate reference crop. Dinitrogen monoxide (N 2 O) from the crop residues for abaca production was calculated using the IPCC Guidelines [109], yield from ESPAC 2022 [97], biomass distribution from Cortez et al. [101], and moisture and nitrogen content from Armecin et al. [110,111]. Indirect N₂O emissions from leaching and runoff were estimated using IPCC Guidelines [109]. The leaching-runoff factor was calculated according S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 5 Fiber Production Abaca Production Resources Sodium hidroxide Alkaline Treatment Water desioned (a) (b) Life cycle inventory taken from Ecoinvent 3.9.1 Life cycle inventory taken from literature and primary data Emisions to soil Emisions to air Emisions to water Abaca plant tuxy, havested Seed Natural Abaca Fiber (NAF) Transport Tuxying Stripping Drying Mixing Inmersion Washing Planting Cultivation Harvesting Tap water Electricity Treated Abaca Fiber (TAF) Waste water Diesel Lubricant oil Biomass Drying Recycled NaOH solution* Building, factory Water, emisions to air Land Support activities Polypropylene fiber production Electricity (c) Polypropylene, granulate Waste plastic Polypropylene Fiber (PPF) Natural flow Emisions to air Fig. 2. Cradle-to-gate system boundary for the production of (a) abaca fiber, (b) abaca fiber treated and (c) polypropylene fiber. * Included in reuse case study. S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 6 to Franke et al. [112] and replaced Frac LEACH in the IPCC calculations. This approach was used in a previous carbon footprint study of bananas in Ecuador [104]. Finally, the inventory of fuel and lubricant consumption for the fiber stripping and drying process was collected from the seven farms visited. The fuel was transformed into energy units using the factors for Ecuadorian diesel (density: 850 kg/m 3 ; heat capacity: 40.8 MJ/kg) [113]. 3.3.2.2. Treated abaca fiber production. The inventory of raw materials for the abaca alkaline treatment fiber process using a 3 % NaOH solution and the proposal for recirculation and reuse of the NaOH solution were taken from the results provided in this study, in Section 2.2 and Section 4.1, respectively. Facilities and energy inventories for the alkaline treatment fiber process were taken from the “mercerizing process, textile – IN” detailed in Ecoinvent 3.9.1 [114] and Faist Emmenegger et al. [67]. This process was adapted to Ecuadorian electricity, and the LCA process of Ramirez et al., [115,116] was used. The NaOH supplier closest to Guayaquil is located in Callao, Peru [117]. Sodium hydroxide and sodium chloride from Ecoinvent 3.9.1 were adapted for Peruvian electricity. 3.3.2.3. Polypropylene fiber production. The life cycle inventory of raw materials for polypropylene fiber production was obtained from studies by Yin et al. [93] and Van den Heede et al. [94]. Processes for polypropylene granulate production, electricity, and waste treatment were obtained from Ecoinvent 3.9.1 [114]. The main parameters and assumptions of this study are summarized in Table A.1 in Appendix A. 3.3.3. Life Cycle Impact Assessment The methodology used for LCIA was the ReCiPe Midpoint (H) version 1.13 [118]. The impact category of global warming potential (GWP100), also known as the carbon footprint, was selected because it is considered the most critical indicator of climate change [119]. OpenLCA 2.1.1 [120] was used for LCA calculations. 4. Results and discussion 4.1. Reuse of NaOH solution in alkali treatment of abaca fiber on a laboratory scale Table 3 shows the experimental results of reusing NaOH solution in the alkaline treatment of natural abaca fibers. Following the methodology described in Section 3.1, by maintaining the same proportions of fiber:solution and fiber:wash water materials, each washing cycle preserved a similar behavior as the conventional treatment of a single-use NaOH solution (T1). The 10 reuse cycles resulted in a loss of sodium hydroxide solution owing to the treatment process (fiber washing and evaporation), ranging from 26.61 % to 30.16 %. As a consequence of alkaline treatment, Fig. 3. Abaca fiber production stages in Santo Domingo-Ecuador: (a) group of abaca trees, (b) stripping process, and (c) drying process. Fig. 4. Average distribution of biomass of (a) abaca plant (Musa textilis) [92,101] and (b) banana plant (Musa spp.) [105,106]. S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 7 natural fibers experience mass reduction owing to the removal of impurities, and wax, hemicellulose, and lignin are removed [121–123]. This behavior was observed in T1, with a mass reduction of 20.5 %; from T2 to T9, similar values between 18.9 % and 20.4 % were obtained. T10 exhibited the lowest mass loss of 15.6 %. Washing cycles were stopped at T10 because the solution was lost. Initially, there was a certain amount of solution, considering the 1:28 fiber:solution ratio; however, as the washing cycles continued, the solution was lost, which resulted in inadequate fiber treatment solution for washing cycle T11. After 10 cycles of NaOH reuse, there was a 70.11 % reduction in NaOH consumption per kilogram of treated abaca fibers. The titration test was performed on all samples from T0 to T10, as shown in Table 3, where the initial solution concentration in T0 was 2.98 % and the NaOH concentration in T10 was 0.31 %. The initial value of 2.98 % can be attributed to the manual procedure of dissolving NaOH in the solution. After applying the first procedure of using and recycling the water solution, the NaOH concentration in T1 was measured to be 2.96 %, which closely resembled the initial concentration of T0. Between T2 and T4, the NaOH concentration showed a slight decrease, ranging from 2.95 % to 2.91 %, indicating that the recycling process effectively maintained the concentration at levels similar to those at T0. From T5 to T7, the average NaOH concentration stabilized at 2.88 %, which remained within an acceptable range, suggesting that the washing and recycling procedures were still effective. It is important to mention that at T8, the NaOH concentration decreased to 2.47 %, representing 17 % of the initial concentration, which could be due to the impurities retained throughout the washing and recycling processes. Between T8, T9, and T10, sharp changes occurred: the value for T9 decreased to 0.84 %, and T10 decreased to 0.31 %. The NaOH concentration further decreased, indicating a substantial loss of NaOH, which could affect the chemical processes required to modify the properties of the natural fibers. This noticeable reduction in NaOH concentration between T9 and T10 was likely due to the cumulative effects of washing and recycling. This reduction may result in insufficient removal of impurities such as hemicellulose, cellulose, and pectin, which are crucial for the proper adhesion of the fiber to the matrix. The significant decrease in the concentration between T8 and T9, by approximately 1.63 % (55 % of Table 3 Experimental inventory of materials in 10 cycles of NaOH solution reuse for alkaline treatment. Cycle Initial NaOH solution (g) Abaca fiber (g) Washing water (g) Wet fiber (g) Dry treated fiber (g) Final NaOH solution (g) NaOH in final solution (%) T0 - - - - - - 2.98 T1 1030 36.1 4326.0 333.59 28.70 719.34 2.96 T2 719.34 25.2 3021.2 243.92 20.04 512.59 2.95 T3 512.59 17.9 2152.9 162.89 13.83 370.29 2.93 T4 370.29 13.0 1555.2 106.74 10.05 271.77 2.91 T5 271.77 9.5 1141.4 85.05 7.37 196.38 2.89 T6 196.38 6.9 824.8 59.66 5.52 142.56 2.88 T7 142.56 5.0 598.7 44.80 3.99 103.38 2.86 T8 103.38 3.6 434.2 31.64 2.92 74.78 2.47 T9 74.78 2.6 314.1 23.85 2.07 52.57 0.84 T10 52.57 1.8 220.8 16.79 1.52 36.91 0.31 Fig. 5. Thermogravimetric analysis (a) and derivative thermogravimetric analysis (b–c) of natural abaca fiber (NAF) and treated abaca fiber (T) in 10 cycles of NaOH solution reuse (1–10). S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 8 the total alkali concentration), may represent a critical threshold at which the effectiveness of the treatment diminishes, potentially compromising the overall outcome. 4.2. Influence of the reuse of NaOH solution on the abaca fiber properties 4.2.1. Thermogravimetry/Derivative Thermogravimetry Analysis (TGA/ DTG) Thermogravimetric analysis was performed to investigate the effect of different cycles of washing and reusing NaOH solution on the composition of the fiber in terms of thermal stability. Fig. 5.a shows the curves of all ten samples subjected to different washing and reuse cycles, including the natural fiber without treatment, described as natural abaca fiber (NAF) in Fig. 5. b. The DTG curve of the NAF fiber shows an initial peak at 60℃ (mass loss around 4 %), which corresponds to the vaporization of absorbed water [124]. After this peak, there are two more peaks at 280℃ and 400℃; according to Shahril et al. [29], between 200℃ and 300℃, the expulsion of hemicellulose lignin and an insignificant amount of cellulose from the fiber occurs. Between 300℃ and 410℃, eliminations of cellulosic components led to mass loss and implied the omission of lignin and wax [78]. Fig. 5.c shows the DTG curves of samples T1–T10, where all samples show similarities in their behavior and two main peaks are visible between 40℃ and 340℃. Sample T1 experiences a mass loss of 3 % at 38℃ owing to vaporization of absorbed water and approximately 60 % mass loss at 335℃ owing to elimination of hemicellulose and cellulosic components, with lignin being eliminated from the samples [125]. During the process of using the recycle solution, the same water solution of 3 % NaOH was used to determine the process efficiency by the non-cellulosic material Fig. 6. Scanning electron microscope (SEM) results of natural abaca fiber (NAF) and treated abaca fiber (T) in 10 cycles of NaOH solution reuse (1–10). S. Alcivar-Bastidas et al. Construction and Building Materials 449 (2024) 138522 9 Table S3 . Life cycle inventories of abaca alkali treated fiber production for 3% NaOH solution Flow Process/Category Amount Unit Reference Input     Building, hall Market for building, hall | APOS, U - GLO 1.56E-05 m 2 [67,114] Electricity, low voltage Electricity, at supply, 2018 mix - EC 1.55 kWh [67,114-116] Fiber, abaca This Study 1.26 kg This study Sodium hydroxide, without water, in 50% solution state Chlor-alkali electrolysis, membrane cell | APOS, U - PE a 1.05 kg This study Tap water Market for tap water | APOS, U - CO 150.73 kg This study Water, deionized b Market for water, deionised | APOS, U - RoW 33.8 kg This study Transport, sea c Market for transport, freight, sea, bulk carrier for dry goods | APOS, U - GLO 1.34 tkm This study Output     Fiber, abaca treated This Study 1 kg This study Wastewater from textile production Direct disposal of wastewater from textile production | APOS, U - GLO 0.165 kg This study Water Emission to air/low population density 1.06E-02 m 3 This study a Sodium hydroxide and sodium chloride production are adapted with Peru’s electricity. b 50% sodium hydroxide solution state water is considered and removed from this flow. c Sodium hydroxide transport. CT: Conventional alkali treatment (1 cycle), RT: Reuse of NaOH solution per 8 cycles of recirculation EC: Ecuador, PE: Peru, CO: Colombia GLO: Global, RoW: Rest-of-World Table S4 . Life cycle inventories of abaca alkali treated fiber production for 3% NaOH solution and reuse of solution (9 cycles). Flow Process/Category Amount Unit Reference Input     Building, hall Market for building, hall | APOS, U - GLO 1.56E-05 m 2 [67,114] Electricity, low voltage Electricity, at supply, 2018 mix - EC 1.55 kWh [67,114-116] Fiber, abaca This Study 1.26 kg This study Sodium hydroxide, without water, in 50% solution state Chlor-alkali electrolysis, membrane cell | APOS, U - PE a 0.325 kg This study Tap water Market for tap water | APOS, U - CO 152.07 kg This study Water, deionized b Market for water, deionised | APOS, U - RoW 10.50 kg This study Transport, sea c Market for transport, freight, sea, bulk carrier for dry goods | APOS, U - GLO 0.414 tkm This study Output     Fiber, abaca treated This Study 1 kg This study Wastewater from textile production Direct disposal of wastewater from textile production | APOS, U - GLO 0.142 kg This study Water Emission to air/low population density 1.06E-02 m 3 This study a Sodium hydroxide and sodium chloride production are adapted with Peru’s electricity. b 50% sodium hydroxide solution state water is considered and removed from this flow. c Sodium hydroxide transport. CT: Conventional NaOH treatment (1 cycle), RT: Reuse of NaOH solution per 8 cycles of recirculation EC: Ecuador, PE: Peru, CO: Colombia GLO: Global, RoW: Rest-of-World Table S5 . Life cycle inventories of polypropylene fiber production Flow Process/Category Amount Unit Reference Input     Electricity, medium voltage Market group for electricity, medium voltage | APOS, U - US 1.517 kWh [93,94] Polypropylene, granulate Market for polypropylene, granulate | APOS, U - GLO 1.05 kg [93,94] Output     Polypropylene fiber This Study 1 kg This Study Waste plastic, mixture Market group for waste plastic, mixture | APOS, U - RER 0.05 kg [93,94] GLO: Global, US: United States RER: Europe References [1] S.O. Ismail, E. Akpan, H.N. Dhakal, Review on natural plant fibres and their hybrid composites for structural applications: recent trends and future perspectives, Compos. Part C. Open Access 9 (2022) 100322, https://doi.org/ 10.1016/j.jcomc.2022.100322. [2] K.L. Pickering, M.G.A. 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