Potential use of sugar cane bagasse ash as sand replacement for durable concrete

Journal of Building Engineering 39 (2021) 102277 Available online 19 February 2021 2352-7102/© 2021 Elsevier Ltd. All rights reserved. Potential use of sugar cane bagasse ash as sand replacement for durable concrete Veronica Torres de Sande a , * , Monower Sadique a , Paloma Pineda b , Ana Bras a , William Atherton c , Mike Riley a a Built Environment and Sustainable Technologies (BEST) Research Institute, Department of Built Environment, Liverpool John Moores University, Byrom Street, Liverpool, L3 3AF, UK b Department of Building Structures and Geotechnical Engineering. School of Architecture. Universidad de Sevilla, Avda. Reina Mercedes, 2, 41012, Seville, Spain c Department of Civil Engineering, Liverpool John Moores University, Peter Jost Centre, Byrom Street, Liverpool L3 3AF, UK ARTICLE INFO Keywords: Building waste materials Sand substitution Bio-ashes Industrial ashes Sugar cane bagasse Circular economy Agricultural waste Bio-concrete Eco-efficiency Sustainability ABSTRACT The increasing urban development, led by concrete, requires a higher availability of materials and energy, and it will be responsible for a high waste generation. To face the exploitation of natural resources, the use of fossil fuels and the reduction of waste disposal, new environmental-friendly strategies emerge accomplishing the circular economy principles. In this research, the use of poor reactive agro-industrial ashes as sand replacement in cement-based materials is investigated. Poor reactive sugar cane bagasse ashes (fly and bottom ash -SCB FA and SCB BA, respectively) from a power plant in Dominican Republic have been used in substitution rates of 10%, 20% and 30% of weight of sand. Physico-chemical characteristics of ashes are investigated and correlated to the performance of the bio-concretes. SCB FA showed being an enhancer of durability-related properties of the concrete even with high content of silica in form of quartz, due to the capability of modifying the microstructure of the concrete and an additional binding capacity of chlorides ions. Durability-related tests (open porosity test, electrical resistivity test, capillary absorption test and chloride migration test) have been conducted at 28, 60, 90 and 240days. Direct correlations exist when compared chloride migration resistance against porosity and electrical resistivity in concretes with SCB FA, not so for capillary absorption. This demonstrates the inadequacy of establishing conclusions about durability performance of bio-concretes based on durability tests when run independently. The use of agro-industrial ashes as substitutes of natural aggregates not only reduces the consumption of natural sand but can deliver bio-concretes with potential benefits in terms of compressive strength and durability. 1. Introduction World population is predicted to overpass 9 billion people by 2050, the demand in energy will increase by 50–60%, in food by 60% and around 70% of world population will live in cities [1,2]. This will result in a high generation of waste and high economic, social and environmental impact. The built environment, which is the main consumer of metallic and non-metallic materials and generate 40% of total carbon emissions in the world, must drive a green role in the years ahead. By 2060, world’s global extracted materials use will double the actual demand overpassing the 167 Gt, Fig. 1. Non-metallic minerals, which constitute the main share, is expected to increase 79% and biomass 68% by 2060. Half of share (44 Gt) of non-metallic minerals -mainly gravel, sand, clay, limestone and gypsum-were used for construction purposes in 2017. As a direct consequence of the urban population and living standards growth, the consumption of non-metallic minerals for construction will double by 2060 reaching 86 Gt [3]. Construction aggregates (sand, gravel and crushed stone) entail the 70% of share of non-metallic ores and 28.8 Gt are employed in concrete production. By 2060, the global demand of construction aggregates will be driven by BRIICS countries -Brazil, Russia, India, Indonesia, China and South Africa- (39 Gt), [3]. Sand and gravel extraction are responsible of severe habitat deterioration in rivers and sea-beds. Further consequences on human lives have been listed such as premature fail of infrastructures, serious disturbances in cultivable land and drinking water, diseases, and human * Corresponding author E-mail address: [email protected] (V. Torres de Sande). Contents lists available at ScienceDirect Journal of Building Engineering journal homepage: http://www.elsevier.com/locate/jobe https://doi.org/10.1016/j.jobe.2021.102277 Received 9 September 2020; Received in revised form 30 December 2020; Accepted 9 February 2021 Journal of Building Engineering 39 (2021) 102277 2 rights abuses due to illegal markets [4–6]. Concrete is expected to promote most of the urban development covering buildings and infrastructures. Since concrete is the second most used material, after water, and it requires a significant use of extracted materials, concrete production is one of the human activities with a higher environmental impact. Production of cement alone entails the 7% of the world’s CO 2 emissions [7]. In this context, finding alternate materials and durable solutions is a priority to face the increasing demand of construction aggregates, minimising the exploitation of natural sand and gravel and the linked consequences. On the other hand, industry is concerned about the necessity of following more sustainable strategies and practices to reduce the environmental impact of waste [8–10]. Focusing on wastes originated by the increasing agricultural industry, one of the developed strategies is the energy production from agriculture biomass, avoiding landfill diversion and covering the higher demand of energy while minimising the use of fossil energies [2,11]. During the biomass burning process, bio-ashes are generated as the resulting waste of this process retained on the bottom of the boiler and on the filters of the combustion chambers. These are known as bottom ashes and fly ashes, respectively. Vassilev et al. [12] estimated the global biomass-ash generation in 476 million tonnes per year considering the total burned biomass to be 7 billion tonnes and 6, 8% mean ash yield on dry basis. According to the authors, this quantity would be comparable to that of coal ash in 2012 (780 million tonnes), giving an idea of the environmental implications. This, combined with the forecasted pressures on the coal ash market and the pozzolanic capacity of some bio-ashes, make biomass ashes a potential materials source in novel low-carbon concrete formulas [13]. Agricultural bio-ashes management is not only an actual disposal and environmental concern, but an increasing problem for the future [14]. When two of the most pollutant human activities converge, construction and agriculture, green strategies emerge to face with this scenario. Agricultural waste become a new source of construction waste materials, meeting the circular economy principles and ensuring better environmental practices. In this research, the use of agro-industrial ashes resulting from the combustion of the bagasse of the most cultivated crop in the world, sugar cane, is investigated. Sugar cane annual production has being increasing on time until reaching 1,907,024,730 tonnes in 2018, Fig. 2, [15]. The entire production is distributed along tropical and subtropical areas in the world, as shown in Fig. 3, where the data are compared with the Gross Net Incoming per capita [15,16]. The 66% of total sugar cane production is held by BRIICS countries led by Brazil (41%) and followed by India (17%) and China (6%). In small countries, such as Dominican Republic, sugar cane is the most cultivated crop in terms of production, and the second one in terms of harvested area (only overpassed by rice which has a yield 14 times lower than that of sugar cane), Fig. 4. Sugar cane is the main source for obtaining sugar and it is also used in alcohol factory and bioethanol production [17]. Its residues, bagasse, have further uses as bio-product with interesting and potential applications in the industry. Bagasse is the term for the residues of a plant after the extraction of the juice. These wastes are normally revalorized and used as animal food, fertilizers, cellulosic industry, and energy recovery or conversion to fuel. The sugar cane bagasse accounts up to the 30–34% by mass of collected sugar cane [18]. According to this, almost 600 million of tonnes of sugar cane waste are obtained. The aforementioned results in a high environmental impact if it would be disposed in landfill, due to the methane gas released during the decomposition. Besides, SCB biomass is an interesting biofuel which does not compete with food crop production [17]. Sugar cane bagasse ash (hereafter SCBA) is a secondary by-product obtained from the combustion process of the biomass for energy from 0 2 4 6 8 10 12 0 50 100 150 200 2017 2020 2030 2060 Billion people Gigatonnes Biomass Fossil fuels Metals Non-metallic minerals Extrac�on total Popula�on Fig. 1. Projections of global materials extraction and population. Data source[3]. 0 250 500 750 1,000 1,250 1,500 1,750 2,000 Produc�on (Mill.Tonnes) Fig. 2. World’s most cultivated crops, 2018. (Data source: [14]). 0 20,000 40,000 60,000 80,000 0 100 200 300 400 500 600 700 800 Current US$ Produc�on (Mill.Tonnes) Produc�on for 2018 (MT) GNI per capita for 2017 (Atlas method current$) Low income Lower middle income Middle income Upper middle income High income Fig. 3. Sugar cane main producer countries in the world in 2018 (Data source: [15,16]). 0 50,000 100,000 150,000 200,000 250,000 0 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000 6,000,000 Area (ha) Produc�on (Tonnes) Produc�on (Tn) Area harvested (ha) Fig. 4. Crops production (tonnes) and harvested area in Dominican Republic, 2018 (Data source: [15]). V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 3 waste plants. The bagasse resulting from the principal agricultural process are reused as fuel for heat generation, leaving 8–10% of ashes by mass. Considering the aforementioned data, up to 48–60 million of tonnes of sugar cane bagasse ashes can be obtained per year if all the residues were burnt. Traditionally, ashes have been treated as waste to be disposed in landfill or accumulated onsite. One of the actual disposing alternatives is the use of ashes as fertilizer for soils due to their contain of mineral fertilizers [19]. Sales and Lima (2010) [20], called into question the nutrient capacity of the SCBA and the real environmental impact due to the capacity of the ashes to retain the agrochemicals in the soil. Zhao et al. (2013) [21] claimed that the use of this type of ashes may be limited by a high presence of harmful poly-aromatic hydrocarbon in ashes with a high content of residual carbon. In response to the environmental awareness, the efforts are focussed on the possible incorporation of bio ashes in the cement-based matrix. In general, during the biomass combustion process two types of residues are generated: bottom ashes (BA) and fly ashes (FA). Bottom ash is made off partial or totally burnt material deposited in the bottom of the boiler and it is commonly mixed with mineral impurities existing in the biomass such as sand, stones and others. Fly ash is composed of particles carried along by the combustion gases out of the combustion chamber that precipitated in filters. The properties of the ashes depend on the material used as a fuel and on the combustion process. Factors such as temperature, time of combustion and intensity will determine the physical and chemical characteristics and some of these can be observed by the appearance of the resulting waste. The main issue in the use of industrial ashes from energy recovery plants in concrete production is to ensure the quality of the by-product, which can have undesirable physical or chemical characteristics and they are, by definition, heterogeneous materials. The addition of bio ashes in cement-based materials may enhance the mechanical strength and durability properties due to the pozzolanic activity and/or the filler effect [22–24]. The pozzolanicity depends on the oxide content, the mineralogical structure (pozzolans have amorphous structure) and the fineness. SCBA can have pozzolanic characteristics since they are rich in silica and alumina oxides mainly that react in the presence of calcium hydroxide when they are mixed in water forming additional retarded cementing products. Nevertheless, pozzolans must be amorphous. Sales and Lima (2010) [20] reported crystalline ashes containing high content of silica (88.2–96.2%). The crystallisation of silica occurs at high temperatures (>800 ◦C) or prolonged burning times [19]. Some authors have studied the effect of the temperature under controlled calcination process in the ultimate properties of the sugar cane bagasse ash [19,22,25]. Cordeiro et al. (2009) [19] observed high pozzolanic activity in the resulting ashes when the SCB biomass is burnt under controlled conditions in the lab. The highest pozzolanicity activity index (PAI) occurred for ashes obtained under two combustion cycles of 3 h: the first one at 350 ◦C and the second one at 600 ◦C after 3 h. Rajasekar et al. (2018) [22] used ashes obtained under controlled conditions (240 min at 550 ◦C) and treated by grinding (120 min). Deshmukh et al. (2011) [25] concluded that, after a burning process of 12 h at 500 ◦C, high reactive silica can be obtained in RHA (rice husk ash). Andrade Neto et al. (2021) [26] assessed the performance of SCBA obtained under controlled laboratory conditions concluding that ashes obtained by burning the bagasse up to 600 ◦C during 8 h had high pozzolanic activity due to the presence of amorphous silica and alumina. In industry, there is almost no control in the operational temperature and burning and cooling processes. In addition, temperature varies depending on the humidity content of the biomass overpassing the crystallisation temperature. Thus agro-industrial ashes are prone to have crystallinity structure and unburnt organic matter as a result of uncontrolled burning processes influencing the resulting by-product [20,27]. Ashes as-received from industrial plants contain higher amounts of carbon and crystalline particles, and both are detrimental characteristics if ashes have to be used as pozzolans [28]. As-received industrial ashes have to be subjected to high energy demanding treatments such us ultra-grinding or re-burning in order to boost the pozzolanic activity of sugar cane bagasse fly ash and minimise the LOI for being used as binders with positive results [17,19,22,29–33]. The optimal substitution rate for ultra-treated ashes in concrete, high performance concrete (HPC), ultra-high performance concrete (UHPC) and recycled aggregate concrete has been declared to be 15–20% (weight of binder) [14,22,33, 34], meanwhile for not ultra-treated ashes, the optimum replacement by cement is between 5% and 10% in mortars and concretes due to lower pozzolanic reaction or higher carbon content [24,32,35]. Arenas-Piedrahita et al. (2016) observed this behaviour at early ages when the substitution rate was above 10% but highlighted an enhancement after 56 days – nevertheless the optimal rate substitution was 10% (weight of binder). Sieving has been pointed as the most suitable treatment due to the lower energy consumption and lesser time requirement and the enhanced properties observed due to the char particles -porous carbonaceous particles-removal. Murugesan et al. (2020) found 20% as the optimal replacement of cement for sieved ashes [14]. At early ages (28 days) compressive strength enhancements in comparison with the control concrete have been observed when ground SCB FA was used as cement replacement in 10%, 20% and 30% substitution rates due to early pozzolanic reaction of fine particles [36]. The required combustion temperature in industry leads to poor reactive SCBA with high content of quartz (mean compound of natural sand) [20]. On the other hand, the energy needed to ultra-treat the ashes in order to boost the pozzolanicity, when possible, increases the embodied energy of bio-ashes. These limitations open a research field in order to find alternate uses further to that of cement substitute material, already widely investigated. The benefits of using ashes as sand replacement can be: i) addressing the exploitation of natural resources, ii) finding a solution for those ashes that cannot be used as cement-replacement due to physico-chemical requirements, iii) reduction of the disposal cost for bio-energy companies, and iv) provision of enhanced properties to the cement-based solutions when used as an improved material in comparison to sand. The use of inert or poor reactive SCBA as sand alternate materials has not been widely explored yet. Sales and Lima (2010) [20] assessed the use of agro-industrial SCBA extracted from the boilers or from conveyor belts with high silica content and high crystallinity as sand substitutes in mortars and concretes. A higher compressive strength than the control specimens was observed at 28 days when 20% and 30% of sand was replaced in mortars. The same authors substituted 30% and 50% of sand in concrete mixes with different type of cement. The best performance was in mixes with slag-modified Portland cement (CEM II) with an increase in compressive strength at 28 days of 17.65% and 20.67% when 30% and 50% of sand is replaced by ashes respectively. Modani and Vyawahare (2013) [37] substituted up to 40% of fine aggregates (by volume) by untreated ashes in concretes observing reductions in compressive strength and tensile strength along with the increase of ashes. Macedo et al. (2014) [38] observed the filler effect and concluded that 10% of substitution (weight of sand) showed the highest compressive strength but 3% was the optimal substitution rate when tensile strength is under consideration. Moretti J. P et al (2016) [39] studied the interaction between SCBA used as sand replacement and waste concrete aggregates in concrete. Few researchers have addressed the impact of the implementation of ashes as cement replacements in the durability properties of concrete and the transport mechanisms. The results showed that the use of treated SCBA is positive in terms of durability since it reduces the permeability and the chloride penetration and promotes the corrosion resistance [22, 26,33,34,36,40–42]. The filler effect of fine particles and the generation of secondary hydrated products have been pointed as the main causes for these due to a modification in the transport stream. Ganessan et al. (2007) [40] reported a twofold and threefold enhancement of the chloride ion penetration resistance in concretes with treated SCBA (up to V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 4 20% cement replacement) at 28 and 90 days and a reduction of the corrosion rate. Chusilp et al. (2009) observed a reduction of 50% and 69% of permeability at 28 and 90 days in concretes with 30% of ground SCBA used as cement replacement due to filler effect of pozzolanic reaction. Similar observations were done by Murugesan et al. (2020) [14] and J.S Andrade et al. (2021) [26], who stated that higher sorptivity index decreases in mixes with higher content of SCBA might be attributed to the refinement of the pore structure due to the pozzolanic reaction, concluding that sieved SCBA demonstrates having positive effects in the durability performance of concrete. A. Bahurudeen et al. (2015) [36] observed that the use of SCBA based blended cement improves the chloride and gas resistance of concretes with increase in ashes replacement and showed higher surface resistivity. V.A Franco-Luj´ an et al. (2019) [42] stated that sieved SCBA increased the chloride diffusion resistance of ternary concretes using FA owed to the pozzolanic reaction promoted by ashes which ultimately reduces the porosity and increases the tortuosity. Although increasing SCBA content reduces the time of chloride ions to diffuse retarding the initiation period and increasing the estimated service life of the structure, it has been declared that it also promotes carbonation due to the consumption of portlandite [26]. Rerkpiboon et al. (2015) [43] stated that ground SCBA can be used up to 50% to replace OPC and concluded that the chloride resistance improvement is more influential than the compressive strength gain. This is due to the higher impact of pozzolanic reaction on diffusivity than on compressive strength [42]. Furthermore, to the knowledge of the authors, no research has been published where the durability properties related with the chloride ions penetration are investigated when SCBA are used as sand replacements in concrete. In this research, the feasibility of using two types of high crystalline agro-industrial SCBA as sand alternate material for concrete is investigated. SCBA were characterised and undesirable particles removed. Mixes with 10%, 20%, 30% of sand substitution rate were casted for each type of ash. In addition, concrete without ashes was produced to comparison. Compacity, fresh and hardened concrete densities, compressive strength, capillary water absorption, open porosity index, surface electrical resistivity and chloride migration resistance tests were conducted. The possible correlations between different durability properties at 28, 60, 90 and 240 days (capillary water absorption, porosity, surface electrical resistivity and chloride diffusion resistance) in bio-concretes were stablished. 2. Materials and methods 2.1. Materials The concrete is made of cement, natural sand, aggregates, two types of bio-ashes and water. Ordinary Portland Cement, (OPC) type CEM I, class of strength 52.5 N, conformed to the Standard BS EN 197–1: 2011 was used [44]. The chemical composition of cement is shown in Table 5. Fine natural dried sand with size under 600 m μ and 10 mm and 20 mm aggregates that accords with the British Standard BS EN 12620:2002+A1:2008 [45] have been used. The Particle Size Distributions (PSD) and densities are shown in Fig. 11 and Table 3. The investigated as-received ashes are shown in Fig. 5. The composition and properties of the ashes highly depend on the burning process. This 30 Mw energy plant uses a travelling grate boiler, where, almost the 80% of bagasse is burnt in suspension, meanwhile the rest sets on the bottom mixed with heavier particles from the soil of the yard were the bagasse is accumulated. Thus, in this case, SCB BA main compound is CaCO 3 from the stones swept away from the soil, and silica is the second compound. SCB FA composition is characterised by the presence of silica whose origin mainly comes from the silica absorbed by plants from soil during their life. Additionally, some silica grains were observed which origin could be in the harvested field. According to the ash producer, SPBE, the combustion temperature of the gases is not constant, and the average is in the range 750–800 ◦C. 2.1.1. Treatment of ashes Ashes were physical-chemically analysed as-received and after being treated. Based on the particle size distribution and on the chloride and contaminants content, the ashes were subjected to minimal treatments according to their requirements, described below. SCB FA was sieved and the portion retained in 200 μ m, mainly charcoal, was discarded since it reduces the workability. The portion of ashes retained in sieve 150 μ m, mix of powder and charcoal, was separated to be partially washed as per Fig. 6(a). Charcoal has further utilization in industry (e.g. in briquettes fabrication or as adsorbent materials). SCB BA was completely water-washed to remove contaminants by solubility and floatability, Fig. 6(a). The porosity and heterogeneity of the grains of SCB BA would have compromised significantly the concrete soundness and the mechanical properties. For this reason, particles greater than 2.00 mm were grinded for 15 min obtaining a well-graded sand, Fig. 6(b and c). In both cases a cheap and low-energy system to wash the ashes was used to remove contaminants and organic matter. In both cases, the ashes were dried at 70 ±5 ◦C in the oven for 24h after washing. 2.2. Methods Two different types of ashes from the same energy plant are studied: Fig. 5. As received ashes: (a) SCB FA, (b) SCB BA. Fig. 6. Treatment of SCB BA: (a) contaminants removal by solubility and floatability; (b) sieving; (c) grinding of SCB BA ashes. V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 5 sugar cane bagasse fly ash (hereafter SCB FA) and sugar cane bagasse bottom ash (hereafter SCB BA). The physico-chemical characterisation of ashes has been conducted and low-energy consumer treatments proposed. The treated ashes were used as fine aggregate to cast different types of bio-concrete and consequently tested according to 2.2.2 Characterisation of concrete. Experimental campaign. The methodology is divided in three distinct stages summarised in Fig. 7: i) the analysis and characterisation of as-received (ar) and treated (t) ashes, ii) the preparation of the bio-concrete and, iii) the characterisation of the concrete at different ages. During the first stage, the ashes were physical-chemically characterised, and the crystallinity and contaminants were determined. The limitations and potentials were drafted. Based on the results, low impact treatments were applied, removing the main portion of contaminants. The treated ashes were characterised again. During the second stage, concretes incorporating 10%, 20% or 30% of SCB FA or SCB BA were casted using the same mix design to enable direct comparison. Constant quantities of cement and w/b ratio (0.45) were used, considering b as the sum of cement and ashes. Testing of hardened concrete is the last stage and covered 28d, 60, 90 and 240 days of wet-curing age, in order to assess the following mechanical and durability properties: compressive strength, porosity, capillary, resistivity and chloride migration resistance. 2.2.1. Characterisation of as-received (ar) and treated (t) ashes Physical-chemical properties of the ashes were analysed. The morphology was observed by means of scanning electron microscope, SEM, using FEI SEM model Inspect S50. The particle size distribution of samples was obtained by dry sieving test as per [46]. Additionally, SCB FA was subjected to the laser diffractometry test using the analyser Beckman Coulter LS 13320. It must be highlighted that this method considers particles as spheres and it can make an approximation to non-spherical particles that could work. However, for many particles, the size distribution obtained is only apparent. This will be the case of the elongated charcoal particles presented in the ashes [47]. Bulk density, real density and specific surface based on the Blaine method were obtained according to the standard BS EN 1097–6:2013 [48]. Some authors pointed out the overestimated results of loss on ignition test due to further chemical reactions at different temperatures [21,49]. Thermogravimetric analyses (TGA) with a TA-Q50 thermogravimetric analyser have been done to have a deeper comprehension of the ashes in the as-received and treated forms. Chemical characteristics are obtained by means of XRD and XRF analyses. Soluble chloride content in ashes were additionally analysed with spectrophotometer DR3900 from HATCH for more accurate results. 2.2.2. Characterisation of concrete. Experimental campaign Durability of concrete depends on the exposure environment and the inherent characteristics of the material. Focusing on the second ones, concrete’s durability relies largely on the microstructure of the material: size, distribution and shape of pores and the porous interconnectivity. In general, a reduction in the interconnectivity results in a reduction of permeability into the concrete so the external ions migration is hampered. These properties can be assessed with different techniques; in this research the following tests were conducted: open porosity, surface electrical resistivity, capillary absorption and chloride migration. The results obtained were correlated pursuing a deeper understanding of the durability properties of concrete due to the addition of ashes. Compositions with 0%, 10%, 20% and 30% of ashes were designed. Three cubes of 150x150 ×150 mm and 6 cylinders of 100 mm of diameter and 200 mm high were casted per composition. To compare the initial data, mixes with both types of ashes were casted considering the water/binder ratio equals 0.45. Due to the XRD and XRF results, no/low reactive reactions were expected. Hence, two families with three different compositions of bio-concrete plus the control one was casted. Fresh and hardened properties were tested according to Table 2. 2.2.3. Mix design and concrete preparation The ashes, SCB FA and SCB BA, were added as a replacement of sand in percentages of substitution of 10%, 20% and 30% (weight of sand). For all the ashes assessed, the water in the mixes designs was estimated considering the possible binder capacity of the ashes. In these cases, the ratio considered is water-binder ratio (hereafter w/b), where binder is the sum of both, cement and ashes. Based on these considerations, the different mixes were designed as following in Table 1. After casting, the concrete samples were kept under lab conditions during 24h before being demoulded and then wet cured in water tanks until the testing date. All samples where oven dried at 60 ◦C until constant mass before capillary absorption, chloride migration and porosity tests. In Fig. 8 the materials used and the cubes and cylinders casted are shown during the SCB FA mixes preparation. It is noteworthy the colour change in the fresh concrete when SCB FA are added becoming darker. 2.2.4. Tests definition Table 2 summarises the tests conducted to assess the physical and chemical properties of concrete at different ages and the applied standards. Chloride migration coefficient of concrete have been measured according to NT Build 492 [56], a non-steady state migration test at 28, 60, 90 and 240 days. An external potential is applied across a cylindrical specimen, (Ø100mm, h ~ 50 mm) that forces the chloride ions in the catholyte solution migrate into the specimen. The specimens were prepared in vacuum and submerged in Ca(OH) 2 , later fitted in a rubber sleeve and this filled with an anolyte solution (0.3 M NaOH) where the anode was introduced, Fig. 9(a). This was put in a reservoir with a catholyte solution (10%NaCl). All the system was connected to the power supply, Fig. 9(b). After 24h, the specimens were split, and the chloride penetration depth assessed with silver nitrate, Fig. 9(c). Finally, the registered data were worked out according to formula (1) where D nssm is the non-steady-state migration coefficient ( ×10–12 m 2 / s); U is the absolute value of the applied voltage, (V); T is the average value of the initial and final temperatures in the anolyte solution, (◦C); L, thickness of the specimen, (mm); x d , average value of the penetration depths, (mm) and t, test duration, (hour). Dnssm =0.0239(273 +T)L (U−2)t(Xd−0.0238  (273 +T)LXd U−2 √)(1) The electrical resistivity of concrete is an inherent property of the material and express the opposition that the concrete mass exerts to the transport of charged ions through the material under an external current [57]. It depends on several factors such as the porosity microstructure, the relative humidity, the pore solution content, etc. In this research the Wenner four probe system with a resistivity meter RESIPOD PROCEQ Fig. 7. Methodology flowchart. V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 6 model with 50 mm spacing was used to measure the surface resistivity. Two internal probes measure the potential difference between these and the external ones where a current is applied and carried by ions in the pore solution through the concrete. The limitation of this system is the not semi-infinite depth condition of the cylinders, affecting the value of the cell constant and consequently the accuracy of the reading. Thus, the readings must be treated in a qualitative way in order to have a better comprehension of the evolution of the concrete permeability on time. An increment on the electrical resistivity values can indicate a reduction on the permeability due to a refinement of the pores microstructure due to further hydration, or a reduction of the hydroxyl ions (OH − ) observed when reactive supplementary cementitious materials are used [36,57]. 3. Results and discussion 3.1. Characterisation of as-received (ar) and treated (t) ashes 3.1.1. Appearance: colour and shape Visually, SCB FA is a dark-grey/black powder with presence of needled unburnt carbon particles, Fig. 10(a). Black colour in ashes indicate uncompleted burning process. SCB BA is a mix of different types of stones with a wide range of densities, colours and sizes and some traces of bagasse, Fig. 10(c). SCB BA colour became whitish after the treatment, but no significative chemical reaction was observed under X- ray diffraction, Fig. 10(d). When the samples are analysed under SEM, different types of particles are observed. SCBA FA is made of four main types of particles: prismatic (≤100 μ m), spherical (≤50 μ m), amorphous particles, unburnt carbonaceous particles (≤300 μ m), small particles adhered to the surface of bigger grains (≤5 μ m) and bubbles, Fig. 11 (a), (b). This is consistent with the observations of Sales and Lima (2010) and Pay´ a et al (2018) [20]. A complex bimodal distribution pointed by Cordeiro et al (2008), Codeiro et altri (2009) and Pay´ a et al (2018) [17,19,30], can be slightly observed in Fig. 13(b) when the differential volume is analysed. In SCBA BA angular particles, spherical ferric particles and light and porous grains are observed, Fig. 11(c) and (d). 3.1.2. Particle size distribution For the dry sieving test of the as-received ashes, two single determinations on separate test portions for each type, heavier of 100 g, were dried in an oven at (105 ±2)◦C until the mass was stabilised. Depending on the particle sizes, different range of sieves were used: 63 μ m to 3.35 mm for SCB FA, meanwhile, for the SCBA BA a larger range had to be used to cover until 14 mm. The assembled stack of sieves with the testing sample was shaken for 10 min, the quantity retained in each sieve and the retainer pan was weight, and the percentage passing, D10, and finally the coefficients of curvature and uniformity were obtained, Fig. 12. It is noteworthy that, for SCB FA, above 200 μ m most of the volume retained is low density char fibrous particles. Additionally, SCB FA was subjected to the laser diffractometric test using the analyser Beckman Coulter LS 13320, Fig. 13. This apparatus determines the particle size distribution suspended in water by measuring the pattern of light scattered by the particles in the sample. It must be noted that this method considers particles as spheres and it makes an approximation to non-spherical particles that could work. However, for many particles, the size distribution obtained is only apparent. This is the case of the elongated charcoal particles presented in the ashes. In addition, due to the small amount of sample tested, the biggest needled particles may drop and not been considered. This will justify the possible discrepancies between both analyses. SSA measurements with laser diffractometer and Blaine method may also differ. SCB FA particle size distribution is represented by a steep curve with the mean value in 140 μ and thinner than the sand used but classified as fine sand. The fineness modulus is 1.06 and 1.05 for as received and treated, respectively. Sizes from 1.18 mm to 150 μ were dominated by the Table 1 Concrete mix design. CEM (kg) Aggregate 20 mm (kg) Aggregate 10 mm (kg) Sand (kg) Ash (kg) Water (l) w/b Control 450 712 610 335 – 202.5 0.45 CEM I +10 ash(t) 450 712 610 301.5 33.5 202.5 0.45 CEM I +20 ash(t) 450 712 610 268 67 217.58 0.45 CEM I +30 ash(t) 450 712 610 234.5 100.5 232.65 0.45 Fig. 8. (a) Concrete preparation of SCB FA-10 [1]: 10 mm aggregates [2], OPC [3], SCB FA [4], Sand [5], 20 mm aggregates; (b) Casted concrete: 0%, 10%, 20% and 30% of replacement. Table 2 Tests summary table. Physical and chemical properties Tests Age (days) Consistency (Slump test) BS EN 12350–2 [50] 0 Density of fresh concrete BS EN 12350–6 [51] 0 Density of hardened concrete BS EN 12390–7 [52] 28 Compressive strength BS EN 12390–3:2019 [53] 28 Water absorption due to capillary BS EN 1015–18:2002 [54] 28, 60, 90, 240 Total open porosity BS EN 1936: 2006 [55] 28, 60, 90, 240 Chloride migration of concrete NT Build 492 [56] 28, 60, 90, 240 Surface electrical resistivity Wenner four-probe system 28, 42, 60, 90, 120, 240 V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 7 presence of unburnt carbon and the content of some white grains - sand from the field. SCB BA (ar) has a distribution covering from medium sand to fine gravel with a fineness modulus of 6.95. SCB BA (t) is a coarse fine aggregate with a fineness modulus of 3.99 and mean value of 550 μ , Fig. 12. 3.1.3. Density and specific surface area (SSA) Table 3 shows the physical properties of bio-ashes. Bulk density of SCB FA is similar to that of cement and slightly lower than that of sand, and SCBA BA bulk density is close to that reported by Sales and Lima (2010) [20]. Nevertheless, the SCB FA real density is lighter than sand and cement and concords with values reported by other authors (1,7–2, 5 g/cm 3 ) [58], meanwhile SCB BA real density is comparable to sand. In the case of SCB FA the specific surface area is much bigger than the normal values for fine sand and still bigger than that of cement, meanwhile SCB BA SSA is a quarter of that of SCB FA. An increase in the treated SCB FA ashes density is observed due to the removal of fibrous carbon particles by means of sieving. The same effect was observed by Murugesan et al. (2020) [14]. 3.1.4. pH of ashes The alkalinity of ashes is shown in Table 4. The decrease in alkaline elements in SCB FA may be due to the charcoal removal which has a higher pH (12–13). The water-washing did not alter the pH of SCB BA probably because minimal amounts of contaminants were removed but not chemical changes occurred. 3.1.5. Mineralogical and chemical analysis Major and minor elements of cement, treated and untreated ashes are expressed as oxides in Table 5. The heterogeneity of particles and the mineralogical structure were observed by means of XRD, and are shown in Figs. 11 and 14. In Fig. 14 it can be observed the crystalline silica that leads the XRD pattern for SCB FA and the presence of silicon and carbon carbonate in SCB BA particles. SCB FA is predominated by SiO 2 which contents in as-received and treated ashes (53.09%/60.92%) are lower than the mean value (63.56%/ 67.90%) reported by Pay´ a et al, (2018) [17] after having compiled 22 authors data who reported SiO 2 contents between 31.41% up to 81.20%. Rajasekar, A. et al. (2018) used high silica content SCBA (86.79%) to Fig. 9. Chloride migration coefficient procedure: (a) vacuum of samples, (b) application of potential, (c) Chloride penetration depth assessed with silver nitrate. Fig. 10. (a) As-received SCB fly ash – SCB FA (ar); (b) Treated SCB fly ash – SCB FA(t); (c) As-received SCB bottom ash – SCB BA (ar); (b) Treated SCB bottom ash – SCB BA(t). V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 8 enhance the properties of UHSC, [22]. The silica content comes from the absorption of the plant from the soil during its life and the content of grain of quartz removed from the field during the harvesting [59]. The amount of silica -as the rest of elements-increase after the sieving due to the removal of charcoal particles, which main element is carbon. The amount of Al 2 O 3 (6.94%/7.62%) and Fe 2 O 3 (5.55%/5.99%) are lower than the mean values gathered by Pay´ a et al (2018): (9.18%/9.80%) and (6,09%/6,50%) respectively. The total amount of main oxides (SiO 2 + Al 2 O 3 +Fe 2 O 3 ) is 65.58% before sieving and 74.53% after sieving, higher than the minimum content of oxides required for Pozzolans type F: 70% [60]. Oxides content is not the only parameter that define the pozzolanic activity of ashes: fineness and crystallinity are also responsible. The crystalline phase detected is quartz, a non-amorphous phase (inert) with non-binding capacity, that makes SCB FA suitable as aggregates substitutes, Fig. 15(a). Cristobalite has not been detected, contrary to Cordeiro et al. (2019) [13] who observed its presence in ashes obtained under controlled combustion at 800 ◦C. This indicates a combustion temperature below 800 ◦C that confirms the declared temperature (750–800 ◦C) by the ash producer, San Pedro BioEnergy. Optimal temperature for pozzolan SCBA have been stablished around Fig. 11. Scanning electron microscopy of ashes. Fig. 12. Particle size distribution. As-received and treated ashes. Fig. 13. Laser diffractometric analysis of SCB FA (ar). (a) “Less than” cumulative volume (b)Differential volume. V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 9 550 ◦C and 600 ◦C [19,22]. The baseline in XRD is slightly displaced showing a minor halo with any major outstanding embossments such as that detected by other authors who investigated the use of SCBA as cement replacements [22,36]. All this indicates a low activity index. Cordeiro et al. (2008) [30] considered as pozzolans those SCBA ashes with a fineness D 80 <60 μ m and Blaine fineness >300 m 2 /kg. Based on the authors correlations and the burning temperature, the reactivity of SCB FA can be considered at 360 mg CaO/g SCBA. When the fineness is considered (575 m 2 /kg) the Chappelle activity is 140 mg/g. These results must be taken with caution since the research was done on ashes obtained under laboratory-controlled conditions. The separated char has been observed under the X-ray diffraction and amorphous structure and the presence of attached particles of silica was detected, Fig. 15(b). SCB BA is clearly crystalline with no presence of amorphous halo, characterised by the presence of calcium carbonate (CaCO 3 ) in the mineral phase of calcite, Table 5 and Fig. 15(c). The main oxides are CaO (65.56%/66.40%) and SiO 2 (23.74%/21.77%). The calcium carbonate indicates the presence of stones from the field in the boiler and added together with the biomass. After the treatment, no important chemical reactions are observed in the X-ray diffractogram, but the removal of some contaminants. Water-washing reduces the water-soluble chloride content. The reduction depends on the initial content, being higher with higher initial Cl-contents. SCB FA and SCB BA have low soluble-chloride content in the as-received forms but it is still higher than the limit stablished by the standards for total chloride in cement or PFA ashes for steel reinforced concrete: 0.10% content by mass of cement [61,62]. The limit for total chloride content in concrete is 0.4% by mass of cement for the sum of all the constituent materials. No limits are stablished for aggregates, nevertheless, natural aggregates from inland deposits have very low water-soluble chloride ion content (<0.01%) [45]. In Fig. 16 the thermogravimetric analysis (TGA) of SCB FA (ar) and SCB FA (t) shows a first loss of mass below 500 ◦C, corresponding to the water removal and the oxidation of residual carbon and organic matter pointed in similar ranges by Zhao et al (2013) [21]. SCB FA (t) showed a higher humidity content derived from environmental circumstances which should be omitted when both ashes are compared. The differential char content between SCB FA (ar) and SCB FA(t) can be observed in the range of 450 ◦C −600 ◦C. Around the 650 ◦C a third mass loss occurs with a new slope likely related with the crystallisation of the amorphous silica. At the end of SCB FA TGA curve, around 900 ◦C, occurs a change that can be related with a change of phase of silica, Fig. 16(a). In the case of SCB BA, Fig. 16(b), there is no major mass loss until 425 ◦C, due to the residual carbon oxidation. The decomposition of CaCO 3 is observed at 780 ◦C where SCB BA (ar) has undergone a greater mass loss. Zhao et al (2013) [21] observed the release of soluble alkali and chlorides between 660 and 900 ◦C. In this case, due to the low content of soluble elements in SCB FA and SCB BA, the removed parts are Table 3 Physical properties of bio-ashes. OPC Sand SCB FA SCB BA Asreceived treated Asreceived treated Real density (kg/m 3 ) 3150 2420 1776.6 1820.2 2349.19 2352 Bulk density (kg/m 3 ) 1400 1635 1422 1492 2432 2447 Fineness modulus – 2.84 1.21 1.01 7.65 3.99 Specific Surface area (cm 2 /g) 3000–4500 360 5853 5750 – 1502 Table 4 pH values of as-received and treated ashes. As-received Treated SCB FA 11.92 11.37 SCB BA 12.67 12.68 Table 5 Chemical properties of ashes. Main oxides. Element, % OPC SCB FA SCB BA As-received Treated As-received Treated SiO 2 26.66 53.09 60.92 23.74 21.77 Al 2 O 3 2.22 6.94 7.62 1.19 0.84 Fe 2 O 3 2.43 5.55 5.99 2.31 1.99 CaO 64.11 9.60 11.84 65.56 66.40 MgO 1.55 2.36 2.78 0.97 0.90 K 2 O 0.64 2.08 2.25 0.89 0.68 Na 2 O 0.27 1.18 1.24 1.31 1.30 TiO 2 1.33 0.57 0.58 0.31 0.32 P 2 O 5 0.87 0.25 0.25 0.25 0.25 MnO 0.15 0.16 0.17 0.04 0.04 Cl- (soluble) – 0.129 0.10 0.128 0.09 Fig. 14. Chemical composition and scanning electron microscopy (SEM) of particles of (a)SCB FA and (b)SCB BA. V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 16 less steep than the control concrete curve and below this one, in other words, the addition of SCB FA increases the open porosity meanwhile deliver concretes with comparable or enhanced results of chloride migration resistance. Only SCB FA-30 at 240d achieve the OP values of control concrete. The evolution on time leads to think about an interruption in the interconnectivity of porous due to the later hydration of pozzolans and cement. Considering that the addition of SCB FA increase the capillary absorption (Fig. 24(e)), there is no option to think about a deposition of the hydrated products from the exterior to the interior. Thus, an additional phenomenon may occur which helps to block the chloride diffusion of ions: a possible physical or chemical chlorides ions binding capacity of concrete promoted by the incorporation of ashes. The alumina within the ashes that promotes the formation of aluminates able to chemically bound chlorides or the adsorption capacity of char particles have been pointed to be the main responsible of this [42,66]. Capillary water absorption is usually related with the diffusivity of aggressive ions into the concrete trough the porous structure [67], consequently it is used as a parameter to assess the adequacy of a mix Fig. 25. Durability-related performance over time. V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 17 exposed to a specific environment. In this research it has been observed that the addition of SCB FA increase the capillary absorption when compared with the control concrete but, in contrast, the chloride migration is reduced, and no direct correlation exists, Fig. 24(e). This means that capillary absorption is not enough to understand the vulnerability of concrete to external aggressive agents and its diffusion through the concrete mass until corrode the embedded rebars. In comparison to SCB FA, the chloride migration and open porosity correlations curves of concretes with SCB BA are above of the control concrete curve due to lower chloride migration resistance due to an excess of water and a minimal reactivity (higher in concretes with higher substitutions rates). As with SCB FA mixes, no clear correlations can be stablished between D nssm and capillary absorption (0.49<R 2 <0.78), except that, at 28 days, higher capillary absorption lead to higher D nssm , Fig. 24(f) . 3.2.7. Performance of concretes over time (from 28 to 240 days) By analysing the evolution of concretes over time, the influence of both ashes due to their disparate nature can be easily assessed. It can be stated that concretes including SCB FA show a clear evolution on time due to the existence of secondary hydration products, with repercussion on the durability performance of the concrete, Fig. 25. This makes SCB FA an enhanced-properties alternate sand, where the amount of rate substitution highly influences the final performance of the concrete. In terms of open porosity, most of the evolution occurs between 28 and 60 days, meanwhile negligible changes take place after 90 days and the velocity rate decreases considerably (between 10 and 20 times). SCB FA 30 mixes rate double that of SCB FA 20 and SCB FA 10, and future reductions might be expected, Fig. 25 (a, c, e). Similar trend is observed in terms of chloride migration rates, where differences between 10% and 20% of substitution rates are minimal in comparison to 30%. The formers follow the tendency (shape and velocity) of the control concrete migration curve meanwhile beyond 30% of replacement a different trend can be observed. This means that, a faster chloride diffusion resistance development can be obtained when higher substitution rates are used. This concords with other authors [42,43] who concluded that the chloride resistance improvement is more influential than the compressive strength gain. In addition, the higher the substitution rate, the steeper the superficial electrical resistivity curves are, turning away from the control concrete curve. The trend shows that even after 240days of wet curing the superficial electrical resistivity increases, thus the conductivity of the concrete is reduced. Considering the trendlines on time, it is clear that SCB BA concretes performance is closer to the control concrete one. The hydration products of SCB BA mixes considering w/b ratio help to counteract the effect of extra water between different mixes. Nonetheless, these mixes increase the open porosity and, as a consequence, the chloride diffusion Fig. 25 (b, d, f). Summarising the experimental campaign, it can be stated that: •The addition of SCB FA reduces consistency due to the existence of char particles and higher water demand meanwhile SCB BA promotes the workability on account of an excess of water. •SCB FA delivers lighter and more porous concretes regardless the higher compacity of mixes; nonetheless, the porosity decreases over time. The density decreases in SCB BA concretes are due to an excess of water. •Lower densities and the amount of char particles are responsible for the decreases observed in concretes with SCB FA 20 and SCB FA 30 at 28d. Later improvements in compressive strength are expected after 56days. SCB BA decreases in compressive strength are due to the excess of water in the mixes. By adjusting the w/b ratio to w/c ratio equivalent results to the control concrete can be achieved. •All ashes increase the capillary absorption of concretes, but improvements in Chloride migration resistance are obtained in mixes with SCB FA proportionally to the amount of ashes. To conclude, Table 8 summarises the concrete characteristics at 28 and 240 days when SCB FA and SCB BA are used as sand replacement. 4. Conclusions This study investigates the feasibility of incorporating low-reactive industrial sugar cane bagasse ashes (fly ash -SCB FA-, rich in silica and bottom -SCB BA-rich in CaCO 3 ), in concrete production as sand alternate Table 8 Summarising chart. V. Torres de Sande et al. Journal of Building Engineering 39 (2021) 102277 18 materials with low-cost and low-energy consumption treatments, concluding that: •The specific surface area of ashes directly affects the workability of the bio-concretes: when SCB FA(t) with higher specific surface area is used, the workability decreases, meanwhile with SCB BA increases. •Concretes with SCB FA achieve the compressive strength of the control concrete when 10% is added at 28 days. Later increases in the compressive strength can be expected due to delayed pozzolanic reaction. SCB BA reduces the compressive strength (between 10 and 17.5%) when w/b =0.45 is considered. By considering the w/c = 0.45 instead of w/b =0.45, higher compressive strengths can be obtained (up to 18.9% and 23.2% for 20% and 30% of substitution rate respectively), achieving the control concrete compressive strength: 56.33 MPa. •In terms of durability, SCB FA(t) enhances the chloride migration resistance showing the best performance at 30% of substitution rate even at 28 days, regardless the decrease of 8.9% in compressive strength. Thus, concretes with 30% of rate substitution are suitable for structural use (C45/50 strength class) and an enhanced durability performance at 28days. At early ages (28 days), SCB FA-10 and SCB FA-20 of replacement provide comparable resistance to the control concrete, and SCB FA-30 improves the control concrete resistance (↑10.9%). At 60 days, all the mixes show a better performance than the control concrete and enhancements can be still observed after 240 days with an average value of 43.1%. A reduction in the connectivity of microstructure of pores, consistent with later hydration of pozzolans and cement, has been pointed as the main cause of the enhancement in the durability properties. •The addition of SCB FA promotes the capillary absorption and the open porosity of concretes. The more ashes are used, the higher reductions occur on time. Based on these results, a possible chloride binding capacity of ashes has been stated as second cause of the enhancement in chloride migration resistance. Consequently, the open porosity test or capillary absorption test cannot be taken as conclusive test when durability properties want to be assessed, in specific, the corrosion vulnerability. •The incorporation of SCB BA(t) with w/b =0.45 increases the capillary water absorption and reduces chloride diffusion resistance of the different mixes, promoted by the excess of mixing water. Based on the resistivity, porosity values and chemical composition, a reduced pozzolanic activity able to counteract the excess of water has been indirectly observed. This research demonstrates that apparent low-reactive SCB FA rich in quartz, can show later pozzolanic activity that makes this waste a potential alternate sand to enhance durability properties of cement-based materials. SCB BA may have potential uses where durability requirements are low, if the amount of mixing water is reduced. This opens a market opportunity for low quality ashes and can be a solution for the increasing housing and material demand in countries with a high urban development where sugar cane is cultivated. This study is mainly focussed in the durability performance. In order to have a thorough comprehension of the performance of an optimised mix design, future works would consider assessing further fresh and mechanical properties (rheology, setting time, compressive strength, tensile strength, flexural strength, elastic modulus) on time. For future studies further electrochemical characterisation is going to be done in order to assess the influence of the ashes in relation to the service life. The main limitation of this study is the heterogeneity of ashes. The results and conclusions obtained are applicable to the concrete made with ashes from the mentioned company and batches with the specifications described before. However, the interest of this study is that the methodology and findings can be extrapolated to other studies with this type of waste. The obtention of optimal ashes through a controlled combustion process is out of the scope of this research. Nevertheless, the authors believe that the implementation of new burning and collecting procedures is necessary in industry where the quality of the resulting ashes is taken into consideration. CRediT authorship contribution statement Veronica Torres de Sande: Conceptualization, Methodology, Investigation, Writing – original draft, and, Writing – review & editing. Monower Sadique: Writing – original draft, and, Writing – review & editing. Paloma Pineda: Conceptualization, Writing – original draft, and, Writing – review & editing. Ana Bras: Conceptualization, Writing – original draft. William Atherton: Conceptualization, Writing – original draft. Mike Riley: 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. 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