Application of biochar from coconut and wood waste to reduce shrinkage and improve physical properties of silica fume-cement mortar
Introduction
Supplementary cementitious materials are widely used to reduce consumption of cement and improve the durability of cementitious composites. One such material is silica fume which is widely used as an admixture to improve early and long-term strength and durability of cement-based construction materials. While silica fume-concrete has distinct advantages over other types of supplementary admixtures, the main challenge associated with its utilization is high autogenous and drying shrinkage leading to micro-cracks especially under warm climatic conditions [1], [2], [3], [4]. High autogenous shrinkage of concrete with silica fume is linked to its pozzolanic action and reduction of pore sizes creating capillary tension inside the small pores, which results in contraction of cementitious paste. The magnitude of contraction and self-desiccation are influenced by the dosage of silica fume and the availability of water [2], [5]. For instance, Al-Amoudi et al. [1] observed that replacement of cement by 8% silica fume resulted in higher shrinkage compared to the control after 15 h (75%) and 28 days (50%) since casting. High early shrinkage suggests that concrete with silica fume may suffer the risk of cracking even when the formwork is not removed during the first 5–7 days after casting. Restraint to shrinkage imposed by formwork may result in cracking at the early stage of the placed concrete. Zhang et al.[2] recorded an increase in autogenous shrinkage at 14 days (1.9 times) and 98 days (2.3 times) because of the replacement of 10% cement by silica fume in cementitious composites prepared with water to cementitious materials (w/c) ratio of 0.35. However, the authors observed an increase in 28-day strength by 19% compared to the control without silica fume. A similar observation was reported by Rao [5] – the addition of 15% silica fume in a mortar with w/c ratio of 0.50 and sand:cement of 1:3 increased drying shrinkage by 3 times compared to the control at 28 days. This indicates a significant increase in shrinkage as a result of the addition of silica fume into concrete even though the mixture was prepared at a relatively high w/c ratio. Furthermore, silica fume can be significantly more expensive than cement, ranging from 2 to 3 times the cost of cement depending on the country and availability.
Several strategies including internal curing by pre-wetted lightweight aggregates [6], superabsorbent polymers [7] and prolonged curing [1] have been explored to obtain improved physical properties and reduce shrinkage of concrete with silica fume. For instance, Zhang and Poon [8] reported that internal curing by bottom ash and lightweight aggregate (100% replacement of normal aggregate) reduced autogenous shrinkage by 60% in concrete with 10% silica fume compared to the control at 100-day age. However, incorporation of lightweight aggregate led to a reduction in 28-day compressive strength and specific strength by 40% and 16% respectively. Al-Amoudi et al.[1] reported that water ponding of concrete with silica fume reduced drying shrinkage strain compared to wet burlap curing over a monitoring period of 100 days, suggesting that extended curing can reduce shrinkage in silica fume-concrete. However, such prolonged curing may not be feasible under actual site conditions.
One of the alternative materials to reduce consumption of silica fume in cementitious composites is biochar, derived from pyrolysis or gasification of waste biomass. Thermal treatment leads to removal of volatiles and organics from waste biomass leading to the formation of pores of wide size ranges in the biochar, i.e. micropores (<2 nm), mesopores (2–100 nm) and macropores (>100 nm) [9]. These pores play an important role in retaining and redistributing moisture within the cement mortar, which may reduce the shrinkage. Muthukrishnan et al. [10] found that combination of rice husk biochar (RHBC) at 2–8 wt% of cement, and rice husk ash (RHA), at 12–18% of cement replacement, prevented sealed shrinkage over the initial 42 days of the monitoring period. This was attributed to improved moisture retention by pores in RHB, leading to the gradual release of water at a later stage and maintaining internal relative humidity. However, the drying shrinkage of mortar with rice husk ash (RHA) was similar to that of control due to higher pozzolanicity of RHA and lower moisture retention capacity due to collapse of the pores. Zeidabadi et al. [11] reported that replacement of 5% cement by RHB in concrete led to approximately 12% improvement in compressive strength compared to control, attributed to filler effect of RHB particles at an early age. The micro-filler effect of biochar has also been highlighted by other researchers, leading to 50–70% improvement in compressive strength and fracture toughness of cementitious composites using 0.1–2% of biochar produced from peanut shell, bamboo and hazelnut shell in cement pastes [12], [13]. Gupta et al. [14] estimated that the cost of biochar, produced from wood waste at 500 °C may vary between 50 and 70 cents/kg, which translates into similar or slightly higher (about 0.70%) cost of mortar per cubic meter, depending on biochar dosage (1–5% of cement). These outcomes show the positive impacts of using biochar on mechanical properties and water tightness of cementitious composites, which suggest its potential to replace or reduce silica fume consumption. However, it must be noted that the impact of biochar on properties of cement composites is influenced by the feedstock, preparation condition and dosage of biochar.
Using biochar as a replacement for cement also offers significant environmental benefits. Depending on feedstock and preparation conditions, 60–90% of carbon from the parent biomass (feedstock) can be sequestered in biochar, leading to a net reduction in greenhouse gas emissions by 530–570 kg CO2-eq [15]. This is particularly important in case of cementitious materials because using biochar as a carbon sequestering material can turn future buildings into ‘carbon sinks’ and reduce the net CO2 emission associated with cement production and processing. However, the environmental benefits and low cost associated with biochar-based building materials can be obtained only if the biochar is sourced and produced from the locally available feedstock, that is abundantly available in that particular region. This would reduce the emission and cost involved in transportation and further processing of biochar. For example, Indonesia, Philippines and India account for 75% of world’s coconut production [16]. Open burning of coconut wastes in pits leads to high emission of greenhouse gases [16], [17]. Due to relatively high calorific value (18–20 MJ/kg), a significant portion of coconut waste is used as an energy source by burning the waste and recovering the heat [17]. Biochars produced as a by-product of this process often suffer from incomplete combustion. Also, disposal of biochar wastes in landfills is another problem because biochar is difficult to be decomposed by microbes due to the hard nature of the shell and fibres [16]. Thus, coconut biochar, produced from the incomplete combustion process, can be further treated and processed to utilize as supplementary admixture in cementitious composites.
Similarly, wood waste is widely generated in Singapore and other South-East Asian countries. Singapore disposed of 131,800 tons of wood waste in 2018 [18], a significant portion of which was incinerated and landfilled. Incineration of wood waste is known to lead to emission of particulate matters and greenhouse gases. Instead, the wood waste can be utilized to produce biochar, which can then be processed as a cementitious admixture in the local construction sector. This will reduce cement consumption and import of silica fume for utilization in concrete.
This research aims to explore the application of biochar from wood waste and coconut waste as partial replacement of cement and silica fume in mortar. Wood waste was sourced from Singapore and coconut shells from an incomplete combustion process were sourced from neighboring countries for retreatment and biochar production. The performances of these biochars were compared to that of commercially produced biochar, made from tropical wood in Southeast Asia. The produced biochars were characterized and used to replace 5 wt% of cement (only biochar), and 15 wt% of cement in combination with 10% silica fume. This would reduce 33% of the silica fume by partially replacing it with biochar. The designed mortars with biochar and biochar-silica fume combination were investigated for rheology, hydration, strength development, shrinkage and water tightness.
Section snippets
Cement and sand used
52.5 N ASTM C150 [19] Type I Portland cement was used in the preparation of mortar samples. The composition of the cement is provided in the supplementary information (Table S1). Natural sand conforming to specification in ASTM C33 [20] was used.
Undensified silica fume with silica content of at least 90% and bulk density (undensified) of 200–350 kg/m3, as per manufacturer’s specification, is used.
Preparation and processing of biochar
Wood waste was collected from a waste recycling company in Singapore. These wood wastes are
Morphology and particle size distribution
Fig. 1 a–d shows the SEM micrograph of CBC, CBC-UT, TBC and SWBC particles. It can be observed from the micrographs that irrespective of feedstock, all the biochars are irregular in shape with a relatively smooth surface. CBC and CBC-UT show similar particle morphology (Fig. 1 a-b) because both are initially ground together before further heat treatment. This means that thermal treatment of CBC-UT does not lead to a significant change in morphology of biochar. Some particles have sharp edges
Conclusion
This study investigated the application of biochar to replace cement and silica fume in mortars. The effect of biochar on hydration, strength, permeability and shrinkage was studied. Based on the experimental outcome, the following conclusions are drawn.
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Replacement of 5 wt% biochar produced from wood waste and coconut shell in cement pastes leads to filler effect, evident from acceleration in hydration and higher peak heat of hydration compared to control. Substituting 33% of silica fume with
CRediT authorship contribution statement
Souradeep Gupta: Conceptualization, Methodology. Padmaja Krishnan: Data curation, Formal analysis, Writing - review & editing. Alireza Kashani: Data curation, Methodology, Writing - review & editing. Harn Wei Kua: Conceptualization, Writing - review & editing, Funding acquisition.
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.
Acknowledgements
The corresponding author thanks the Ministry of Education, Singapore, for providing the Tier-1 Academic Research Fund (R-296-000185-114). The authors also thank the Center for Infrastructure Engineering and Safety (CIES) at UNSW, Sydneym for providing some of the resources used in this study.
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