Application of biochar from coconut and wood waste to reduce shrinkage and improve physical properties of silica fume-cement mortar

https://doi.org/10.1016/j.conbuildmat.2020.120688Get rights and content

Highlights

  • Biochar from uncontrolled combustion was thermally treated to improve its properties.

  • Plastic and dynamic viscosity of cement composite increase with biochar porosity.

  • 33% silica fume can be replaced by biochar without affecting permeability and strength.

  • Addition of biochar reduces autogenous and drying shrinkage of silica fume-mortar.

  • Biochar added at 5% cement replacement improve water tightness and strength.

Abstract

High autogenous and drying shrinkage associated with the use of silica fume increases the likelihood of cracking under warm climatic conditions and limits its application as a supplementary cementitious material. This research investigates the influence of biochar, added as a partial replacement of silica fume and cement, on shrinkage, hydration, strength and permeability of cement mortar. Biochars were prepared from wood waste and coconut shell and used as 5 wt% cement replacement and 33% replacement of silica fume, respectively. It was observed that combination of biochar (5 wt% of cement) and silica fume (10 wt% of cement) can reduce autogenous shrinkage and drying shrinkage by 61% and 23% compared to the mortar with only silica fume (15 wt%) at 91-day age. By comparison, it is found that biochar with higher permeability and lower pore tortuosity lead to a higher reduction in autogenous shrinkage. Biochar from wood and coconut shell added at 5 wt% of cement has been found to improve hydration, strength and water permeability of mortar compared to control. Combination of biochar and silica fume shows improved 28-day strength compared to control mortar and offers similar strength and water permeability as that of mortar with only silica fume (15 wt%). The research outcomes highlight the benefits of using biochar in high-performance concrete which can reduce the demand for Portland cement and silica fume.

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.

  • 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.

References (69)

  • D. Hatungimana et al.

    Compressive strength, water absorption, water sorptivity and surface radon exhalation rate of silica fume and fly ash based mortar

    J. Build. Eng.

    (2019)
  • A. Dixit et al.

    Waste Valorisation using biochar for cement replacement and internal curing in ultra-high performance concrete

    J. Cleaner Prod.

    (2019)
  • P.J. Gleize et al.

    Effects of metakaolin on autogenous shrinkage of cement pastes

    Cem. Concr. Compos.

    (2007)
  • L. Senff et al.

    Effect of nano-silica on rheology and fresh properties of cement pastes and mortars

    Constr. Build. Mater.

    (2009)
  • A. Shaaban et al.

    Characterization of biochar derived from rubber wood sawdust through slow pyrolysis on surface porosities and functional groups

    Procedia Eng.

    (2013)
  • R.K. Biswas et al.

    Study of short range structure of amorphous Silica from PDF using Ag radiation in laboratory XRD system, RAMAN and NEXAFS

    J. Non-Cryst. Solids

    (2018)
  • E.E. Kwon et al.

    Effects of calcium carbonate on pyrolysis of sewage sludge

    Energy

    (2018)
  • D. Angın

    Effect of pyrolysis temperature and heating rate on biochar obtained from pyrolysis of safflower seed press cake

    Bioresour. Technol.

    (2013)
  • D. Jiao et al.

    Effect of constituents on rheological properties of fresh concrete-a review

    Cem. Concr. Compos.

    (2017)
  • H. Vikan et al.

    Rheology of cementitious paste with silica fume or limestone

    Cem. Concr. Res.

    (2007)
  • K.L. Scrivener et al.

    Advances in understanding hydration of Portland cement

    Cem. Concr. Res.

    (2015)
  • F. Liu et al.

    Internal curing of high performance concrete using cenospheres

    Cem. Concr. Res.

    (2017)
  • A. Quennoz et al.

    Interactions between alite and C3A-gypsum hydrations in model cements

    Cem. Concr. Res.

    (2013)
  • M. Boháč et al.

    Investigation on early hydration of ternary Portland cement-blast-furnace slag–metakaolin blends

    Constr. Build. Mater.

    (2014)
  • G.A. Rao

    Role of water–binder ratio on the strength development in mortars incorporated with silica fume

    Cem. Concr. Res.

    (2001)
  • Z. Qiu et al.

    Biochar-based carbons with hierarchical micro-meso-macro porosity for high rate and long cycle life supercapacitors

    J. Power Sources

    (2018)
  • R. Siddique

    Utilization of wood ash in concrete manufacturing

    Resour. Conserv. Recycl.

    (2012)
  • M. Radlinski et al.

    Investigation into the synergistic effects in ternary cementitious systems containing portland cement, fly ash and silica fume

    Cem. Concr. Compos.

    (2012)
  • H. Du et al.

    Improvement in concrete resistance against water and chloride ingress by adding graphene nanoplatelet

    Cem. Concr. Res.

    (2016)
  • C. Tasdemir

    Combined effects of mineral admixtures and curing conditions on the sorptivity coefficient of concrete

    Cem. Concr. Res.

    (2003)
  • S. Caré

    Influence of aggregates on chloride diffusion coefficient into mortar

    Cem. Concr. Res.

    (2003)
  • A. Itim et al.

    Compressive strength and shrinkage of mortar containing various amounts of mineral additions

    Constr. Build. Mater.

    (2011)
  • K. De Weerdt et al.

    Hydration mechanisms of ternary Portland cements containing limestone powder and fly ash

    Cem. Concr. Res.

    (2011)
  • S. Gupta et al.

    Effect of water entrainment by pre-soaked biochar particles on strength and permeability of cement mortar

    Constr. Build. Mater.

    (2018)
  • Cited by (59)

    • Application of biochar in concrete – A review

      2023, Cement and Concrete Composites
    View all citing articles on Scopus
    View full text