Biochar Addition Affects the Performance of Portland Cement Composites: Meta-analysis and Laboratory Experiments
Creators: Zhao, Zhihao
Publisher: name: University of Alberta Library
Year: 2024

<p>Portland cement is the most widely used binder in construction. However, the production of Portland cement emits a large amount of CO2, and one strategy to decrease such anthropogenic CO2 emissions is to reduce the amount of Portland cement produced by partially replacing it with supplementary cementitious materials (SCMs). Biochar is a potential SCM that is a stable porous pyrolytic material, which may improve the performance of Portland cement composites and increase carbon sequestration simultaneously. However, due to limited research, the effects of biochar addition on the performances of Portland cement composites are not fully understood. The performance of different forms of biochar, when mixed with Portland cement composites, has also not been fully synthesized. Such knowledge gaps require a quantitative assessment of the effects of biochar addition, particularly the addition of different biochar types.</p>

<p>Meta-analysis was used to investigate the impact of biochar addition on the 7- and 28-day compressive strength of Portland cement composites based on 606 paired observations. Biochar feedstock type, pyrolysis condition, pre-treatment and modifications, biochar dosage, and curing type all influenced the compressive strength of Portland cement composites. Biochars obtained from plant-based feedstocks (except rice and hardwood) improved the 28-day compressive strength of Portland cement composites by 3-13%. Biochars produced at pyrolysis temperatures higher than 450 °C, with a heating rate of around 10 °C min-1, increased the 28-day compressive strength more effectively. Furthermore, adding biochar with small particle sizes (D90 is around 40 μm) increased the compressive strength of Portland cement composites by 2-7% compared to those without biochar addition. Biochar dosage &lt; 2.5% of the binder weight enhanced both the 7- and 28-day compressive strengths, and common curing methods maintained the effect of biochar addition. However, when mixing the cement, adding fine and coarse aggregates such as sand and gravel affects the concrete and mortar’s compressive strength, diminishing the effect of biochar addition and making the biochar effect nonsignificant. </p>

<p>Laboratory experiments were conducted to explore the effects of the addition of different forms of biochar on the performances of Portland cement composites. This study compared oat hull biochar with sawdust biochar, with different particle sizes, to explore their effects on concrete’s mechanical performances and durability, including fresh properties, compressive strength, and water sorptivity. The quality of cement paste under the same batching design was also assessed to exclude the aggregate effects. This study found that biochar addition decreased slump by 29% and increased fresh air content by 18% on average. Although it did not affect the overall compressive strength, the smaller particle size and lower biochar dosage significantly increased 28-day compressive strength by more than 20%. In addition, adding biochar decreased initial sorptivity by 39% on average and increased secondary sorptivity by 31% on average but did not affect total water absorption. Moreover, biochar addition altered the quality of cement paste, including setting time, compressive strength, and hardened density. However, since there were only one or two significant relationships among performances of cement paste and concrete and their effect sizes, based on Pearson correlation analysis, the changes in the quality of cement paste did not directly reflect changes in concrete performance. </p>

<p>I conclude that the appropriate addition of biochar could partially improve the mechanical performance and durability of concrete and alter the quality of cement paste. However, it is important to note that not all changes in the quality of cement paste were correlated with performance changes in concrete. Future research should explore and fully understand the mechanisms of biochar effects on the performance of Portland cement composites and how biochar works in the cementitious matrix, especially considering how biochar cooperates with aggregates, and should also consider expanding the selection of different biochars to optimize biochar application.</p>