Author: Chen, Liang
Title: Application of biochar in sustainable cement-based composites
Advisors: Tsang, Dan (CEE)
Wang, Lei (CEE)
Degree: Ph.D.
Year: 2022
Subject: Biochar
Cement composites -- Environmental aspects
Composite materials -- Environmental aspects
Hong Kong Polytechnic University -- Dissertations
Department: Department of Civil and Environmental Engineering
Pages: xix, 158 pages : color illustrations
Language: English
Abstract: Biochar is a known product to permanently remove carbon from its cycle. It is essential to find high-quality and large-quantity utilization for biochar. Application of biochar in cement-based composite to produce low carbon and even carbon-negative construction material is a promising technology to achieve circular economy and carbon neutrality.
This study investigated the roles of carbon-negative rice husk biochar (RBC) and yard waste biochar (YBC) as green additives in the cement system. Experimental results illustrated that the addition of both biochars promoted cement hydration reaction via pozzolanic reaction and internal curing. In particular, the incorporation of 10 wt.% RBC (rich in activated Si) significantly increased the content of calcium-silicate-hydrate (C-S-H) gel from 41.6 wt.% (control sample) to 52.0 wt.% and increased the average degree of connectivity of C-S-H gel from 1.43 to 1.52 as indicated by quantitative X-ray diffraction and 29Si nuclear magnetic resonance analysis. In addition, the low-carbon biochar cement binder was applied for stabilization/solidification (S/S) of municipal solid waste incineration (MSWI) fly ash. The incorporation of RBC and YBC (20 or 30 wt.%) enhanced the immobilization efficiency of potentially toxic elements in MSWI fly ash due to the additional hydration products and high adsorption ability of biochar. For instance, in R-80FA and Y-80FA samples (namely, 20 wt.% binder dosage, of which RBC or YBC accounted for 10 wt.% of binder), the immobilization efficiency for Pb could reach 96.2% and comply with the leachability limit. The biochar-modified S/S blocks achieved comparable strength to the cement-based S/S blocks, presenting a mechanically stable solidified matrix for engineering application. Therefore, this study expands the emerging application of biochar and demonstrates that biochar-augmented binder can ensure low-carbon and high-performance S/S of hazardous materials.
This study also assessed the efficacy of biochar on the hydration of magnesia cement (MC) and magnesia cement−Portland binary cement (MP)-based pastes and evaluated the synergistic effect of biochar and CO2 curing on the pastes. The thermogravimetric and X-ray diffraction analyses showed that the incorporation of biochar, especially CO2 gasification biochar, promoted the generation of hydration products due to the internal curing effect. The use of CO2 curing effectively accelerated the carbonation of pastes. Hydrated magnesium carbonates were preferentially formed in CO2-cured MC pastes, whereas CaCO3 was preferentially generated in CO2-cured MP pastes. Moreover, the incorporation of biochar, especially porous CO2 gasification biochar, could further facilitate CO2 diffusion and promote carbonation. As a result, the synchronous use of biochar and CO2 curing significantly enhanced the mechanical strength of blocks. Therefore, biochar-augmented and CO2-enhanced composites could be novel and low-carbon construction materials for sustainable engineering applications.
Based on the above findings, this study proposed a revolutionary design of carbon-negative concrete with a large volume of biochar incorporation and elaborated the roles of biochar in the cement hydration and microstructure development of biochar-augmented concrete. The total CO2 emissions and economic values of biochar-augmented concrete were, for the first time, quantified by conducting life cycle assessment and cost-benefit analysis. Pre-soaked biochar as aggregate in concrete promoted the cement hydration process, facilitating the formation of C-S-H gel and enhancing the polymerization degree of C-S-H gel via internal curing. The incorporation of supplementary cementitious materials (SCMs) in the binder further enhanced the mechanical strength of biochar-augmented concrete via time-dependent pozzolanic reaction. The life cycle assessment confirmed that the biochar incorporation significantly reduced CO2 emissions, and most importantly, the combined use of biochar and SCMs successfully achieved carbon-negative concrete production. Preliminary cost and benefit analysis illustrated that the biochar-augmented concrete could yield satisfactory overall economic profits. Considering the mechanical performance, resource availability, negative CO2 emissions, and economic profits, the 30BC-MK (with biochar as aggregate and metakaolin as a binder representing 30 wt.% and 9 wt.%, respectively) was the most promising mixture, which could sequester 59 kg CO2 tonne-1 and potentially generate the overall profit of 35.4 USD m−3. In summary, our novel design of biochar-augmented concrete can open up a new field of biochar application that produces technically feasible and financially profitable carbon-negative construction materials.
Rights: All rights reserved
Access: open access

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