Developing Si/Al-rich biochar for higher incorporation in building materials: Enhanced mechanics and reduced carbon emissions

Incorporating biochar into building materials offered a viable strategy for reducing carbon footprint. However, high dosages of biochar addition usually degraded mechanical properties of cementitious materials due to raising porosity and weakened biochar-cement interfacial transition zone (ITZ). To overcome this limitation, this study provided a mechanochemical modification strategy to load Si/Al-rich additives onto biochar, enabling it to participate in pozzolanic reaction and thus strengthen the ITZ. Compared to the pristine biochar, the modified biochar exhibited a rougher surface morphology and higher pozzolanic reactivity. At biochar contents of 5 wt% and 20 wt%, the biochar-cement composites showed 8-16% and 12-26% boost in compressive strength, respectively. Hydration kinetics analysis revealed that the modified biochar underwent the pozzolanic reaction to generate additional calcium aluminosilicate hydrate (C-(A)-S-H) gel, which filled the pores and defects in ITZ and enhance the interfacial bonding. Microscopic characterization further confirmed a denser ITZ and the elastic modulus of ITZ in 5BC-RHB and 5BC-MK increased by 12% and 35%, respectively. Besides, the carbon emission and cost assessments showed that the addition of BC-RHB reduced the carbon emission and cost per unit strength of cementitious composites. Overall, this research provided multiscale experimental evidence that using Si/Al-rich biochar improved the ITZ with cement matrix, mitigating the strength loss caused by biochar incorporation and further decreasing the building materials’ carbon footprint.