Pozzolanic activity of biochar with high carbon content and its influence on comprehensive strength-emission performance of biochar-cement composite paste

Abstract

Biochar has complex influences on the mechanical properties and carbon emissions of biochar-cement composite concrete. Aiming to prove the beneficial effects of biochar with high carbon content on the compressive strength and carbon-emission reduction of concrete, this study tested the pozzolanic ability of four biochars with their carbon content as high as 78 wt% - 90 wt%, evaluated their pozzolanic activity levels, and further estimated their integrated influences on the compressive strength and carbon emission of biochar-cement composite pastes. The result shows that biochar with high carbon content has pozzolanic activity which is determined by its amorphous SiO₂ content as well as carbon content. Biochar-cement composite pastes especially adding biochar with high pozzolanic activity level present good performances, even has the biochar dosage as high as 20 wt%. The compressive strength of composite paste is ideally linked with the sum amount of hydration products from both cement hydration reaction and biochar pozzolanic reaction. The parameter, relative strength-to-emission ratio, is used to evaluate the comprehensive performance of compressive strength and carbon emission of composite paste. For composite paste with biochar dosage of 20 wt%, its value can reach several times to even tens of times higher than that of biochar-free cement paste, showing that biochar with high carbon content has a huge positive effect on the comprehensive strength-emission performance of biochar-cement composite paste.