Effect of crystalline admixtures on shrinkage and alkali-silica reaction of biochar-cementitious composites

This study investigated effects of crystalline admixture (CA) on shrinkage and alkali-silica reaction behaviours of biochar-cementitious composites. Addition of 1–1.5 wt% superabsorbent polymer (SAP) completely mitigated autogenous shrinkage while slightly increasing the 120-day total shrinkage of the SP-cement composite by 5.7%, resulting in the highest apparent porosity. 1–1.5 wt% CA addition did not affect autogenous shrinkage while slightly reducing the 120-day total shrinkage by 10.1%. The combination of CA and waste wood biochar (WWB) reduced autogenous shrinkage by 24.23% and 120-day total shrinkage by 23.6%, resulting in the lowest apparent porosity. The formation of hydration products in the WWB pores and on WWB surface densified the cementitious matrix, leading to a reduction in water evaporation. Furthermore, for specimens exposed to 1 M NaOH solution at 80 °C, CA addition significantly reduced the 120-day expansion by 50.6%, while the combination of CA and WWB addition reduced the 120-day expansion by 42.9%. •CA addition has no effect on the mitigation of autogenous shrinkage of the cementitious composites.•Apparent porosity is highly correlated with the results of total shrinkage for all cementitious samples.•1.0 wt% CA addition leads to 10.1% total shrinkage reduction by preventing water moisture evaporation.•CO group showed the highest mortar expansion, due to excessive formations of ASR products.