Multiscale biochar as cement and sand replacement in mortar

Biochar, with its high carbon sequestration potential and porous structure, offers a promising way for reducing the carbon footprint of concrete. This study investigates the dual role of biochar in mortar as a chemically inert but physically active cement micro filler (MF) and as a fine lightweight aggregate (LWA). The effects on hydration kinetics, flowability, and compressive strength were evaluated, while internal relative humidity (IRH) and autogenous shrinkage were monitored to evaluate internal curing performance. Results show that, when used as cement replacement, biochar acts as an inert MF, improving flowability due to a thicker water film on cementitious particles. Packing density was measured using the wet packing method to determine the water film thickness (WFT), which was used to model and explain flow behavior. When used as sand replacement, biochar acts as fine LWA, improving hydration, sustaining IRH, and mitigating autogenous shrinkage through internal curing. As LWA, unground particles provide more effective internal curing than ground ones. However, strength loss is evident in both applications due to the inherent porosity of biochar.