Converting Beechwood Biomass Into Building Materials: Properties of Mortars With Biochar as a Sand Substitute

The construction sector is responsible for a significant share of global greenhouse gas emissions, while the extraction of natural aggregates is increasingly unsustainable. In this context, biochar, a porous carbon-rich material obtained by biomass pyrolysis, has attracted attention as an ecofriendly additive in cementitious composites. This study investigates the effects of beechwood biochar as a partial replacement for natural sand in mortars. Five mortar series were prepared with biochar contents ranging from 0 to 20 wt.%, while keeping a constant cement mass and adjusting the water content according to the mixture composition. Specimens were tested for density, compressive and flexural strength, pull-off adhesion strength, and capillary water absorption at different curing ages. The results revealed a systematic decrease in density and strength with increasing biochar content, reflecting the low density and high porosity of biochar particles. Nevertheless, mortars with up to 10 wt.% biochar exhibited acceptable mechanical performance for potential application in non-structural elements, while offering benefits in terms of lower density and valorisation of renewable biomass waste. The findings highlight the potential of beechwood biochar as a sustainable partial aggregate replacement, contributing to circular economy strategies in the construction industry.