Effect of biochar as filler and partial cement replacement in sustainable concrete

Biochar, a carbon-rich by-product of biomass pyrolysis, has attracted growing attention as a sustainable material for cementitious composites due to its carbon sequestration potential. This study presents an experimental investigation into the use of biochar as both a filler and a partial cement replacement in high-strength concrete, with the aim of reducing the environmental impact associated with cement production. Biochar powder was separated into two particle-size fractions (0–63 μm for cement replacement and 63–125 μm for filler application) and incorporated into concrete mixtures at different proportions (BF0%, BF5%, and BF10%). The experimental programme evaluated fresh properties (workability and setting time), hardened properties (density, compressive strength, stress–strain behaviour, and modulus of elasticity), alkalinity, and microstructural characteristics through Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis. The results indicate that increasing biochar content leads to reductions in compressive strength, modulus of elasticity, density, and workability, primarily due to the porous structure and high-water absorption capacity of biochar. At the highest replacement level, compressive strength decreased by up to 61.6% and density by up to 14.2% compared with the control mix. However, the incorporation of biochar significantly reduced the embodied carbon of the concrete, achieving reductions of up to 88.5%. These findings demonstrate the potential of biochar as a sustainable concrete component capable of substantially lowering carbon emissions, although optimisation of biochar dosage is necessary to balance environmental benefits with mechanical performance.