Empty Fruit Bunch Biochar as a Green Additive in Cement Mortar: Engineering Performance and Potential for Biogenic Carbon Retention

Empty fruit bunch (EFB) biochar was investigated as a partial replacement for fine aggregate (sand) in cement mortar, with emphasis on early-age engineering performance and the potential retention of biogenic carbon in a cement-based matrix. EFB was thermally treated at 280, 320, and 360 °C to compare yield, carbon-rich composition, and pore development; 320 °C was subsequently selected as a practical processing condition within the investigated range. Mortars with EFB biochar replacement levels ranging from 0 to 11 wt.% of sand were prepared as 50 × 50 × 50 mm cubes and water-cured for 7 and 28 days. At 28 days, the 3% mixture exhibited the highest measured mean compressive strength of 38.3 MPa, compared with 32.3 MPa for the control. Although the mean strength was generally lower at higher replacement levels, a non-monotonic response was observed, with the 9% mixture reaching 35.6 MPa before decreasing markedly at 11% replacement. Density decreased and bulk accessible porosity increased with increasing biochar content. SEM/EDS observations showed carbon-rich biochar-associated regions within the examined mortar fracture surfaces; however, these localized observations do not by themselves establish specific hydration phases, pozzolanic reactions, uniform dispersion, or long-term durability. The environmental contribution discussed in this study is therefore framed as the potential retention of biogenic carbon rather than quantified net carbon sequestration, which would require dedicated lifecycle carbon accounting.