Development and Performance Evaluation of Biochar–Fly Ash Concrete for Sustainable Construction Applications

Abstract

Portland cement production contributes roughly 7–8% of global CO₂ emissions, which has driven interest in supplementary cementitious materials and carbon-sequestering additives. This study evaluates coconut-shell-derived biochar, used as a cement-weight additive at 0–5%, in a fly ash blended concrete system. Biochar was characterized by EDX (carbon 77–80%, oxygen 15–16%) and physical tests (specific gravity 1.60, fineness modulus 1.83, water absorption 15.8%). Concrete specimens were tested for compressive, split tensile and flexural strength at 7, 14 and 28 days, and for strength loss after sulphate attack, chloride attack and alternate wet–dry cycles. A 1% biochar dose gave the highest 28-day compressive strength (33.8 MPa, +7.0% over the 31.6 MPa control), and 2% gave 33.2 MPa (+5.1%). Doses of 3% and above gave no gain or a reduction. Early-age (7- and 14-day) split tensile and flexural strength improved at 1–3%, but 28-day tensile and flexural strengths were below the control for all dosages. Durability was best at 2% biochar, where strength loss was 2.2% (sulphate), 1.8% (chloride) and 2.8% (wet–dry), against 8.5%, 7.0% and 9.0% for the control. The results indicate that a low biochar dosage of about 1–2% can improve compressive strength and durability, while higher dosages are detrimental.