An Experimental Performance Evaluation: Optimizing Biochar and TiO₂ Synergies in Concrete

Abstract

This study presents an experimental investigation into the fresh and hardened properties of M30 grade concrete incorporating two supplementary admixtures — biochar (BC) and anatase-phase titanium dioxide (TiO₂ nanoparticles) — as partial cement replacements. Three mix variants were evaluated: Specimen(S1) (1% BC + 0.5% TiO₂), Specimen(S2) (1% BC + 1.75% TiO₂), and Specimen(S3) (2% BC + 3% TiO₂) by weight of OPC Grade 43 cement. Fresh workability was assessed via slump testing (IS 1199:1959), while hardened mechanical performance was evaluated through compressive strength on 150 mm cubes (IS 516) and split tensile strength on 300×150 mm cylinders at 7-day and 28-day curing ages. The optimum mix (S1) attained a 7-day compressive strength of 41.19 MPa — exceeding the M30 target — and a 7-day split tensile strength of 23.86 MPa. The observed slump of 110 mm, classified as medium-to-high workability (IS 456:2000), confirms the effectiveness of pre-saturation of biochar and optimised mix proportioning. An atypical strength regression between 7 and 28 days is documented and mechanistically attributed to accelerated early hydration by nano-TiO₂, subsequent thermal eigenstress accumulation, and micro-crack propagation at biochar–paste interfaces.