Mechanical Properties, Microstructural Characterisation and Durability of High-Performance Concrete Incorporating Rice Husk Ash and Silica Fume as Supplementary Cementitious Materials

The partial replacement of Ordinary Portland Cement (OPC) with pozzolanic supplementary cementitious materials (SCMs) addresses simultaneously the construction industry's twin imperatives of structural performance enhancement and carbon footprint reduction, given that OPC production contributes approximately 8% of global CO₂ emissions. Rice Husk Ash (RHA), the siliceous residue from controlled combustion of agricultural rice husks — generated at approximately 25 million tonnes per year in India alone — and condensed Silica Fume (SF), a by-product of silicon and ferrosilicon alloy production, are both established high-performance SCMs whose individual effects on concrete properties are well documented. Ternary blends combining RHA and SF at optimised proportions have been proposed in the literature as potentially synergistic, but comparative systematic data under Indian construction material conditions and temperature regimes remain limited. This study investigates the fresh, hardened mechanical, and durability properties of M25 grade concrete incorporating RHA (10%, 20%, 30% cement replacement by weight) and SF (10%, 20% replacement) and a ternary blend (10% RHA + 10% SF) across five mix designs. Properties evaluated include workability (Vebe time, compacting factor), compressive strength at 28, 56, and 90 days, flexural and split tensile strength, water absorption, chloride permeability by RCPT, and SEM/EDX microstructural analysis at 28 days. Load-deflection response of reinforced beams (150×200×1200 mm) and Mercury Intrusion Porosimetry (MIP) pore structure evolution at ages 3-90 days provide structural performance and microstructural development data. M25+20%SF achieves 90-day compressive strength of 44.6 MPa (32% above control), outperforming all RHA mixes. The ternary blend achieves 90-day strength of 42.1 MPa with chloride permeability of 312 C (RCPT) — a 24% reduction versus control — and CO₂ emissions of 312 kg/m³ (24% reduction). SEM analysis confirms dense interfacial transition zones and reduced calcium hydroxide crystallinity in SF-modified specimens. EDX reveals higher Si/Ca ratios in SF mixes, consistent with enhanced secondary pozzolanic reaction product formation.