Synergistic effects of rice husk ash and neutral silicate activator on rheology and strength of ambient-cured self-compacting geopolymer concrete

This study investigated the development of ambient-cured self-compacting geopolymer concrete ((SCGC)) using a neutral-grade sodium silicate solution as the sole activator. The binder system comprised ground granulated blast furnace slag (GGBFS), fly ash (FA), and micro silica (MS), with partial substitution of MS by rice husk ash (RHA) at 5–15% to improve sustainability and pozzolanic reactivity. Mixes were evaluated at two binder contents (500 and 600 kg/m³) and two solution-to-binder (S/B) ratios (0.6 and 0.5), with assessments of fresh properties, mechanical performance, and microstructural characteristics. All formulations demonstrated satisfactory workability and cohesive rheological behavior. The early age strength development was significant, with selected RHA-based mixes achieving over 50% of their 28-day compressive strength within 3 days and exceeding 85% by 7 days. The optimal mix, incorporating 600 kg/m³ binder and 15% RHA at 0.5 S/B, exhibited a 26% strength gain over the control mix, with corresponding improvements in tensile and flexural strengths. The SEM analysis revealed dense gel matrices with interpenetrating N–A–S–H and C–A–S–H phases, reduced porosity, and improved gel continuity. The results demonstrate that ambient-cured SCGC based on a hybrid industrial-waste binder constitutes a promising, lower-clinker geopolymer binder system with enhanced fresh, mechanical, and microstructural performance, while long-term durability and structural behavior require further investigation.