Influence of Hydrothermal Synthesis Parameters of Rice Husk Ash-Derived Sodium Silicate on the Mechanical and Durability Properties of Alkali-Activated Slag Mortars

Ordinary Portland cement (OPC) production accounts for ~8% of global anthropogenic CO₂ emissions, underscoring the urgent need for sustainable binder alternatives. This study develops a low-energy route to sodium silicate by hydrothermally synthesising it directly from Malaysian rice husk ash (RHA), employed in its as-received form without pretreatment. This approach redefines RHA as a biochar-type residue with applications beyond soil management, enabling its valorisation in high-performance construction. Unlike previous methods requiring multi-stage alkali blending, the proposed synthesis at 12M–14M NaOH and 40–80 °C generates sodium silicate suitable as a single-step activator for ground granulated blast furnace slag (GGBFS) mortars. Mortars activated with the 14M–80 °C system achieved a 28-day compressive strength of 76.8 MPa and flexural strength of 8.6 MPa, surpassing OPC and other RHA-derived systems. Durability was simultaneously enhanced, with up to 30% reductions in water absorption and porosity, and ultrasonic pulse velocity values above 4.5 km/s, confirming dense, refined microstructures. One-way ANOVA established the statistical significance of both NaOH concentration and synthesis temperature across mechanical and durability outcomes. By simplifying sodium silicate synthesis and validating its superior performance, this work addresses critical gaps in circular binder development while reducing dependency on carbon-intensive industrial activators. The findings position untreated RHA as a viable waste-derived precursor for alkali silicate production, reinforcing its role in waste-to-resource strategies. Future work on life-cycle performance and field-scale deployment is expected to accelerate its adoption in sustainable, high-strength construction systems.