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 contributes approximately 8% of global anthropogenic CO₂ emissions, driving the need for sustainable binder alternatives. This study investigates a comparatively lower-temperature hydrothermal route for synthesizing sodium silicate (Na₂SiO₃) directly from Malaysian rice husk ash (RHA) in its as-received form, without pretreatment. The process, conducted at 12–14 M NaOH and 40–80 °C, produces sodium silicate suitable as a single-step activator for blast furnace slag (BFS) mortars. Mortars activated with the 14 M–80 °C solution achieved compressive strengths of 76.8 MPa at 28 days and 85.3 MPa at 91 days, with flexural strength up to 8.6 MPa. Durability performance improved, showing reductions of up to ~ 30% in water absorption and porosity, and ultrasonic pulse velocity exceeding 4.5 km/s. Two-way ANOVA confirmed the statistical significance of NaOH concentration and synthesis temperature on both mechanical and durability properties. The findings indicate that hydrothermally synthesized RHA-derived sodium silicate can effectively activate BFS under the specific ambient curing conditions investigated. By simplifying silicate synthesis and valorizing untreated RHA. By simplifying silicate synthesis and valorising untreated RHA, this study provides preliminary evidence supporting the potential of lower-temperature silicate production routes. However, comprehensive life-cycle and techno-economic assessments are required to confirm overall environmental benefits.