Rice Husk Ash in Metakaolin-Derived Geopolymer Binders
Maria Kaka Etete Enoh *
University of Cross River State, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Geopolymers are inorganic aluminosilicate binders produced through alkali activation of precursors such as metakaolin, fly ash, and slag using alkaline activators commonly based on sodium hydroxide and sodium silicate. Owing to their lower greenhouse gas emissions and improved durability characteristics, they are increasingly investigated as sustainable alternatives to ordinary Portland cement. Current research efforts are focused on reducing dependence on commercially manufactured activators and increasing the utilisation of biomass-derived waste materials within geopolymer systems.
Rice husk ash (RHA), a silica-rich agricultural by-product containing predominantly amorphous SiO₂, has emerged as a promising supplementary material for modifying geopolymer chemistry through both precursor substitution and activator enhancement. This chapter reviews the chemistry and functional role of RHA in metakaolin-based geopolymer systems and experimentally evaluates the combined influence of RHA incorporation and alkaline activator composition on mechanical performance.
Metakaolin was produced by calcining kaolin at 700 °C, while RHA was obtained through controlled combustion at 600 °C. The experimental programme investigated RHA incorporation levels of 0–10 wt.% and sodium silicate-to-sodium hydroxide (SS/SH) ratios ranging from 0 to 1.0 using a factorial experimental design. Compressive and flexural strengths were evaluated after ambient curing, and statistical analysis was performed using analysis of variance (ANOVA).
The results demonstrated that RHA incorporation significantly improved mechanical performance, with compressive strength increasing by approximately 41% at 10 wt.% RHA and an SS/SH ratio of 1.0. Strength development was governed by the combined effects of silica availability from RHA and soluble silicate concentration within the activator system.
Overall, the findings demonstrate that RHA can serve as an effective and sustainable silica source for tailoring reaction chemistry and enhancing the performance of metakaolin-based geopolymer binders while simultaneously promoting agricultural waste valorisation.
Keywords: Geopolymer, metakaolin, activators, rice husk ash, sustainable materials