Abstract
This study evaluates the valorization of rice husk ash (RHA) as a silica-rich precursor in metakaolin (MK)-based geopolymers, transforming a low-density agricultural residue into a functional mineral additive that reduces dependence on costly, energy-intensive synthetic silica sources. Three binary mix designs partially replacing MK with 5, 25, and 50% RHA, along with a control sample containing 100 wt % MK, were synthesized using a 10-M NaOH activator with a fixed silica modulus (Ms) of 1.68. Mechanical performance was assessed via compressive and flexural testing after 7 days of early curing. Results indicate that the optimum design, containing 25 wt % RHA, enhanced mechanical strength through supplemental reactive silica, whereas incorporation of 5 wt % RHA led to a strength decline due to increased porosity, as evidenced by microcracks. These findings were corroborated by XRD, FTIR, and SEM, which showed that the geopolymerization process was optimized by creating a more silica-rich amorphous network and a dense microstructural matrix, despite the presence of non-reactive quartz and cristobalite fillers. The findings establish an optimal substitution threshold for MK-RHA geopolymer binders, integrating biomass waste into a circular construction economy.
