Comparative Performance of Rice Husk Ash and Palm Oil Fuel Ash in Metakaolin Geopolymers

Abstract

The growing demand for sustainable construction materials and effective waste valorisation has stimulated interest in the utilisation of agricultural wastes as partial substitutes for aluminosilicate precursors in geopolymer production. Despite growing interest in biomass ashes for geopolymer synthesis, direct comparisons of rice husk ash (RHA) and palm oil fuel ash (POFA) under identical metakaolin-based geopolymer conditions remain limited. This study investigated the influence of POFA and RHA on the physical and mechanical properties of metakaolin-based geopolymers. Geopolymer specimens containing 0–30 wt.% POFA or RHA were synthesised using a 10 M sodium hydroxide solution, a sodium silicate-to-sodium hydroxide ratio of 1, and an activator-to-binder (liquid-to-solid) ratio of 0.80, followed by ambient curing at 25 ± 3 °C for 28 days. The materials were characterised using X-ray fluorescence (XRF), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy, while density, water absorption, and compressive and flexural strengths were evaluated. The characterisation results revealed that RHA was predominantly composed of amorphous silica, whereas POFA contained a higher calcium concentration and a greater proportion of crystalline phases. Increasing the biomass ash content reduced the density and increased the water absorption of the geopolymer composites. However, moderate biomass ash incorporation enhanced mechanical performance. Maximum compressive strengths of 23.83 MPa and 23.90 MPa were achieved at 20 wt.% POFA and 15 wt.% RHA, respectively, while peak flexural strengths of 5.71 MPa and 5.51 MPa were obtained at 15 wt.% replacement. Further increases in biomass ash content resulted in a gradual reduction in strength. Overall, both POFA and RHA demonstrated comparable potential as sustainable reinforcement materials for metakaolin-based geopolymer composites, with an optimum replacement range of 15–20 wt.%, providing an effective balance between physical and mechanical performance. The findings demonstrate the potential for utilising locally available agricultural wastes to develop lower-carbon geopolymer binders for sustainable construction applications.