Sustainable Reuse of Waste Palm Oil Fuel Ash in Geopolymer Stabilization of Silty Soil: a Comprehensive Assessment of Mechanical Strength and Leaching Toxicity for Environmentally Conscious Infrastructure Development

Soil plays a vital role in infrastructure development, but certain types, such as silty soil, require stabilization due to their poor load-bearing capacity. With a growing emphasis on sustainable construction highlights palm oil fuel ash (POFA), a byproduct of biomass combustion in palm oil mills, as a viable alternative for soil stabilization, particularly in regions such as Sultan Kudarat, Philippines, where it is readily accessible. This study addresses the limited research on using POFA-based geopolymer for stabilizing silty soils, particularly in evaluating its environmental impact, and applicability in infrastructure development. It aims to assess the effectiveness of POFA-based geopolymer in enhancing the strength, stability, and load-bearing capacity of silty soil in Sultan Kudarat. A post-test-only controlled group experimental design was used to evaluate the effectiveness of POFA-based geopolymer in stabilizing silty soil, and mechanical strength tests were conducted on both the natural soil and treated samples. The study assessed shear strength, compressive strength, and California bearing ratio (CBR) using ASTM-standard methods alongside a Toxicity Characteristic and Leaching Procedure (TCLP) to evaluate environmental safety. Results showed that POFA-based geopolymer significantly improved shear strength, with optimum performance at 27.5% and 30% of POFA after 28 days of curing. Whereas compressive strength and CBR values were lower than in natural soil, treated samples showed enhanced ductility, reduced swell potential, and stable failure modes. TCLP confirmed that all heavy metals tested were below regulatory limits, with a non-corrosive pH of 10.95 and no hazardous reactivity or flammability observed. These results highlight POFA-based geopolymer as a safe, environmentally friendly option for stabilizing silty soils and offer potential for sustainable infrastructure development.