Characterization and Splitting Tensile Strength of Marawi City Laterite-Based Geopolymers: Effect of Calcination Temperature

Geopolymers are a sustainable class of inorganic polymeric binders that are synthesized through the alkaline or acidic activation of aluminosilicate precursors. Although various materials have been explored as geopolymer precursors, the use of laterite from Mindanao, Philippines remains limited and underexplored. This study evaluated local laterite from Marawi City in the Philippines as a geopolymer precursor, determining the optimal calcination temperature to enhance reactivity. The laterite samples were calcined at 600, 700, and 800 °C for five hours, then activated with a blended potassium hydroxide (KOH)/sodium hydroxide (NaOH)/sodium silicate solution; and cast into disc molds. The resulting geopolymers were then tested using splitting tensile testing, and their physical, mineralogical, and microstructural properties were characterized. Experimental results showed that thermal activation at 700 °C completely dehydroxylated the kaolinite, resulting in highly reactive amorphous metakaolin, while maintaining the structural stability of the hematite. In contrast, calcination at 600 °C resulted in incomplete dehydroxylation, while thermal treatment at 800 °C induced over-calcination, promoting particle sintering, structural recrystallization, and cristobalite formation. The geopolymer synthesized from the 700 °C precursor (LT700) exhibited the greatest mechanical performance, with a mean splitting tensile strength of 0.6854 MPa, peaking at 0.7903 MPa. Analysis of variance (ANOVA) confirmed that the calcination temperature significantly influenced this indirect tensile performance (F-value = 6.55, p-value = 0.0310). This enhanced strength was consistent with the favorable textural properties of LT700, which had a specific surface area of 52.604 m²/g and a pore volume of 0.1703 cm³/g. These textural characteristics facilitated the penetration of the alkaline activator and the dissolution of the metakaolin, promoting the development of a relatively homogeneous, interconnected (N,K)-A-S-H gel matrix. This matrix is reinforced by stable hematite micro-fillers and secondary crystalline nepheline phases. Calcination at 700 °C is recommended to maximize precursor reactivity and mechanical performance in laterite-based geopolymers. Nevertheless, subsequent enhancement of activator levels, solid-to-liquid ratios, and curing parameters is necessary to reduce the substantial apparent porosity of 43.04%, eradicate drying-induced microcracking, and establish fundamental durability for structural applications.