Effect of Granodiorite Sand Content and Particle Size on the Mechanical and Thermal Performance of Metakaolin-Based Geopolymer Mortar.

In this study, the coupled effects of granodiorite sand particle size and content on the mechanical and thermal performance of metakaolin-based geopolymer systems are systematically investigated using a holistic, geometry-sensitive approach. For block-type specimens compressive and flexural strengths, together with postheating mass loss after exposure to 300 °C, are evaluated to elucidate strength retention and thermal stability. The results demonstrate that granodiorite particle size plays a decisive role in governing the balance between mechanical integrity and thermal insulation. Coarse particles (0.5-1.0 mm) at 30 wt % content provided the highest mechanical performance, achieving a compressive strength of approximately 50-68 MPa depending on the skeletal structure formation. Thermal stability tests at 300 °C showed that incorporating granodiorite effectively controlled degradation, with mass loss ranging from 3.4% to 9.1%, where the BG1-20 composition outperformed the control sample's 5.7% loss. For plate-type specimens, thermal insulation was assessed under 700 °C flame loading. Fine granodiorite particles (<0.25 mm) markedly improved insulation by reducing back-surface temperatures to as low as 93 °C, significantly lower than the 207 °C observed with coarse particles. Overall, the findings reveal an inverse yet predictable relationship between mechanical strength and thermal insulation, providing a rational framework for designing geopolymer blocks and plates tailored to application-specific requirements.