Effect of elevated temperature on strength and microstructure of metakaolin-based geopolymer composites having fireclay and silica sands

This study investigates the impact of elevated temperatures on the strength and microstructure of metakaolin-based geopolymer composites incorporating fireclay and silica sands as fillers. The experimental program assessed the mechanical and thermal performance of four geopolymer composite mixes with varying filler types and particle sizes, alongside a reference ordinary Portland cement mix. Specimens underwent exposure to temperatures ranging from 100 °C to 1000 °C for durations of 30 and 120 minutes. Comprehensive evaluations, including compressive strength, flexural strength, scanning electron microscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis, were conducted. The results highlighted that the geopolymer composites exhibited superior thermal stability compared to ordinary Portland cement. At 1000 °C for 30 minutes, the compressive strength of the Portland cement mix dropped by 81.6%, whereas the geopolymer mix with coarse fireclay showed a 74.5% reduction, and the mix with fine fireclay showed a 3.77% increase. After 120 minutes at the same temperature, the Portland cement mix experienced a 79.6% reduction, while the coarse and fine fireclay mixes showed a 26.78% reduction and a 75.4% increase, respectively. Despite some reduction in flexural strength with higher temperature and exposure duration, geopolymer mixes consistently outperformed Portland cement. The critical transition for metakaolin-based geopolymers was observed around 900 °C, where partial melting and void formation began to affect strength. Filler type and particle size played a key role in enhancing thermal resistance, with fine fireclay fillers contributing to improved matrix densification and sintering behavior. Scanning electron microscopy and X-ray diffraction analyses revealed fewer microstructural defects in geopolymer composites compared to ordinary Portland cement, emphasizing their superior thermal resistance in high-temperature environments. This study underscores the potential of tailored formulations of geopolymer composites for applications requiring robust performance under extreme thermal conditions. • Fine fireclay filler enhances compressive strength of geopolymer composite by 75.4% at 1000 °C. • Geopolymer composites show superior thermal resistance to OPC at high temperatures. • SEM analysis confirms fewer microstructural defects in geopolymer composites. • FTIR and XRD reveal structural densification and leucite formation in GC samples. • GC mixes exhibit lower mass loss and better thermal stability in TGA analysis.