Characteristics of rice husk ash-sodium aluminate geopolymer at elevated temperature

This investigation was focused on the effect of elevated temperature on rice husk ash (RHA) geopolymers activated with sodium (Na) aluminate. Geopolymers were synthesised by varying the silicon (Si)/aluminium (Al) atomic ratio from 1.5 up to 3.5 and maintaining a sodium/aluminium ratio of 1. The specimens were exposed to 300, 600 and 900 degrees C. The effects of elevated temperature were studied in terms of physical changes, weight loss, volumetric shrinkage and residual strength. The composition and microstructure were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and thermogravimetric analysis. The specimen colour changed gradually from grey to white with increase in temperature. The specimens showed no tangible surface cracks up to 300 degrees C. Macrocracks on the order of 0.1-0.3mm were noticed after 300 degrees C, which further increased up to 0.3-1.0mm at 900 degrees C. The specimens showed rapid shrinkage and weight loss up to 300 degrees C. The geopolymer specimen with a silicon/aluminium ratio of 2 showed the highest residual compressive strength of 79.11% for 900 degrees C. XRD analysis indicated quartz, cristobalite and gibbsite as the main constituents of the specimen with phase transformation of cristobalite to tridymite at around 900 degrees C. SEM micrographs of exposed specimens show sintering of gel and unreacted RHA grains.