Geopolymer with improved thermal stability by incorporating high-magnesium nickel slag

The present study investigates the thermal stability of binary geopolymers prepared with fly ash and high magnesium nickel slag (HMNS). TGA, XRD and SEM techniques are adopted to investigate the chemical composition and microstructure of the binder gel phase exposed to high temperatures. The results show that incorporating HMNS mitigates dehydration induced mass loss and volume shrinkage of the geopolymer samples between room temperature and 250 °C, and increases their residual strengths when exposed to high temperatures between 600 °C and 800 °C. The key role that HMNS substitution of improving the thermal stability of geopolymers is associated with the formation of sodium-alumina(magnesia)-silica hydrate (N-A(M)-S-H) gel phase, which turns to be a less porous microstructure after thermal exposure when compared with the N-A-S-H gel phase, which is formed in the pure fly ash sample. The binary geopolymers exhibit higher residual strengths and better maintaining of volume stability in the range from 250 °C to 550 °C compared to Portland cements. However, both the geopolymer and Portland cement show severer volume shrinkage after exposure of 600 °C, because of the viscous sintering of geopolymer gels and the decomposition of cement hydration products, respectively.