Influence of the activator concentration of sodium silicate on the thermal properties of alkali-activated slag pastes

Ground granulated blast-furnace slag (slag) pastes were chemically activated using sodium silicate with different Na2O concentrations of 3.5%, 5.5%, 6.5% and 10.5%, by slag weight. The specimens were subjected to elevated temperatures ranging from 200 °C to 1000 °C with an increment of 200 °C for 2 h. Compressive strength before and after firing were measured. Water quenching test was applied to determine thermal shock resistance. The various decomposition phases formed and the morphology of formed hydrates were identified using X-ray diffraction (XRD), thermogravimetric analysis (TGA), derivative thermogravimetric analysis (DTG), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The results indicated that compressive strength before or after firing increased with increasing Na2O concentration up to a certain level, then no extra improvement and deterioration was observed. Higher Na2O concentration had adverse impact on the thermal shock resistance.