Effect of sillimanite sand on the mechanical property and thermal resistance of alkali-activated slag mortar

The change in the properties of fine aggregate has significant potential in altering the efficacy of alkali-activated slag. Therefore, this study investigated the influence of sillimanite sand on the mechanical and microstructural properties of alkali-activated slag (AAS) mortar at ambient temperature, and after exposure to elevated temperatures (250, 500, 750, and 1050 °C). In addition, the effect of the concentration of sodium hydroxide solution (6, 8, 10, and 12 M) was evaluated. The silicious normal sand was replaced with sillimanite sand at 25, 50, 75, and 100 % by volume. The results show that the superior physical properties of sillimanite sand improved the strength of the AAS mortar, compared with silicious normal sand. After heating at 250 °C, all AAS mortar mixes showed a significantly higher increase in compressive strength compared to unheated samples. Utilization of the largest concentration of sodium hydroxide solution (12 M) as well as sillimanite sand resulted in a residual compressive strength equivalent to the control sample after 500 °C temperature exposure. A significant drop in residual compressive strength was observed for all mixes at 750 °C. Visual observation of the heated samples revealed that casting the mortar samples with sillimanite sand limited the number, width and extent of thermal cracks. XRD analysis revealed the decomposition of C-S-H and the formation of silica oxide at 750 °C. The results of this study suggest that replacing silicious normal sand with sillimanite sand can help the modification of the engineering properties of AAS mortar at ambient temperature, and after exposure to elevated temperatures.