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

•The effect of sillimanite sand on the properties of AAS mortar at elevated temperatures was investigated.•The strength properties of AAS mortar improved by substituting the silicious normal sand with sillimanite sand.•The compressive strength of AAS mortar prepared using sillimanite sand improved up to the 250 °C exposure.•Sillimanite sand significantly reduced the cracks on the surface of AAS mortar after being exposed to elevated temperatures.