Effect of alum sludge ash on the high-temperature resistance of mortar

The effect of high temperature on the mechanical, durability, and microstructural properties of mortar containing alum sludge ash (ASA) was investigated in this paper. The ASA was derived from grinding and calcinating alum sludge, a typical by-product of the drinking water treatment processes. Four different mortar mixtures with ASA content at weight percentages of 0%, 10%, 20%, and 30% (as cement replacement) were exposed to high temperatures of 300 °C, 550 °C, and 800 °C, respectively. The experimental results showed that mortar samples containing up to 20% of cement replaced with ASA exhibited superior high-temperature resistance to the reference ones (without ASA), especially after exposure to 800 °C. The thermal analysis determined the portlandite consumption because of ASA pozzolanic reaction, and the x-ray diffraction pattern showed that the ASA reaction might contribute to the formation of aluminum-bearing phases with excellent refractoriness in the binder matrix. In addition, crack examination conducted by backscattered electron images evidenced that the ASA addition mitigated the binder paste degradation.