Modification of microstructure and physical properties of cement-based mortar made with limestone and alum sludge

Alum sludge is a typical by-product of the drinking water industry. Due to the rapid population growth and improved living standard, the amount of stockpiled sludge has increased significantly. In this study, the effects of limestone and alum sludge as cement replacement on the properties of mortar were investigated. 30% of cement was replaced with sludge and limestone, and the mass ratio of sludge to limestone was designed as 1:2, 1:1, and 2:1. The obtained results indicated that the mechanical performance of mortar samples, e.g., compressive strength, flexural strength, and water sorptivity, was improved with the combination of sludge and limestone at the mass ratio of 1:1 and 2:1. Through x-ray diffraction, thermalgravimetric analysis, backscattered electrons images, and mercury intrusion porosimetry tests, such an improvement could be explained by the fact that limestone reactivity raised in a limestone-sludge-cement system due to enrichment with aluminates, and the formation of hemicarboaluminate was promoted. The reaction between limestone and sludge, along with the pozzolanic reaction of sludge, contributed to improved strength and densified pore structure, especially for the refinement of capillary pores. Therefore, the combination of sludge and limestone might increase the cement substitution rate than using one of them alone.