Effect of Phosphate Slag on the Properties of Silicate Cement Mortars

In this paper, solid waste phosphate slag is introduced into cement to use its phosphates to retard the setting time of cement, which is helpful for improving the mechanical properties of cement while saving energy and environmental protection. In this paper, the mechanism of action of phosphate slag on the cement system was investigated by analyzing the properties of cement setting time, phase composition, microscopic morphology, pore structure, and mortar strength using different phosphate slag contents. The research shows that the iron phosphate contained in the phosphate slag can react with calcium hydroxide—the cement hydration product—and a calcium phosphate barrier layer is formed on the cement particles that effectively prevents the rapid hydration reaction and prolongs the cement setting time. When the content of phosphate slag is 5%, the total porosity of the sample can be reduced by 61.6%, and the 28-d compressive strength can be increased by 24.5%. This discovery gives new value to phosphate slag and turns waste into treasure, which is a typical case of waste recycling.