Hydration reaction and microstructural characteristics of hemihydrate phosphogypsum with variable pH

The hydration of hemihydrate phosphogypsum in the alkaline environment was investigated aiming to recycle phosphogypsum-a promising gypsum resource in gypsum-based materials that generally incorporated alkaline binders. Isothermal conduction calorimetry and transmission electron microscopy showed that the hydration reaction of hemihydrate phosphogypsum was significantly changed by alkali. Both the dissolution of hemihydrate particles and crystallization of gypsum were impeded, as a result of phosphate precipitation from neutralization reaction. Consequently, the setting times decreased with the increase of pH and the time span of heat liberation was extended. The impurities in hemihydrate phosphogypsum, especially phosphate, served as setting retarders and crystal modifiers in alkaline environment, contributing to loose crystal microstructure with lower strength. This important finding called for the extra attention to the utilization of hemihydrate phosphogypsum in an alkaline environment.