Performance Impairment of Greener Phosphogypsum Binder under Alkaline Conditions: Phenomenon and Mechanism Analysis

The sustainable utilization of hemihydrate phosphogypsum (HPG) in building materials is crucial for reducing industrial waste and promoting eco-friendly practices. However, its performance is sensitive to pH variations, which can impede its engineering applications. To understand the effect of pH on phosphogypsum hydration, this study examined the strength, hydration heat, hydration rate, ion concentration, phase composition, and microstructure of HPG and hemihydrate flue gas desulfurization gypsum (HFGD) under varying pH conditions. The results showed that pH had little effect on HFGD but significantly affected HPG. At pH 6.13, HPG had a 2 h strength of 6.57 MPa, a single hydration peak, and a 90% hydration rate in 1 h. At pH 8.53, the strength dropped to 0.93 MPa, the hydration peak almost disappeared, and the hydration rate was 57.62% in 10 h. At pH 11.62, strength increased to 5.67 MPa, with two hydration peaks and a 90% hydration rate in 2 h. Further ion analysis in the slurry indicates that the release and transformation of HPO42– under different pH conditions mainly affect phosphogypsum properties. In low acidity (pH = 5–7), low HPO42– content minimally impacts hydration. In low alkalinity (pH = 7–10), substantial HPO42– release severely hinders hydration. At higher alkalinity (pH = 10–12), abundant HPO42– gradually converts to insoluble calcium phosphate, reducing inhibition and causing a second exothermic peak. This research highlights the importance of controlling alkalinity and HPO42– content to optimize the HPG cementitious performance, thereby supporting cleaner production methods and advancing sustainable construction practices.