Effect of oxalic acid pretreatment on the mechanical properties and microstructure of phosphogypsum

In China, the amount of phosphogypsum (PG) accumulated has been increasing rapidly with the development of the phosphate fertilizer industry. PG exists mainly in the form of dihydrate gypsum, which can replace natural gypsum to produce hemihydrate gypsum and is used in the building materials industry. However, it contains impurities, such as phosphate, fluoride, sulfate, and natural radioactive elements, which limit its application and seriously pollute the surrounding environment. To promote the application of PG, it is crucial to understand how pretreatment affects the properties of hemihydrate phosphogypsum (HPG). In this research, the mechanical properties and microstructure of HPG after pretreatment was investigated for improving the reuse of PG as a promising resource. The harmful phosphorus impurity in PG was efficiently removed with different concentra-tions of by H2C2O4. Then the hydration process and the hardening characteristics of HPG were studied in detail. The findings indicate that H2C2O4 alters the particle characteristics of HPG and has an impact on hemihydrate gypsum dissolution and dihydrate gypsum precipitation. According to studies, small amounts of H2C2O4 can remove impurities, promoting hydration and enhancing strength by opening the closed orifice of HPG, which would promote the dissolution of HPG and alter the morphology of gypsum from a tiny crystal to a thickened columnar structure. However, the addition of excessive H2C2O4 impacts the compression strength of HPG plaster and reduces the rehydration of HPG due to the formation of calcium oxalate on the gypsum crystal surface. These new findings may give insight into how H2C2O4 affects the performance of HPG. Therefore, to ensure the efficient utilization of PG, quality stabilization and improvement are crucial.