Upcycling of phosphogypsum as anhydrite plaster: The positive effect of soluble phosphorus impurities

The utilization of phosphogypsum (PG) to prepare anhydrite could massively consume its stockpiling and obtain stable and excellent gypsum materials. In order to effectively recycle and reuse PG, the influence mechanism of varying forms of soluble phosphorus impurities on the properties of anhydrite should be evaluated. In this paper, original PG respectively containing H3PO4 and Ca(H2PO4)2·H2O were proposed to prepare anhydrite, and their hydration, mechanical strength, volume stability, microstructure, and pore structure were investigated. Further, the mechanism behind soluble phosphorus impurities from the insight of crystal defects was discussed. The results show that H3PO4 and Ca(H2PO4)2·H2O could accelerate the anhydrite dissolution and dihydrate gypsum nucleation due to the increased crystal defects and the formation of the nucleation site. The early mechanical strength of anhydrite significantly increased due to its high hydration degree, compact microstructure, and fine pore size. H3PO4 and Ca(H2PO4)2·H2O possessed a similar beneficial effect regardless of the content. Remarkably, H3PO4 and Ca(H2PO4)2·H2O had slight effect on the late properties of anhydrite, and all the samples presented similar mechanical strength, hydration degree, microstructure, and pore structure. This work indicates that preparing anhydrite from PG is a feasible solution to recycle PG with high efficiency and high added value without considering its unstable and diverse phosphorus impurities.