Preparation of anhydrite from phosphogypsum: Influence of phosphorus and fluorine impurities on the performances

Phosphogypsum (PG), as a bulk industrial solid waste with high and complicated impurities content, makes it difficult to be utilized. In this paper, raw PG by adding the soluble phosphorus (P2O5) and fluorine (F) content of 0–1.5 wt% was used to prepare anhydrite with calcination treatment method, and the influence of P2O5 and F impurities on the hydration and hardening properties of anhydrite was investigated. Results show that obvious defects appeared in anhydrite facets due to the formation of calcium pyrophosphate (Ca2P2O7), calcium fluoride (CaF2). Besides, sodium sulfate (Na2SO4) was formed, resulting in the accelerated hydration process. Moreover, Na2SO4 presents a better improvement, which made a higher hydration degree at an early age (3 days). However, the dihydrate crystal size was refined due to the formation of CaF2, resulting in reduced early strength. Remarkably, the later strength (28 days) of all the samples shows similar values, mainly due to the similar hydration degree and microstructure. Besides, the later strength and microstructure of anhydrite prepared from PG2 with different impurities and morphology are similar to PG1. The work shows a potential application for PG utilization and lays a foundation for preparing anhydrite from PG.