Effect of phosphorus on the properties of phosphogypsum-based cemented backfill

When phosphogypsum (PG) is used as an aggregate for backfill, phosphate in the PG might influence the hydration process and escape into the environment. The current study aimed to understand phosphate dynamics during the PG-based backfilling process by adding different amounts and types of phosphates (H3PO4, KH2PO4, K3PO4, and Ca3(PO4)2). The results indicate that the majority of the phosphate was first immobilized by PG depending on the types, and the residual dissolved phosphate (RDP) could be further stabilized/solidified (S/S) in the backfill via the hydration process. However, increasing RDP content lowered unconfined compressive strength of the backfill, attributing to the suppression of the hydration process and a loosened backfill structure. Furthermore, the environmental behavior of phosphate was studied by measuring dissolved phosphate in bleeding water and leachate. For bleeding water, a high RDP content might lead to the phosphate concentration exceeding the national standard limit (GB 8978-1996) depending on the phosphate types, and it was recommended that the RDP content should be controlled or converted to Ca3(PO4)2 or K3PO4 before PG inclusion into in the backfill. For leachate, the phosphate concentration was always below the standard limit, indicating that the cemented backfill ensured long-term S/S of the phosphate.