Water resistance and hydration mechanism of phosphogypsum cemented paste backfill under composite curing agent...

Cemented paste backfill has made outstanding contributions to the large-scale consumption of phosphogypsum (PG), but poor water resistance significantly weakens the mechanical strength, promotes the leaching of total soluble phosphate (TP) and fluoride ions (F), and reduces its attractiveness in mine engineering. This research synthesized a curing agent (CA) using sodium methylsilicate, sodium silicate, and polyaluminum chloride (PAC). PG produced from Deyang Haohua Qingping Phosphate Mine Co., Ltd. in Sichuan Province, China was modified by CA. Concomitantly, the effects of CA content and soaking time on the softening coefficient, water absorption, mechanical properties, and leaching behavior of phosphogypsum-based cemented paste backfill (PCPB) were investigated. The results indicated that CA displayed advantages in promoting hydration reaction and improving pore structure. Upon the addition of CA, the water absorption of PCPB diminished rapidly, and this trend persisted with the extension of soaking time. More exactly, when the CA content was 3 % and the soaking time was 1 day, the water absorption and softening coefficient obtained the minimum value (4.62 %) and maximum value (0.97), respectively. The softening coefficient rose with increasing CA content within 7 days of soaking time. Nonetheless, when prolonged beyond 14 days, PCPB with higher content CA exhibited a low softening coefficient, which was ascribed to the negative effects of microcracks and pores in the later stage. The enhancement of water resistance is primarily attributed to two aspects. On the one hand, the hydrophobic film formed by sodium methylsilicate played an isolation role. On the other hand, the network structure created by intertwining ettringite and C-(A)-S-H gels optimized the distribution of connected pores and further prevented the migration of water molecules. Encouragingly, the 28-day compressive strength with added CA far exceeded the 0.5 MPa requirement in GB/T 32124-2024. Herein, the mechanical parameters showed a rising and then declining tendency with 2 % CA as the critical content, and the peak was reached at 56 days (compressive strength: 6.08 MPa, flexural strength: 2.01 MPa, tensile strength: 440.32 kPa, elastic modulus: 340.28 MPa). The leaching toxicity test demonstrated that the leaching concentrations of TP and F at various curing ages and different soaking times met the Class I standard limits in GB 8978-1996. These findings can provide theoretical support for the performance regulation of PCPB in mine backfill engineering.