Preparation of phosphogypsum-based filling materials and leachability of harmful substances from cemented bodies

The industrial waste phosphogypsum of phosphorus chemical enterprises has a low utilization rate and a large stockpile, causing serious pollution to the surrounding environment. To analyze the effects of mass concentration, phosphogypsum dosing, and fly ash dosing on the working performance and strength of phosphogypsum to effectively utilize this resource and minimize its adverse impact on the environment, the study uses phosphogypsum as the primary component while controlling variables. When the mass concentration is 75%-76%, the phosphogypsum doping is less than 70%, and the fly ash doping is 18%-20%, the fluidity is 180-220 mm, the bleeding rate is 1%-6%, the yield stress is decreased by 37% and 19% respectively, and the plastic viscosity is all lower than 1.60 \mathrmPa\cdot \mathrmS , which can satisfy the working performance of the filling material and the filling strength of the mine. Simultaneously, the test using an X-ray diffractometer（XRD） and scanning electron microscope（SEM） examines the hardening of the microstructure of the filling material and its morphology. As the amount of phosphogypsum doping increases, the number of calcium alunite crystals; and hydrated calcium silicate gel hydration products decrease gradually, leading to a decrease in structural compactness. Through analyzing the working performance and strength of the filling material, the optimal composition for phosphogypsum-based filling slurry is determined as follows: 75% mass concentration, 65%-70% phosphogypsum doping, and 18%-20% fly ash doping. It is found through the harmful substance immersion test that the contents of total hardness, sulfate, and fluoride in the leachate of the filling body increase with the increase of immersion age in the initial stage, but then stabilize. In contrast, the content of total phosphorus continues to increase with the age of immersion. The test results reveal that the leachate of phosphogypsum-based filling materials comply with the specified concentration standards for total hardness, total phosphorus, sulfate, and fluoride. Moreover, heavy metals like Fe, Zn, Pb, and As are absent in the leachate, demonstrating the environmentally friendly nature of the filling materials with low pollution potential.