Effect Of Phosphorus On The Material Properties And Environmental Behavior Of Phosphogypsum-based Cemented Backfill

The high phosphate content in phosphogypsum（PG）enhances eutrophication,therefore posing a threat to the ecological stability of the surroundings.As a typical solidification/stabilization（s/s）process,the PG-based cemented backfill technology can effectively immobilize the phosphate in PG.Besides,the hardened backfill can function as the support for the stope roof and alleviate surface subsidence,which will help the safe and efficient mining of phosphate ore.However,residual phosphate in PG may interfere with the hydration process.It is necessary to systematically explore the dynamic evolution of phosphorus in the backfill process,and clarify the impacts of phosphate on the material properties and environmental behavior of the PG-based backfill.Based on experiments and combined with theoretical analysis,this study is to comprehensively evaluate the impacts of phosphate on the properties and environmental performance of the PG-based backfill.The main conclusions are drawn as follows:（1）The contents of dissolved phosphate and total phosphate in 37batches of PG were determined,and the relationship with PG’s physical and chemical properties was clarified.The phosphate content in PG fluctuated wildly,and the content of dissolved phosphate is closely correlated with the water content of PG,fluorine content,p H value,electrical conductivity,total dissolved solids,and salt content of the PG leachate;（2）The influence of phosphate on the material properties and the environmental behavior of the PG-based backfill was studied by taking the phosphorus content in PG as variables.The results indicate that the high content of dissolved phosphate aggravates the deterioration of the material properties of the backfill and the harm to the environment;（3）The effects of different phosphates on the properties and environmental behavior of backfill materials were studied.The results show that the negative impact of phosphate on the properties of backfill materials and environmental hazards are in the following order:H₃PO₄>KH₂PO₄>K₃PO₄≈Ca₃（PO₄）₂;（4）The control standard for dissolved phosphate when PG is used as aggregate is proposed for the first time.The residual dissolved phosphate content in PG should be limited to 87 mmol/kg to reduce the influence of phosphorus.Besides,to improve the engineering applicability,the mathematical correlation model between phy-chemical properties of PG and dissolved phosphate contents was used to select PG;（5）The dynamic performance of phosphorus was investigated by drying-wetting cycle test under different deterioration mechanisms.In an acidic solution,the release of phosphate was greatly enhanced by H+,while the saline solutions have little impact by releasing eutectic phosphate.In addition to the environmental impact,the strength reduction of PG-based backfill was also observed.The mechanical damage of salinity was caused by the degradation of hydration production and micro-cracking,while in an acidic solution,the H⁺broke the backfill by dissolving hydration products.