Study on water resistance of phosphogypsum solidified by magnesium oxychloride cement (MOC) and NaHCO3

Phosphogypsum (PG) is a byproduct of the phosphoric acid industry and is often disposed of as solid waste instead of being efficiently recycled. However, long-term accumulation of PG can lead to environmental pollution and pose risks to human health. Currently, the primary method for solidifying PG involves using cement, but cement production generates significant carbon dioxide emissions. In this study, magnesium oxychloride cement (MOC) and NaHCO3 were combined to solidify PG, and a new sustainable PG treatment method was developed. The effects of MgO/MgCl2 molar ratios, NaHCO3 dosages, and curing age on the mechanical properties and microstructure of the solidified PG were discussed through unconfined compressive strength (UCS) test, water resistance test and microstructure analysis. The fixation efficiency of phosphorus (P), fluorine (F), and heavy metal ions of PG was evaluated. The results showed that with an MOC content of 12 %, an MgO/MgCl2 molar ratio of 3:1 and a NaHCO3 dosage of 0.5 %, the UCS and softening coefficient of solidified PG reached the maximum of 6.25 MPa and 74.28 %, respectively. Adding NaHCO3 improved the water resistance of MOCsolidified PG and enhanced the reaction rate of MOC. MOC and NaHCO3 were combined to form acicular Phase 5 and columnar nesquehonite crystal structure. The reaction products filled the pores and interweave to create a dense network structure, increasing the resulting UCS and water resistance of MOC-solidified PG. MOC combined with NaHCO3 had better fixation efficiency for P, F and heavy metal ions in PG. This study provides a novel approach to PG solidification for use as roadbed material, promoting the resource utilization of solid waste while contributing to environmental protection and aligning with sustainable development goals.