Enhancing the sustainable immobilization of phosphogypsum by cemented paste backfill with the activation of γ-Al2O3

The in-situ solidification/stabilization of phosphogypsum (PG) has been deemed to be an efficient technology for large-scale stockpiled waste remediation. But there is still great uncertainty and controversy concerning fluorine and phosphorus transformation. Further, with a great concern for the high carbon footprint of the typical Portland cement binder, developing a low-carbon binder is of increasing importance for sustainable development. Herein, γ-Al2O3, an alternative with lower CO2 emissions and energy consumption, was preliminarily explored in PG remediation. Results showed that γ-Al2O3 promoted ettringite and amorphous precipitation. The SEM/EDS evidenced the role of aluminum-containing assemblages in pollutants retention. The pH-dependent leaching tests illustrated that the addition of γ-Al2O3 increases the fluorine retention capacity, but it is less dominant in phosphorus and sulfate immobilization. The modeling approach verified Al-F, Ca-F, and Ca-P phases as the primary phases for pollutants retention. The deviation in experimental and modeling leaching results indicates that the physical encapsulation cannot be ignored, which is evidenced by the higher mechanical strength that γ-Al2O3 modified mixtures achieved, presenting mechanically stable assemblages for engineering application and pollutants retention. This study proved that the utilization of γ-Al2O3 can expand the scalable application of PG, both in view of low-carbon strategy and high-performance solidification/stabilization.