A study of the mechanical properties, environmental effect, and microscopic mechanism of phosphorus slag-based uranium tailings backfilling materials

Heap leaching uranium tailings (HLUTs) contain radionuclides and sulfuric acid, presenting some challenges for the disposal of cemented HLUTs backfill (CUTB), including low stability and potential environmental contamination. In light of these issues, using a blend of cement clinker (CL) and high content phosphorus slag (PS) as binders to replace traditional cement for the preparation of green and cost-effective CUTB were proposed. The fluidity, setting time (ST), mechanical properties, Radon (Rn) exhalation rate, uranium (U) occurrence form, U (VI) leaching behavior, and microstructure of CUTB with different PS contents were investigated. The results showed that the fluidity, bleeding rate and ST of slurry (PS content >40%) met the filling requirements. The strength of PS-based CUTB was initially low, but rapidly increased in subsequent periods, surpassing 2 MPa at 28 d and 4.5 MPa at 90 d (with 50-70% PS content). Moreover, the Rn exhalation rates and U (VI) leaching levels of CUTB were far lower than the stipulated limits. When PS content exceeded 50%, the migratory uranium content in CUTB was below 10% with low migration risks. With the addition of PS, the amount and degree of polymerization of C-S-H gels in the matrix increased, while gypsum and CH content decreased. The S/S of U (VI) and SO42- was attributed to the combined effects of chemical immobilization of nascent minerals and physical adsorption/encapsulation of hydration products. Additionally, these hydrates effectively sealed micro-cracks on HLUTs particles surfaces and filled interlocking pores to decrease Rn exhalation. PS-based CUTB exhibited a high capacity increasing ratio and low cost. This research can promote the utilization of hazardous solid wastes such as HLUTs, reduce carbon dioxide emissions, and provide a theoretical basis for ensuring green and safe production in mining.