Recycling gold mine tailings into eco-friendly backfill material for a coal mine goaf: Performance insights, hydration mechanism, and engineering applications

Recycling gold mine overflow tailings for coal mine filling is crucial for sustainable mining. In this work, an eco-friendly, performance-controllable overflow tailings-fly ash-based backfill material is developed for coal mine filling. The effects of three critical factors, namely, the slurry concentration (SC), cement-sand ratio (C:S), and tailings-fly ash ratio (T:F), on the workability and uniaxial compressive strength (UCS) properties of the novel backfill material are thoroughly investigated, and an optimization of the corresponding formulation is conducted. The optimal formula for the backfill is determined to be a CS of 60 %, a C:S of 0.10, and a T:F of 6:6. The hydration mechanism of the chosen typical mixtures is analyzed via X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy, and the results show that a needle-like Aft gel, identified as the major gelatinous product, is intricately intertwined to create an intricate network structure. As the T:F increases, the content of calcium and silicon oxide initially decreases but then increases, and the optimal mixture reaches a minimum value of 63.66 %. The optimum specimen exhibits a peak wavenumber at 1109.46 cm⁻¹ involving a Si-O stretching vibration bond. A comprehensive filling program at the Liangjia Coal Mine is successfully implemented. Approximately 0.27 tons of overflow tailings are utilized for every ton of backfills. The underground core-pulling backfill achieves a peak uniaxial compressive strength (UCS) of 7.56 MPa after 28 d, surpassing design requirements and showing promise for coal mine filling applications. This study is expected to achieve the transformation of a coal mine goaf into a gold mine tailings pond.