Bleeding, flowabilities, rheology, mechanical properties and strength deterioration mechanism of sulphide-rich cemented paste backfill

At present, research on the sulphide-rich cemented paste backfill (CPB) performance is not comprehensive, and the mechanism by which the hydration process of sulfate on cement leads to the deterioration of CPB strength is still unclear. In this study, a comprehensive study was conducted on the sulphide-rich CPB performance, including bleeding rate, slump, slump flow, shear yield stress, viscosity, setting time, and unconfined compressive strength (UCS). The experiment mainly investigated the effects of different sulphur contents (30.7%, 24.7%, and 20.7%), cement-to-tailings ratios (1:8 and 1:12), and mass concentrations (73% and 75%). The experimental results show that the bleeding rate increases with the growth of sulphur content, and the slump and slump flow also increase with the growth of sulphur content. The shear yield stress and viscosity decrease with the growth of sulphur content, and the mass concentration has a higher impact on the bleeding rate, shear yield stress, viscosity, slump, and slump flow. The UCS under different experimental conditions shows an increasing-decreasing-increasing trend with the prolongation of curing time. Finally, X-ray Diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the strength deterioration mechanism of the CPB, and it was found that the pyrite mineral in the tailings is an important substance causing the strength deterioration of the CPB. Analysis of the hydration process of sulphide-rich CPB shows that the oxidation and hydration products of pyrite generate expansive gypsum and a small amount of ettringite. Internal stress causes the CPB to expand and form penetrating cracks, leading to strength deterioration. Moreover, the oxidation of pyrite inhibits the cement hydration process, resulting in the loss and deformation of the CPB skeleton.