Performance activation and strength evolution mechanism of carbide slag on anhydrous phosphogypsum backfill material

Phosphogypsum backfill has the advantage of low cost and has been gradually applied to backfill mining. However, due to the low activity, long setting time, and hardening time of anhydrous phosphogypsum backfill (APB), it needs to be activated and modified. Carbide slag is an industrial solid waste with strong alkalinity and can be used as an alkali activator. In this study, the effects of carbide slag on the setting time, fluidity, bleeding rate, pH value, and strength of anhydrous phosphogypsum backfill materials were analyzed, and the damage evolution process of APB under different carbide slag content was explored. The microstructure and phase structure of the APB were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD), and the influence mechanism of carbide slag on the APB was revealed. The results showed that: With the increase of carbide slag content, the setting time of APB decreased rapidly and then increased gradually. The initial fluidity, 30 min fluidity, and bleeding rate gradually decreased, and the pH value gradually increased. The flexural strength of the APB increased first and then decreased with the increase of carbide slag, and reached the maximum when the content of carbide slag was 10%. Carbide slag can be used as a good activator for anhydrous phosphogypsum. When the content was less than 10%, it promoted the hydration of anhydrous phosphogypsum into columnar dihydrate gypsum crystals, and the strength of the APB was enhanced. When more than 10%, the dihydrate gypsum crystal gradually transformed into a loose needle shape, and the APB continued to deteriorate. According to the peak stress, peak strain, and elastic modulus, the damage model of APB under uniaxial compression was established. The research results could provide a theoretical reference for the preparation and promotion of phosphogypsum backfill materials.