Study on the Microscale Mechanisms of Multi-Source Solid Waste Synergy in Enhancing Physicochemical Properties of Red Mud-Based Backfill Materials

To address the issues of the low pozzolanic activity and high pollution potential of red mud (RM), this study utilizes different industrial solid wastes to synergistically enhance the physicochemical properties of red mud-based filling materials. The compressive strengths of red mud-based filling materials activated by three types of solid wastes—desulfurized gypsum (DG), carbide slag (CS), and steel slag (SS)—were compared, revealing the differences in their effects on the physicochemical properties of the materials. The results showed that DG significantly enhanced the compressive strength of the backfill material. The composite system composed of 65.8% RM, 18.8% FA, 9.4% cement, and 6% DG achieved a compressive strength of 7.36 MPa after 28 days of curing, demonstrating a 97.8% increase compared to the control group. Techniques such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) analysis were employed to characterize the microstructural evolution of the red mud-based filling materials activated by different solid wastes. This study investigated the differences in the pore structure, microscopic morphology, and chemical composition of the materials containing different solid wastes. The results indicated that DG effectively promotes the formation of ettringite and C(-A)-S-H gel, optimizes the pore structure of the filling materials, and forms a dense matrix, thereby enhancing the stiffness and strength of the materials. Additionally, the red mud-based filling materials developed in this study exhibit excellent environmental performance. This not only provides theoretical support for the development of red mud-based filling materials but also offers new insights for mine backfilling and the co-disposal of solid wastes.