Synergistic effects of steel slag and red mud on the properties and microstructure of backfill materials

The work aimed to reduce cement consumption in coal mine paste backfill and enhance the resource utilization of industrial solid wastes. Steel slag (SS) and red mud (RM) were used as composite substitutes for cement to systematically investigate the effects of different substitution ratios and SS/RM proportions on the performance of backfill materials. Acoustic emission (AE) technology was employed to monitor the damage evolution behavior of the backfill body during loading. Besides, X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) were used to elucidate the hydration mechanism and microstructural evolution. The composite substitution of cement with SS and RM limited slurry workability, with only one mixture failing to meet the pipeline transport requirements. In terms of mechanical properties, SS and RM exhibited a pronounced synergistic effect at 28 d. When the substitution ratio was 20% with an SS/RM ratio of 5:5, the compressive strength of the backfill body reached 6.4 MPa, which was comparable to that of the control group. AE monitoring further revealed that the SS-RM composite system modified the failure mode of the backfill body. The mode transformed from abrupt tensile failure to a more progressive failure process accompanied by a higher proportion of shear damage. Microstructural analyses demonstrated that the SS-RM composite system promoted the formation of a dense C-(A)-S-H gel network, which filled and refined the pore structure, thereby enhancing the compactness and continuity of the backfill body and improving its mechanical performance. The findings provide valuable experimental insights for applying multi-solid-waste synergistic cementitious materials in mine backfill engineering.