Response Surface Optimization of Fly Ash–Carbide Slag–Bentonite-Based Whole-Tailings Backfill: Multi-Objective Mix Design and Microstructural Mechanism

Reducing cement consumption in mine backfill materials and improving the synergistic utilization of multi-source solid wastes are important for developing low-carbon backfill systems. In this study, fly ash–carbide slag–bentonite-based whole-tailings backfill (FCB) was optimized using a Box–Behnken response surface design. The effects of replacement ratio, FA/CS ratio, and bentonite content on fluidity, setting time, and 28 d uniaxial compressive strength were investigated, and the microstructural mechanism was analyzed by XRD and SEM. The results show that all quadratic models are statistically significant, with the strength model exhibiting the best fitting and predictive performance. The replacement ratio and bentonite content are the main factors affecting FCB performance, while the FA/CS ratio plays a secondary role. Strength decreases with increasing replacement ratio, whereas bentonite content shows a quadratic effect on fluidity and strength. Multi-objective optimization gives an optimal mix of A = 0.58, B = 2.71, and C = 4.99%, with validation errors below 5%. XRD and SEM results indicate that C-(A)-S-H gel and AFt are the main hydration products. Lower replacement ratios and appropriate bentonite contents were associated with a more continuous matrix morphology in the selected SEM regions and higher compressive strength.