Response Surface–Fuzzy Comprehensive Evaluation-Based Inverse Mix Design and Strength Mechanisms of Waste Rock–Unclassified Tailings Cemented Backfill

This study developed a coupled response surface methodology–fuzzy comprehensive evaluation (RSM–FCE) approach for the inverse mix design of waste rock–unclassified tailings cemented backfill under a target-strength constraint. The unclassified tailings replacement ratio, aggregate-to-cement ratio, and slurry solids concentration were selected as the key factors. A Box–Behnken design (BBD) was used to establish a response-surface model for 28 d uniaxial compressive strength (UCS). Fuzzy comprehensive evaluation was then used to screen candidate mix-design ranges, while low-field nuclear magnetic resonance (LF-NMR) and scanning electron microscopy (SEM) were used to investigate microstructural features associated with strength development. The main effects on UCS decreased in the order of aggregate-to-cement ratio, slurry solids concentration, and unclassified tailings replacement ratio. The strongest interaction occurred between the aggregate-to-cement ratio and slurry solids concentration. Fine-particle filling, hydration-product infilling, interparticle bonding, reduced free water, and suppressed bleeding jointly refined the pore structure and promoted a continuous bonding network and matrix densification. The coupled RSM–FCE analysis identified candidate ranges of 35–40% for the unclassified tailings replacement ratio, 4–6 for the aggregate-to-cement ratio, and 84–86% for the slurry solids concentration. The measured 28 d UCS values ranged from 5.24 to 9.21 MPa and all met the mine’s target-strength requirement.