Performance analysis and design optimization of modified coal gasification slag-based backfill materials

Large-scale stockpiling of coal gasification slag (CGS) has posed pressing environmental challenges, while its industrial-scale valorization remains severely constrained by energy-intensive pretreatment processes (drying, grinding, and calcination) conventionally required to activate latent pozzolanic reactivity. To address this bottleneck, this work develops a modified coal gasification slag-based backfill material (MCB), in which coal gasification fine slag (CGFS) and coal gasification coarse slag (CGCS) are directly utilized in their as-received state without any pretreatment, acting as the primary cementitious component and aggregate skeleton, respectively. Modified magnesium slag (MMS) and flue gas desulfurization gypsum (FDG) are incorporated as an alkali-salt composite activator to synergistically trigger the pozzolanic activity of CGS. An integrated optimization framework coupling Response Surface Methodology (RSM) and the Whale Optimization Algorithm (WOA) is established for dual-objective optimization, in which the Pareto front is generated by non-dominated sorting and representative solutions are selected from it according to engineering preference. The established RSM model exhibits excellent fitting accuracy, enabling precise quantification of correlations between component proportions and performance responses. The compressive strength of MCB increases consistently with curing age, reaching 15.44 MPa at 28 days and 26.09 MPa at 90 days, with slump values of 102∼142 mm that fully satisfy pumping requirements for underground mine backfill. The strength contribution of each solid waste shows distinct age dependence, MMS dominates early-age (3-day) strength development, while CGFS pozzolanic activity governs later-stage strength growth; the optimal FDG dosage rises from 4% to 6% with extended curing age, yet excessive incorporation inhibits strength evolution. Microscopic characterization identifies ettringite (AFt) and C-(A)-S-H gel as the dominant hydration products. High MMS and CGFS dosages paired with moderate FDG promote the formation of a dense three-dimensional network structure, contributing to enhanced compressive strength. The WOA-derived Pareto optimal set achieves a well-balanced trade-off between strength and cost. Compared with conventional cement-based backfill materials, the optimized formulation delivers a 122.1% improvement in cost efficiency, cuts carbon emissions by 99.1%, and requires only 4.8% of energy input, with heavy metal leaching concentrations all below national standard limits. This study provides a pretreatment-free, all-solid-waste technical route for sustainable mine backfill, achieving the synergistic unity of superior mechanical performance, outstanding environmental benefits, and favorable economic competitiveness.