Given the challenges in quantitatively analyzing complex ionic species within high-salinity mine water and the dominance of SO?2? in both concentration and material erosivity, this research investigates how SO?2? concentration governs the mechanical behavior of coal gangue-based backfill materials to establish a theoretical foundation for co-processing both wastes into dual-waste backfill materials. We prepared specimens using coal gangue and simulated mine water (sodium sulfate solution at 0–30,000 mg/L SO?2?) as core components, adding a curing agent for consolidation, then tested compressive strength and performed SEM microscopy across curing ages. Results demonstrate that during days 1–7, moderate SO?2? boosts strength whereas excess concentrations weaken it, and by days 14–28, the 30,000 mg/L SO?2? group achieves optimal enhancement; SEM imaging at 28 days reveals escalating SO?2? concentrations drive denser interlaced networks of ettringite crystals and C-S-H gels while visibly reducing pores, aligning consistently with macroscopic strength progression. Specimens fabricated with sodium sulfate solution outperform tap-water counterparts in compressive strength, confirming the feasibility of dual-waste backfill synthesis.