Strength evolution and chemical product transformation of coal gangue cement backfilling paste under magnesium sulfate erosion

This study investigates the strength evolution and chemical product transformation of coal gangue cement backfilling paste (CGCBP) in the context of mine water containing highly erosive SO42- and Mg2+ ions, which significantly impacts CGCBP stability. To address the need for a nuanced understanding of sulfate attack dynamics, we employed experimental soaking tests with varying MgSO4 concentrations (0~10%) and durations (0~120 days), combined with gray correlation analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results indicate that sulfate erosion initially enhances CGCBP strength (0~60 days) through pore-filling by ettringite and gypsum, with the 10% MgSO4 group reaching a peak compressive strength of 24.35 MPa at 60 days, accompanied by a dynamic anti-erosion coefficient of up to 1.35. Beyond 60 days, the strength decreases as decalcification of C-S-H and cracking from superimposed expansion products become dominant, with the 10% concentration group showing a 29.32% strength reduction by 90 days. Gray correlation analysis reveals erosion age as the primary factor affecting strength (correlation coefficients: 0.7908 at 60d, 0.7676 at 90d, 0.7427 at 120d), with its influence decreasing linearly over time, while sulfate concentration has a lesser impact (correlation coefficients consistently around 0.61). Microstructural analysis shows that early-stage erosion products (ettringite, gypsum) fill voids and rough interfaces, increasing compactness, whereas late-stage degradation involves C-S-H decalcification, calcium hydroxide consumption, and crack propagation from internal and external stresses. These findings suggest that the negative effects of sulfate erosion may be overstated, particularly in short-term (0~60 days) and low-to-moderate concentration scenarios, and propose three strategies: adjusting raw material proportions to control reactive Al2O3 and CaO content, increasing paste porosity to accommodate expansion products, and using isolation films to prevent acid ion penetration, thereby balancing short-term strength gains and long-term stability in sulfate-rich mining environments.