Degradation of Confined Compressibility in Weakly Cemented Coal Gangue Backfill Exposed to High Humidity

Coal gangue (CG) backfill, utilizing magnesium slag as a cost-effective alternative to ordinary Portland cement, represents a critical technology for underground waste management in China. Crucially, the confined compressibility of this matrix governs its long-term deformation resistance. However, since CG is enriched with hydrophilic clay minerals that trigger intense water uptake, the compressibility of this weakly cemented backfill is highly susceptible to deterioration under the pervasive high-humidity conditions of underground coal mines, which remains poorly understood. To address this gap, this study investigated the multi-scale confined compression behavior of magnesium slag-based (MPB) and conventional cement-based (CPB) fine CG aggregate backfill cured under a high-humidity environment. One-dimensional confined compression tests using a triple-lever oedometer and scanning electron microscopy (SEM) observations were integrated across varied curing ages. The results indicate that the macroscopic confined compression response is governed by the coupling effects of the moisture-induced softening of fine CG aggregates and the progressive evolution of hydration minerals. With prolonged curing, increased moisture absorption elevates the void ratio variation and compression index (Cc), thereby increasing matrix compressibility. The post-yield Cc after the bond breaks increases with the higher binder content because abundant hydration products encapsulate aggregates, which undergo catastrophic shearing after the cement bond breaks. Conversely, the consolidation coefficient continuously decreases under advancing loads as matrices densify. Although CPB consistently maintains lower compressibility than MPB, both backfills exhibit a time-dependent decline in the yield stress of the cement bond under high-humidity curing. These macro-mechanical degradations are supported by SEM observations; the experimental accuracy was validated through benchmark tests on standard quartz sand.