Durability Performance and Microstructural Evolution of Carbonation-Solidified Red Mud-Based Backfill Under Wetting–Drying Cycles and Alkaline Red Mud Liquor Immersion

Red mud-based backfill containing circulating fluidized bed (CFB) ash and reactive MgO must maintain adequate mechanical integrity and environmental stability under moisture and alkaline exposure. This study investigated the durability of carbonation-solidified red mud-based backfill subjected to wetting–drying cycles and alkaline red mud liquor (RML) immersion. Specimens carbonated for 0, 2, 4, 6, 8, and 10 h were evaluated in terms of macroscopic morphology, mass loss, linear shrinkage, unconfined compressive strength (UCS), electrical resistivity, pH, hazardous-element leaching, scanning electron microscopy, and X-ray diffraction. Increasing carbonation duration generally reduced mass loss and linear shrinkage while improving UCS and electrical resistivity within the investigated exposure range. Wetting–drying cycling resulted in progressive surface erosion, shrinkage, and strength deterioration. By contrast, RML immersion produced an initial increase in UCS and resistivity, followed by stabilization or a slight decline at later ages. The observed changes were consistent with pore filling by carbonate-bearing products and low-crystallinity reaction products, followed by local pore development and disruption of the cemented structure during prolonged exposure. At a UCS threshold of 0.8 MPa, the response-surface models yielded estimated threshold exposures of 6.92–13.98 cycles under wetting–drying conditions and 45.69–75.05 d under RML immersion, with the RML estimates extrapolated from the 28 d dataset. Within the tested conditions, carbonation improved the resistance of the backfill material to cyclic moisture disturbance and alkaline immersion, with the 8–10 h groups retaining relatively higher mechanical and dimensional performance.