Strength Evolution and Heavy Metal Solidification/Stabilization Mechanisms of Weakly Cemented Backfill Under Groundwater Erosion

Instead of standard curing conditions, underground backfill materials are continuously exposed to a complex aqueous environment comprising mine seepage, bleeding water, and backfill mixing water. This unique environment not only influences strength development but also governs the release and immobilization behaviors of heavy metals during the hydration process.This study systematically investigated Weakly-cemented backfill  (WCB) specimens with binder-to-tailings (B/T) ratios of 1:10, 1:15, and 1:20.Following a 1-day initial curing and demoulding, the specimens were subjected to long-term immersion in solutions at pH levels of 4, 6, 7, 8, and 10.Macroscopic and microscopic tests, including UCS, SEM-EDS, XRD, FTIR, TG/DTG, NMR, and ICP-OES, were conducted to evaluate mechanical degradation and heavy metal leaching behaviors.The results indicate that the WCB with a 1:20 B/T ratio achieves an optimal balance between long-term mechanical reliability and environmental safety.At pH=7, the UCS decreased by only 14.41% over 120 days.Under acidic erosion (pH=4), the 120-day UCS dropped to 0.65 MPa, which still satisfied underground backfilling requirements.Conversely, under alkaline conditions (pH = 10), the UCS increased to 1.96 MPa, driven by the combined effects of continued hydration and localized physical filling by carbonate precipitates.Within the pH 6-8 range, the 120-day leaching concentrations of Cu, Zn, Pb, and Ni remained strictly below the limits of the GB/T 14848-2017 Class III groundwater standard.Microstructural analyses reveal that a neutral environment (pH=7) preserves the C-(A)-S-H gel network integrity, facilitating synergistic physico-chemical immobilization.Acidic erosion (pH=4) triggers a decalcification and depolymerization chain reaction.Notably, alkaline conditions (pH=10) induce a paradoxical phenomenon: macroscopic strength enhancement coincides with a deterioration in chemical immobilization capacity.These findings provide a theoretical foundation for backfill engineering design with ultra-low binder dosages in complex groundwater environments.