Mechanistic insights into red mud-slag geopolymer stabilization of multi-metal contaminated soil under coupled environmental attacks: Multiscale assessment from macro-engineering performance to micro-structural evolution

This study systematically evaluates the long-term stability of red mud-slag geopolymer (RM-SGP) solidified heavy metal contaminated soil under combined Dry-Wet-Freeze-Thaw (D-W-F-T) cycles through a multi-method analytical framework integrating macroscopic mechanical tests (unconfined compressive strength, shear parameters), chemical monitoring (pH, electrical conductivity, resistivity, toxicity leaching), and microstructural characterization (SEM/NMR). The results demonstrate superior D-W-F-T resistance of RM-SGP solidified soil, with mechanical performance evolution governed by initial contamination levels: Lower concentration specimens exhibit characteristic UCS “initial increase followed by decrease” transitions mediated by competing hydration enhancement and microcrack propagation, while higher concentration groups display progressive strength deterioration from hydration inhibition. Shear parameters exhibit nonlinear attenuation. Environmental safety assessments reveal concentration-dependent leaching kinetics, where physical-chemical barrier failure triggers heavy metal remobilization. pH values briefly decrease before stabilizing, EC gradually increases to equilibrium, and resistivity declines linearly with structural damage. Through the coupling analysis of resistivity and UCS, the quantitative predictive effect of resistivity on UCS, was established, thereby achieving nondestructive field monitoring. Microstructural evolution demonstrates RM-SGP solidified contaminated soil has the characteristic of a synergistic “triple barrier” mechanism (physical-chemical curing, hydration remediation, and multi-scale pore structure), which can alleviate the connectivity problem of cracks.The mechanism enables dynamic equilibrium between mechanical integrity and environmental safety, surpassing conventional passive stabilization approaches. This work pioneers an active-adaptation paradigm for contaminated soil remediation, simultaneously addressing industrial solid waste reuse, carbon footprint reduction, and environmental risk mitigation challenges in cold-region geotechnical engineering.