Macroscopic properties and microstructure of stabilized contaminated soil under the combined action of chloride salt erosion and freeze-thaw cycles

Contaminated soil, post-stabilization/solidification, is frequently subjected to complex engineering environments. In Northwest China, the combined effects of chloride salt erosion and freeze–thaw cycles present significant challenges to foundation stability and construction projects by impairing the engineering performance of geopolymer-stabilized/solidified contaminated soil. To address this, a study was conducted involving coupled tests of chloride ion erosion and freeze–thaw cycles to systematically analyze the deterioration mechanisms of stabilized contaminated soil under these combined effects. Macroscopic performance was evaluated by monitoring the evolution of mass loss rate, unconfined compressive strength (with its loss rate), shear strength parameters (cohesion and friction angle), hydraulic conductivity, and toxic leaching concentration in the stabilized contaminated soil under varying salt solution concentrations and numbers of freeze–thaw cycles. Simultaneously, microscopic testing methods (XRD, SEM, NMR) and the modified BCR sequential extraction were employed to investigate the chemical fraction transformation and underlying mechanisms. The results show that chloride exposure produced a non-monotonic concentration-dependent response. Among the tested conditions, the 3% NaCl solution caused the most pronounced mechanical deterioration, whereas Pb2+ leaching generally increased under chloride exposure. The Pb2+ leaching concentration was first observed to exceed the regulatory limit of 5 mg/L after 8, 4, and 6 freeze–thaw cycles in 0%, 3%, and 5% NaCl solutions, respectively. Because measurements were conducted at two-cycle intervals, these values represent the experimentally observed stages of first exceedance rather than exact critical thresholds or field service-life limits. Microscopic observations indicate that freeze–thaw cycling and chloride exposure degraded cementitious products, exposed soil particles, and promoted crack development. The combined PCAS and NMR results suggest that newly formed fine pores and microcracks developed concurrently with the enlargement, interconnection, and coalescence of pre-existing pores, resulting in increased porosity, pore coarsening, and deterioration of macroscopic performance.