Effects of Freeze–Thaw Cycling on Pb–Cd Contaminated Soil Solidified with MgO and Fly Ash

To simulate the actual freezing conditions of soils in seasonal frozen regions where shallow groundwater recharge is present during the freezing process, this study employs reactive MgO-fly ash-solidified Pb–Cd cocontaminated soil as the research object. Freeze–-thaw (F–T) cycling tests under unidirectional freezing conditions, toxicity leaching tests, and heavy metal speciation analyses were conducted in both open systems (with external water supply) and closed systems. The results indicated that F–T cycling caused Pb and Cd to gradually transform from stable fractions to weak acid–extractable fractions. Under different freezing modes, the degree of degradation in Pb–Cd solidification effectiveness decreased in the following order: open-system unidirectional freezing > closed-system unidirectional freezing > thermostatic freezing. After 11 F–T cycles, under open-system unidirectional freezing modes, the leaching concentrations of Pb and Cd in the solidified soil increased by 5.08 and 0.95 mg/L, respectively, compared with pre-F–T levels. These values were 0.99 and 0.58 times those under closed-system unidirectional freezing and 4.20 and 1.53 times those under thermostatic freezing, respectively, and both exceeded the soil standard limits. X-ray diffraction and scanning electron microscopy analyses revealed that F–T cycles reduced the M-S-H hydration products in solidified soil and enlarged the pore structure, leading to an increased risk of secondary release of Pb and Cd. This effect was particularly evident under open-system unidirectional freezing modes. The findings of this study provide a reference for the long-term stability assessment and engineering application of in situ solidification remediation technologies for heavy metal–contaminated soils in shallow groundwater environments within seasonal frozen ground zones.