2.2.1. Preparation of Artificial Composite Contaminated Soil

In order to improve the observability of ion changes with F–T cycles in this study, as well as to make the test soils more closely resemble the form of contamination in the project, the tertiary standard limits of Pb, Zn, and Cd in the Soil Environmental Quality Standard (GB36000-2018) were enlarged by 16, 10, and 400 times, respectively, to obtain test yields of 8000 mg/kg, 5000 mg/kg, and 400 mg/kg for Pb, Zn and Cd, respectively.

At the same time, the soil moisture content was set at 120% of the optimal moisture content to ensure adequate F–T effects, and then, according to the soil water content and the HM mixture, a certain amount of deionized water, analytical-grade Pb(NO3)2, analyticalgrade Zn(NO3)2·6H2O, and high-purity Cd(NO3)2·4H2O was weighed and placed in a magnetic mixer for ten minutes to mix the solution well.

After that, the mixed solution was added to the sieved soil, stirred with a magnetic mixer for 10 min, then wrapped in a plastic bag and placed in a humidor with a temperature of 22 ◦C and a relative humidity of 95% for 30 days to make an artificial composite HMscontaminated soil.

## 2.2.2. Stabilization/Solidification of Contaminated Soil

According to the previous results of our research group [41], the better designed mixing ratio of cement, lime, and fly ash on S/S of HMs-contaminated soil is 5%, 2.5%, and 2.5%, respectively, so this ratio was adopted in our current study. After weighing a certain amount of binder according to the binders' set ratio, the soil-added binders were stirred by a mechanical mixer for 10 min, then sealed by a plastic membrane and kept for 56 days in a standard curing chamber (22 ◦C, 95% relative humidity) for use.
