2.1.2. Binders

Cement, lime, and fly ash were used simultaneously in this study for the following three reasons. (1) Cement costs a lot of energy and releases a lot of carbon according to previous studies [45,46]. Therefore, the use of cement should be reduced; (2) the active substances in fly ash and lime react with the hydration products of cement to further improve properties of the soil and enhance the S/S effects of the HMs; (3) fly ash, as a kind of industrial byproduct, used in this way can reduce the pressure of waste treatment.

Figure 2 shows the results of the X-ray fluorescence analysis of cement, lime, and fly ash, containing the chemical composition and content of each component of three binders. Analytical-grade calcium oxide is the main component of quicklime, which was tested as high as 88.73 percent; the model of the ordinary Portland cement was 325; the fly ash was taken from the Chongqing power plant, and the grade of it was determined as two, according to GB/T 1594-2017. To ensure the uniform mixing of binders and soil, the binders needed to pass a 200-mesh sieve before use.

## 2.1.3. Heavy Metals

To minimize the influence of anions of the added HM compounds on the test results, analytical-grade Zn(NO3)2·6H2O, analytical-grade Pb(NO3)2, and high-purity Cd(NO3)2·4H2O were used simultaneously as HM contaminants to provide the required HM ions. Reactions are difficult to take place between binders, hydration products, and nitrate, while the concentration of nitrate was not high in this research. The side effects of Zn(NO3)2·6H2O, Pb(NO3)2, and Cd(NO3)2·4H2O were, thus, negligible [47,48].

**Figure 2.** The chemical components of three binders.
