## 1 Introduction  
Because of considerable migration depth, complex migration mechanism, sizeable spatial variability, strong concealment, and resistance to degradation of heavy metals (HMs), its high-concentration and high-risk pollutants in the soil are extremely difficult to repair and manage and are called \"chemical time bombs\" [1, 2, 3]. In recent years, the remediation of soil HM pollution has been an environmental topic worthy of attention. Studies have shown that HM pollutants may cause changes in soil structure, such as colloidal corrosion between soil particles, decrease in cementation strength, increase in clay content, resulting in increased soil compression, reduced liquid, and plastic limits, reduced shear strength (the reduction can reach about 35%), increased permeability, and the decreased bearing capacity [4, 5, 6]. In addition, the non-degradability of HMs leads to their accumulation in plants, animals, and humans along the biological chain [7, 8, 9, 10, 11]. Both pose a threat to human safety and health; therefore, it is imperative to control soil HM pollution [12]. There are two kinds of solutions to manage HM pollution in the soil. The first one is to directly reduce the concentration of HMs in the soil, and the second one is to reduce the possibility of HM migration. That means fixing them in place and reducing their biotoxicity by special treatments [13; 14]. In this context, a series of treatment methods for HM-contaminated soils have emerged, such as S/S technology, displacement technology, soil leaching, incineration, solvent extraction, glass curing, and physical separation, emerged. Among them, solidification/stabilization (S/S) technology is the most widely used because of its cost-effective and simple operation [14; 15]. The S/S technology of soil usually provides the ability to handle HMs through two aspects. On the one hand, the S/S technology has a direct effect on HMs. Chemical precipitation, physical encapsulation, adsorption, and other means occur between HMs and hydrations products of binders; on the other hand, the S/S technology directly affects soil, and indirectly affects HMs by changing the soil structure. The reduced permeability leads to reduced migration of HMs, then the stability of HMs in the soil can be achieved [15; 16]. However, while enjoying the restorative power of S/S technology, one must also consider its environmental impact. Considering this problem, three inorganic binders, cement, lime, and fly ash, were used in the current study to improve S/S efficiency while reducing carbon emissions and easing the burden of industrial waste disposal [17; 18; 19]. However, it should be noted that the use of binders should involve reasonable control of the amount: too little S/S has poor effect, but if the amount is too much it will not only increase the cost, but also change the physical and chemical properties of the soil, and even cause secondary pollution. Starting from the mechanism of the S/S technology, it can be realized that this technology cannot reduce the concentration of HMs in the soil, but restricts the migration of pollution, or limits the effectiveness of pollutants. Based on this, it is necessary to track the follow-up progress of HMs after the use of S/S technology. In the internal system of S/S HM-contaminated soil, the type of soil [20], the type and content of HMs [21], the type and content of binders [22; 23], the curing conditions [23], and the moisture content [24] all have an impact on the long-term stability of HMs. In various complex environments, such as carbonation [25], acid rain leaching [25; 26; 27], high salt groundwater infiltration [28; 29], wetting and drying cycles [30; 31; 32], and F-T cycles [33; 34; 35; 36], the stable forms of the contaminants in the S/S-contaminated soil is very likely to change or even fail, which causes the activation of HM pollutants in the soil, causing secondary pollution and affecting its engineering mechanical properties.