## 1 Introduction  
Mercury (Ag), lead (Pb), cadmium (Cd), arsenic (As), and chromium (Cr) are the five most toxic chemical elements. Cd is highly mobile, resistant to corrosion, and causes irreversible damage to the soil, making preparation of the contaminated soil difficult. Therefore, the management of cadmium toxicity is a crucial field of soil environment research [1; 2; 3]. According to the Bulletin of the National Soil Pollution Survey in 2014, the current total excess rate of soil heavy metal pollution in China is 19.4%; among the causes, Cd is a primary pollutant with a rate exceeding 7.0% [1; 4; 5]. Being a global environmental problem with high and persistent toxicity, the American Agency for Toxic Substances and Disease Registry (ATSDR) and the European Union classify cadmium as a highly toxic, dangerous, and carcinogenic substance [5]. Cadmium is taken up by plant roots and enters the food chain mainly due to its high mobility and bioavailability, causing harmful effects on the human body and animal health [6; 7; 8]. Cd can easily bind to albumin (high-molecular-weight proteins) in the human body through non-protein sulfhydryl groups (R-SH) [9] leading to pulmonary adenocarcinomas, bone diseases (osteomalacia and osteoporosis), prostate cancer, kidney dysfunction, and hypertension [10; 11; 12; 13]. There is an urgent need for the effective remediation of cadmium-contaminated soils. In situ heavy metal immobilization is a standard method, among various technologies, for the remediation of contaminated soil. In in situ immobilization technology, inorganic remediated materials, such as cement, zeolite, and fly ash, are often used on Cd-contaminated soils to reduce Cd toxicity, bioavailability, and mobility [14; 15; 16; 17]. Highly alkaline ordinary Portland cement has efficiently decreased the bioavailability of multiple heavy metals by increasing the soil pH [18]. However, excessive use of highly alkaline materials has been reported to increase soil alkalinity risk and ultimately affect plant growth [19]. Waste cement is the heaviest and most voluminous waste stream generated from the construction, renovation, and demolition of buildings, roads, and other infrastructure [20; 21]. It is a hydrated cement, which is far less alkaline than ordinary cement and does not cause soil compaction. In particular, recent studies have revealed the mechanisms of cement waste in immobilizing Cd via its application as an aqueous solution to the contaminated soil [22; 23]. Furthermore, waste cement can reduce Cd bioavailability, increase soil pH and trigger the precipitation of Cd carbonates, oxides, or hydroxides. It can potentially decrease Cd solubility, as shown by incubation experiments and leaching tests [23]. Soil enzyme activity is a critical soil microbial growth and activity indicator. It plays an essential role in maintaining soil health, ecology, physical and chemical properties, and energy flow, and controlling organic matter decomposition. Heavy metals can directly affect soil enzymes and change their activity and diversity. A large number of studies have confirmed that heavy metals are harmful to soil enzyme activities [2; 24; 25; 26]. Soil enzymatic activities represent the degree to which the terrestrial ecosystems are affected by heavy metals [27; 28; 29]. Soil enzyme activities such as fluorescein diacetate hydrolysis, catalase, and urease are susceptible to even small heavy metals concentrations in the soil system and have been used as biomarker indicators [28; 29; 30; 31; 32]. Enzyme activity is influenced by soil physicochemical properties [33], and the availability of heavy metals are also correlated with soil enzyme activity [34]. Heavy metals can affect plant growth and even lead to plant death when their concentration exceeds a certain threshold.