Performance and influence mechanism of Cd-contaminated soil solidified by rice husk ash-cement

In order to realize resource utilization of contaminated soil and rice husk ash, and solve the problem of high emission of cement-solidified materials, the rice husk ash (RHA)-cement is used as the solidified agent to treat heavy metal cadmium (Cd)-contaminated soil. The unconfined compressive strength (UCS), toxicity leaching, XRD and SEM tests are carried out on the solidified Cd-contaminated soil with different curing ages, curing agent types and Cd contents. The UCS, leaching concentration, failure modes, micro-morphology and mineral composition are used to analyze the macro-micro characteristics. The micro-mechanism of the RHA-cement solidified Cd-contaminated soil is revealed. The results demonstrate that the RHA can accelerate the hydration process of cement and improve the UCS of solidified soil, while adding 5%~10% RHA at low cement content has better effect. The strength of solidified soil increases first and then decreases with the increase of Cd content, and there is a critical value of 100~400 mg/kg. When the RHA is added into the soil, the brittle failure characteristics of the soil are weakened. The soil cracks and irregular failure surfaces increase under Cd pollution. The leaching concentration of solidified soil decreases with the increase of age, and reaches the standard limit when the Cd content is 100 mg/kg. The leaching concentration of solidified soil after partial replacement by the RHA cement is similar. The RHA-cement mainly uses hydration calcium aluminosilicate polymer gel (C-A-S-H) and ettringite (AFt) to support soil pores, and continuously agglomerates and cements to form a spatial network structure, and finally to form the skeleton structure and to adsorb Cd²⁺.