Evaluation and mechanistic analysis of the effect of the addition of alkaline earth metal CaO on Cd solidification enhancement in lightweight aggregate preparation

Abstract

The volatilization of Cd during the preparation of lightweight aggregates (LWAs) can cause serious damage to the environment, so a method to harmlessly transform Cd during this process is required. In this regard, the alkaline earth metal CaO was added to Cd-containing aggregate raw materials for treatment, and the effect of CaO addition on the properties of LWAs in the presence of chlorine and sulfate was investigated. Kinetic models of the Cd volatilization were established by using the Arrhenius equation to predict the volatilization of Cd at different sintering stages. The results showed that 0.8% wt of CaO under the influence of chlorine can reduce the Cd volatilization rate from 84.9% to 12.64%, corresponding to an increase in the reaction activation energy (E_a) from 22.62 to 49.55 kJ mol⁻¹. Additionally, the Cd volatilization rate under the influence of sulfate was reduced from 30% to 8%, with an increase in the E_a from 33.25 to 42.62 kJ mol⁻¹. The activation energy increase suggests that the addition of CaO is beneficial because it increases the energy required for Cd volatilization. According to the Cd leaching experiments conducted on the LWAs, it was found that the solidification ratio of Cd was higher than 99.9% for all samples after the addition of CaO. The addition of CaO promotes the formation of CdFe₂O₄ and anorthite for effective solidification of Cd, thus optimizing the structures of the LWAs. This work may provide a new idea for Cd waste recycling.