Phase‑resolved metal immobilization during co-processing of heavy metal-contaminated soil in cement kilns

Heavy metal-contaminated soil (HMS) poses persistent environmental risks, while its heterogeneous composition and the generation of secondary solid wastes limit the effectiveness of conventional remediation methods. Cement-kiln co-processing offers a potential route for the safety disposal and resource utilization of HMS; however, suitable addition dosages and kiln operational windows that preserve clinker quality while stabilizing metals remain insufficiently defined. This study therefore investigated the effects of HMS addition dosage, calcination temperature, and residence time on clinker characteristics, heavy metal speciation, and leachability under kiln-relevant conditions. Microstructural and elemental analyses, XPS, sequential extraction, and leaching tests were combined with thermodynamic calculation to evaluate possible metal-phase evolution during calcination. The addition of 7.5% HMS caused no obvious changes in clinker micromorphology or surface elemental distribution, and available CaO facilitated the formation of the target C₃S crystalline phase. At this dosage, the solidification rates of Cr, Ni, Pb, As, and Cd increased by approximately 6-13% compared with C_L0. Thermodynamic calculation suggested that HMS dosage had limited influence on the predicted dominant metal-bearing phases. HMS dosage and calcination temperature significantly affected heavy metal solidification, whereas residence time had only a minor effect. An HMS dosage of 7.5%, a calcination temperature of 1400 °C, and a residence time of 10 min provided effective metal stabilization. Cd, Pb, As, and Ni retained stable valence states, whereas Cr underwent valence evolution and ultimately stabilized as Cr³⁺. Sequential extraction showed that the residual fraction proportions increased by 45.56-123.47% relative to those in the raw materials, and the leaching concentrations of all investigated heavy metals remained below the applicable standard limits. Overall, this study defines a practical operating window for the environmentally safe co-processing of HMS in cement kilns and provides mechanistic support for using cement manufacturing as a risk-controlled disposal route for heavy metal-contaminated soil.