Hydration and physicochemical immobilization mechanisms of pozzolanic-hazardous waste in supersulfated cement

Ordinary Portland cement (OPC) is a versatile cement binder suitable for solidifying hazardous wastes, while its production induces significant carbon emissions. This study developed a novel low-carbon supersulphated phosphogypsum slag cement (SPSC) with the inclusion of pozzolanic-hazardous waste and investigated the synergetic mechanism of its pozzolanic reaction and immobilization behavior. The hydration characteristics of SPSC were explored using ionic chromatography, XRD, and hydration heat tests. The study also assessed the effectiveness of SPSC in immobilizing hazardous waste using toxicity characteristic leaching procedure and sequential extraction procedure tests, and explored these mechanisms through XRD, SEM, Zeta potential, NMR, and progressive leaching tests. The results indicate that the SPSC system is more effective in immobilizing hazardous waste than OPC. This superior performance is attributed to the lower Ca/Si ratio and higher Al/Si ratio in SPSC, which results in the formation of hydrates with more negative charges than OPC and thus stronger physical adsorption of heavy metals. Additionally, the longer gel chains with larger interlayer zones in SPSC contribute to better solidification of heavy metals. Larger amounts of ettringite in SPSC also aids the immobilization by facilitating the exchange of Al ions for heavy metals. The excess SO₄²⁻ in the pore solution of SPSC binder could help immobilize heavy metals by sulfate precipitation. Overall, this study provides new insights into the sustainable immobilization of hazardous waste by adopting SPSC.