MgO-based binders with different formulations for solidifying Pb and Cd in MSWI fly ash: Solidification effect and related mechanisms

The MgO-based binder is a novel and effective material for solidifying municipal solid waste incineration fly ash (MSWI FA). In this study, the effect of MgO-based binders with different MgO contents on the solidification and stabilization of MSWI FA was investigated by adjusting the proportions of MgO, silica fume, and MSWI FA in the solidified system. The leaching behavior and immobilization mechanisms of Cd and Pb in the solidified bodies were focused on. The hydration products generated were characterized by XRD, FTIR, and TG analysis. The results demonstrated that the compressive strength of the solidified bodies and the immobilization efficiencies of heavy metals improved with the increase in MgO addition. The MSWI FA solidified by MgO-based binders could maintain low leaching toxicity over a wider pH range. The generated Mg-containing hydration products, including Mg(OH)2 and M-C-S-H gels, were believed to contribute to the reduction of the leaching concentrations of heavy metals. The immobilization mechanisms of Pb and Cd were different: the immobilization of Pb came from chemical chelation, and Cd was stabilized from alkaline precipitation. In conclusion, this work provides new insights into applying MgO-based binders in MSWI FA solidification.