Preparation of magnesium potassium phosphate cement from municipal solid waste incineration fly ash and lead slag co-blended: Ca-induced crystal reconstruction process and Pb-Cl synergistic solidification mechanism.

Higher chlorine (Cl) content than lead (Pb) content in municipal solid waste incineration fly ash (MSWIFA) impeded the practical application of Pb(PO)Cl-derived magnesium potassium phosphate cement (MKPC) preparation strategy. Herein, Pb/Ca-rich lead slag (LS) was co-blended with MSWIFA to prepare MKPC for the synergistic treatment of both two solid wastes and the Pb-Cl solidification. The results showed that the resulting 15-15 (15 wt% MSWIFA and 15 wt% LS incorporation) sample achieved 25.44 MPa compressive strength, and Pb and Cl leaching toxicity was reduced by 99.18 % and 92.80 %, respectively. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses showed that Pb, Ca, phosphate and Cl formed PbCa(PO)Cl in samples. The formation of PbCa(PO)Cl was also demonstrated by the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM), while differences in the lattice characteristics of PbCa(PO)Cl and Pb(PO)Cl were found. In-situ XRD indicated that Ca accelerated the transformation of Pb to Pb(PO)Cl. After co-precipitating with Ca to form PbCa(PO)Cl, Pb continuously substituted Ca to eventually transform to Pb(PO)Cl. This work informs the synergistic treatment of MSWIFA and LS and offers new insights into the reaction mechanism between Pb, phosphate and Cl under Ca induction.