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 5 (PO 4 ) 3 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 2+ , Ca 2+ , phosphate and Cl - formed Pb x Ca 5−x (PO 4 ) 3 Cl in samples. The formation of Pb x Ca 5−x (PO 4 ) 3 Cl was also demonstrated by the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM), while differences in the lattice characteristics of Pb x Ca 5−x (PO 4 ) 3 Cl and Pb 5 (PO 4 ) 3 Cl were found. In-situ XRD indicated that Ca 2+ accelerated the transformation of Pb 2+ to Pb 5 (PO 4 ) 3 Cl. After co-precipitating with Ca 2+ to form Pb x Ca 5−x (PO 4 ) 3 Cl, Pb 2+ continuously substituted Ca 2+ to eventually transform to Pb 5 (PO 4 ) 3 Cl. This work informs the synergistic treatment of MSWIFA and LS and offers new insights into the reaction mechanism between Pb 2+ , phosphate and Cl - under Ca 2+ induction.