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 Pb5(PO4)3Cl-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 Pb2+, Ca2+, phosphate and Cl- formed PbxCa5-x(PO4)3Cl in samples. The formation of PbxCa5- x(PO4)3Cl was also demonstrated by the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM), while differences in the lattice characteristics of PbxCa5_x(PO4)3Cl and Pb5(PO4)3Cl were found. In-situ XRD indicated that Ca2+ accelerated the transformation of Pb2+ to Pb5(PO4)3Cl. After co-precipitating with Ca2+ to form PbxCa5_x(PO4)3Cl, Pb2+ continuously substituted Ca2+ to eventually transform to Pb5(PO4)3Cl. This work informs the synergistic treat- ment of MSWIFA and LS and offers new insights into the reaction mechanism between Pb2+, phosphate and Cl- under Ca2+ induction.