Magnesium phosphate cement coupling with graphene nanosheets enhances the synchronized self-cementation of magnesium oxychloride cement and immobilization of heavy metals and chloride salts in the municipal solid waste incineration fly ashes

In this study, the magnesium phosphate cement (MPC) coupling with graphene nanosheets (GNP) was employed to solidify municipal solid waste incineration fly ashes (MSWI-FA) and stabilize the contaminants of heavy metals (HMs) and chloride salts in the matrix. The maximum compressive strength of 45.62 MPa and the minimum leaching concentration of 0.012, 0.0064, 0.00079, 0.0045, and 0.048 mg/L for Zn, Cu, Cd, Pb, and total Cr and 0.86 g/L for chloride were obtained from the GNP-MPC-MSWI-FA-cemented matrix in the orthogonal tests and toxicity-leaching experiments, respectively. MSWI-FA significantly changed the MPC hydration process. The influence of MPC on the speciation transformation of HMs was more prominent than that of GNP. Fick's theory model was appropriate in fitting the whole leaching process of HMs and dissolvable chloride species from the matrix under acid-immersion conditions. Both K-struvite and Phase-5 were formed in the matrix in mineral analysis, verifying the formation of both MPC and magnesium oxychloride cement (MOC). The chloride salts in the MSWI-FA facilitated the self-cementation of MOC from light-burnt magnesia in MPC. The mechanisms including the formation of phosphate precipitation, gelation encapsulation, and chemical hydration brought by MPC attributed to effective solidification/stabilization of HMs and chloride in the MSWI-FA.