Dual-reactivity MgO-mediated structural evolution of magnesium potassium phosphate cement-based matrices for heavy metal immobilization in municipal solid waste incineration fly ash

The stabilization of hazardous contaminants in municipal solid waste incineration fly ash using magnesium potassium phosphate cement is highly dependent on the regulation of MgO hydration kinetics. Herein, a dual-reactivity MgO strategy integrating light-burned MgO and dead-burned MgO was developed to regulate the time-dependent Mg 2+ supply for the solidification of municipal solid waste incineration fly ash using magnesium potassium phosphate cement. The effects of MgO reactivity regulation on hydration kinetics, phase evolution, microstructural development, mechanical performance, and contaminant immobilization were systematically investigated. The results revealed that light-burned MgO contributed to rapid initial Mg 2+ availability and early K-struvite nucleation, whereas dead-burned MgO may have provided sustained Mg 2+ availability to facilitate crystal growth and matrix densification. An optimized dead-burned MgO/light-burned MgO ratio of 4:6 achieved a crystalline–amorphous synergistic structure, resulting in the highest K-struvite content (38.3%), reduced porosity (16.89%), and enhanced 28-day compressive strength (48.92 MPa). The optimized magnesium potassium phosphate cement matrix effectively immobilized Cu 2+ , Pb 2+ , Zn 2+ , and Sr 2+ through synergistic mechanisms involving phosphate precipitation, lattice incorporation, adsorption, and physical encapsulation. Furthermore, the compressive strength of the regulated solidified matrix still decreases under wet–dry and freeze–thaw cycling conditions. This study establishes the intrinsic relationship between MgO reactivity, time-dependent Mg 2+ supply, hydrate structure evolution, and hierarchical contaminant immobilization, providing a new strategy for designing durable magnesium potassium phosphate cement-based materials for hazardous waste management.