Evolution pathway of heavy metal speciation during ultrasonic carbonation and subsequent cementitious utilization of MSWI fly ash

The low-carbon utilization of carbonated municipal solid waste incineration fly ash (MSWI FA) as a cementitious material represents a promising strategy for achieving both harmless disposal and resource recovery. However, the occurrence and transformation mechanisms of heavy metals within cementitious systems remain poorly understood, constraining their environmental safety and long-term stability. In this study, ultrasonic-assisted carbonation was employed to pretreat FA, enhancing dechlorination, carbonation, and dealkalization, thereby reconstructing its mineral composition and chemical environment. The ultrasonically carbonated FA (UFA) was subsequently incorporated into cement paste. pH-dependent leaching experiments combined with geochemical modeling elucidated the mineral phase transformations of major and trace elements in RFA, UFA, and FA-blended systems. In UFA-cementitious systems, ultrasonic carbonation reduced the alkalinity and ionic strength of FA, stabilized Ca and Mg as carbonate phases, transformed sulfur from gypsum to ettringite (AFt), and promoted the synergistic formation and densification of C-S-A-H gels and AFt phases. Anionic elements (As, Cr) were immobilized through adsorption on Fe oxides, complexation with organic ligands, and lattice substitution within AFt, while cationic metals (Cd, Pb, Zn, Cu) were stabilized via the formation of insoluble carbonates and hydroxides, as well as encapsulation within C-S-A-H gels. Through these synergistic mechanisms, heavy metal solubility was markedly suppressed over the pH range of 6–13, thereby maintaining the OPTI of UFA and cementitious matrices at a Low-risk level. This study reveals the elemental stabilization mechanisms involved in ultrasonic carbonation and hydration-gelation processes in FA, underpinning the improved environmental stability of FA-derived cementitious systems.