Heavy metals leaching control in fly ash through cement and chelating agent solidification: A microstructural evolution kinetics study

Heavy metals exhibit high leachability in municipal solid waste incineration fly ash (MSWI FA), which brings severe environmental threats. The solidification/stabilization (S/S) of heavy metals can not be achieved by using cement alone to comply with the regulatory standard of China. The study explored the effect and mechanism of S/S heavy metals in FA with different dosages of piperazine chelating agent (TS300) and ordinary Portland cement (OPC525). The kinetic results additionally reveal surface wash-off as the predominant leaching mechanism. This study quantified the regulatory effects of TS300 on physical transport properties of OPC525 matrix through non-reactive tracer technique. Despite the more open pore network, heavy metals were effectively immobilized, mainly due to two synergistic chemical mechanisms: the formation of stable heavy metal–thiol coordination bonds and the crystallographic transformation of hydration products from ettringite to Friedel’s salt. This chemical S/S mechanism enhanced chemical retention factors by 9–3458 fold, demonstrating that the synergistic chemical effects of coordination bond formation and hydration products optimization far exceed traditional physical encapsulation mechanisms. These findings provide new insights into dual-mechanism design strategies for heavy metals S/S.