Study on hydration and heavy metal leaching of washed MSWI FA as a green cementitious material

Despite municipal solid waste incineration fly ash (MSWI FA) being classified as hazardous waste, there is a consensus on its significant potential for resource utilization. The high content of soluble chlorides in MSWI FA not only limits its co-disposal in cement kilns but also hinders its application in the field of construction materials. The research aimed to investigate the viability of MSWI FA as a binding material in the construction engineering field. Through examining the impact of liquid/solid ratio and pH levels on the chemical composition of MSWI FA during the washing procedure, the study delved into the mechanisms affecting hydration and the characteristics of heavy metal contamination before and after washing pretreatment. The results indicate the feasibility of developing MSWI FA into construction engineering materials. The liquid/solid ratio is a significant factor affecting the removal of chlorides, with an optimal ratio of 30. pH plays a crucial role in the mineral composition during the water-washing process of MSWI FA. An acidic environment can remove a higher proportion of chlorides, while an alkaline environment can form more calcium salts. The compressive strength of pure MSWI FA at 28 days is 3.92 MPa, which can be increased by 0.99 MPa after washing. The washed MSWI FA not only exhibits heightened reactivity but also demonstrates reduced leaching concentrations of heavy metals, and its leaching toxicity was much lower than the safe landfill threshold. Therefore, MSWI FA, after washing, is a very promising alternative material for construction engineering. •MSWI FA can be used as an alternative to building gelling materials.•The water--washing process can limit the toxicity of heavy metal leaching to some extent.•Alkaline washing pretreatment did not affect the reactivity of MSWI FA as the precursor of geopolymers.