The accelerated ageing tests of fly ash solidified/stabilized by calcium aluminate cement combined materials

The safe disposal of fly ash is a weak link in the pollution control of municipal solid waste incineration. There are still some disadvantages in the solidification/stabilization (S/S) technique based on ordinary Portland cement. In this study, sodium dihydrogen phosphate (NaH2PO4) and sodium diethyl dithiocarbamate (DDTC) were added to calcium aluminate cement (CAC) to obtain CAC combined materials for S/S of fly ash. The long-term stability of the cured fly ash was investigated by accelerated laboratory ageing tests. The leaching toxicity of Cd, Pb, and Zn was studied by leaching tests. Sequential extraction tests were conducted to investigate the chemical species of heavy metals. X-ray diffraction and scanning electron microscopy tests were used to analyze the mineral composition and microstructural of the fly ash. The results show that the leaching concentrations of Cd and Pb are the lowest at the 26th year, with the concentrations of 0.10 and 0.13 mg/L, respectively. The concentrations increase afterward and exceed the limits at the 78th simulated year. The leaching concentration of Zn increases continuously and reaches 47.89 mg/L in the 104th year. After cured by the CAC-based materials, the stable chemical fractions, F4 and F5, of Cd, Pb, and Zn in the fly ash increase, and the metastable F3 decreased. Along with ageing, F3 increases and F5 decreases, resulting in an increase in leaching concentration. The CAC hydration products, phosphate precipitation, and complexes increase the amount of aggregates and reduce the pores, thus reducing the leachability of heavy metals. After the accelerated ageing tests, these materials decrease or even disappear, resulting in a decrease in encapsulation effect. This study reveals the effectiveness and ageing rule of the CAC combined materials for S/S of fly ash, and provide theoretical basis and technical support for the long-term safe disposal of fly ash.