Research on long-term stability of fly ash solidified by calcium aluminate cement-based materials through accelerated ageing test

The leaching under acid rain is an important factor that affects the long-term stability of solidified fly ash. The sodium dihydrogen phosphate (NaH₂PO₄) and sodium diethyl dithiocarbamate (DDTC) are added to the calcium aluminate cement (CAC) to obtain CAC-based materials for solidification of fly ash. The long-term stability of the cured fly ash is investigated through the accelerated ageing tests. The leaching toxicities of Cd, Pb and Zn are studied by leaching tests. The sequential extraction tests are conducted to investigate the chemical species of heavy metals. X-ray diffraction and scanning electron microscopy tests are used to analyze the mechanism involved. The results show that the leaching concentrations of Cd and Pb are the lowest at the 26th year. The concentrations increase afterward and exceed the limits at the 78th simulated year. The leaching concentration of Zn increases continuously during the simulated 104 years. After cured by the CAC-based materials, the heavy metal bound to organic and residual increases, and that bound to Fe-Mn oxides decreases. Along with ageing, the heavy metal bound to Fe-Mn oxides increases and the residual decreases, resulting in an increase in the 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 the encapsulation effects. This study provides theoretical basis and technical reference for the long-term safe disposal of fly ash.