Solidification/Stabilization of Waste Incineration Fly Ash by Modified Calcium Aluminate Cement

Ordinary Portland cement (OPC) is the most common curing agent for solidification/stabilization (S/S) of fly ash (FA), however, the treatment is not efficient. This study aims to investigate the S/S of FA with the modified calcium aluminate cement (CAC) as a binder. The modified CAC was obtained by adding sodium dihydrogen phosphate (NaH2PO4), an inorganic chemical agent, and sodium diethyl dithiocarbamate (DDTC), an organic chemical agent, into CAC. Toxicity characteristic leaching procedure and unconfined compressive strength (UCS) test were carried out to evaluate the treatment effect of the proposed binder. The sequential extraction procedure (SEP), X-ray diffraction and scanning electron microscopy (SEM) were used to study the chemical speciation, mineralogical characterization and microstructure of the solidified/stabilized FA. The treatment results of pure CAC and OPC showed that the immobilizing efficiency of CAC was much higher than that of OPC. When curing with 10% CAC and 1% DDTC, the leaching concentrations of Cd, Pb and Zn decreased to 0.06, 0.12 and 18.95 mg/L, respectively. With the combination of 10% CAC and 0.5% DDTC + 0.5% NaH2PO4, the leaching concentrations of Cd, Pb and Zn were 0.12, 0.16 and 10.21 mg/L, respectively. The regulated leaching limits can be met under both of the above combinations. The UCS of the solidified body was 3.02 MPa under the combination of CAC + NaH2PO4 + DDTC, which was much higher than the 1.34 MPa of the CAC + DDTC combination. SEP shows that most unstable forms of Cd, Pb and Zn in FA were converted to stable forms after the treatment by modified CAC. The immobilization mechanisms of the proposed binder included encapsulation of heavy metals and the formation of heavy metal complexes or precipitate. The CAC hydration products, the precipitate and complexes reduced the pores in FA particles and therefore reduced the leachability of heavy metals. This study demonstrated the effectiveness of the proposed modified CAC as a binder for FA treatment. As NaH2PO4 is inexpensive than DDTC, the scheme of CAC + NaH2PO4 + DDTC is recommended.