Study of zinc in cementitious material stabilised/solidified wastes by sequential chemical extraction and microstructural analysis

In the present study, a five step sequential chemical extraction (SCE) procedure was employed to investigate the binding mechanisms and chemical partitioning of zinc in cement-based stabilised waste materials. After each step of SCE, scanning electronic microscope with energy dispersive spectrometer (SEM/EDS) and X-ray diffraction (XRD) were used to assess the microstructural and chemical changes of the waste materials. The results showed that the leaching of Zn from the cement based stabilised waste mainly took place at the second and the third extraction steps at pH 5.0 and 2.0, respectively. The addition of pulverised fly ash (PFA) increased the leachability of Zn at the second extraction step. The SCE results also indicated the hydration of cement was greatly retarded by the incorporation of zinc waste. The SEM and XRD analysis of the samples after each extraction step demonstrated that zinc leached at the second step extraction was partially due to the dissolution of the new crystalline phase (ZnO) in the waste materials. Since large amounts of zinc were leached out only at the third extraction step at pH 2.0, these stablised waste materials may be relatively stable under a normal disposal environment.