Leaching behaviors and binding chemistry of industrial sludge by solidification/stabilization techniques

Currently, the success of a solidification/stabilization (S/S) process is determined by physical and leach tests. These tests might not provide sufficient information for determining the actual performance of the S/S waste in a long run since they do not provide information on the physical and chemical changes between the waste and the additives. This research project utilizes a combined approach to investigate the leaching behaviors and binding chemistry of a solidified/stabilized industrial waste. The combined approach includes physical test, leach tests and microstructural analysis. Two cement-based solidification systems were considered: one with cement alone (OPC system) and the other with cement and fly ash (OPC/PFA system). The OPC system shows satisfactory results in unconfined compressive strength (UCS), toxicity characteristic leaching procedure (TCLP), and dynamic leach test (DLT). For the OPC/PFA system, the use of the coarse type PFA fails the UCS, and this system has higher zinc leaching than the OPC system. Sequential chemical extraction (SCE) shows that zinc has the highest leaching in the oxide phase. Microstructural analyses of the solidified samples were examined by scanning electron microscopy (SEM) associated with an energy dispersive spectrometry (EDAX), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). SEM images of solidified samples suggest that the presence of zinc might retard the hydration of OPC and PFA because identifiable hydration products are no longer observed. Moreover, zincite (ZnO) and calcium zincate [CaZn2(OH)6H2O] are found in the OPC system but only ZnO is found in the OPC/PFA system. It is believed that the addition of cement in the zinc sludge might alter zinc from its amorphous form to a crystalline form. The addition of PFA fails to help in the fixation of zinc since EDAX shows it has no interaction with the sludge. Also, a certain amount of zinc is believed to be on the surface. In summary, the OPC system is more effective than the OPC/PFA system in this S/S process. The use of microstructural analysis has been proved to be useful and essential in providing information on the physical and chemical changes within the cement matrix