Carbonation of Cement-Solidified Hazardous Waste
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Solidification technology can be an effective process for treating a variety of difficult to manage waste materials containing heavy metals prior to reuse or disposal. There are numerous commercial solidification techniques spanning a spectrum of technical complexity and cost. The most common methods include those based on cement or cement/pozzolanic materials. These materials, which are used in many solidification processes, make the technology appear simple and inexpensive. However, there are significant challenges to the successful application of this technique. The morphology and chemistry of the solidified waste forms are complex, specially when the waste streams used contain components other than the metals that are likely to be effectively immobilised. Also, the selection of the binder, depends upon an understanding of the chemistry of both the contaminants and the binder itself, to ensure efficient and reliable results. Nevertheless,a number of complex interactions are known to cause significant retardation on normal hydraulic reactions of cement-based materials, causing numerous and controversial problems. In recent years there has been renewed interest in elucidating the binding mechanisms responsible for the fixation of waste species. Carbonation, which is known to affect a wide range of cementitious materials, is a phenomenon observed by many scientists and has received very little attention. The aim of this work has been to investigate the effects of natural and accelerated carbonation on the development of mechanical and microstructural properties of solidified products as well as on the binding of metallic waste components. Particular emphasis was paid to examine the influence of different binders on the properties of carbonated solidified waste forms. The kinetics of the carbonation reaction was thoroughly examined, particularly when mix parameters such as binder/waste type and water content were varied. An examination of the resulting products showed that carbonated solidified waste materials had improved mechanical properties and increased metal binding capacity, when compared to specimens cured in nitrogen or normal atmospheric conditions. Microstructural analysis showed that large amounts of calcite where characteristics of carbonated samples. The increased formation of calcite as a result of carbonation appeared to be directly linked with the development of strength and enhanced metals fixation. NMR and FTIR spectroscopy indicated that carbonation has a significant influence on the hydration of waste forms by increasing the degree of polymerisation of the silicate hydration phases, with a consequent acceleration of the hydration of the cement paste. Examination by SEM analysis confirmed an acceleration of C3S hydration, typified by a de-calcified hydration rims and a matrix of dense calcite intergrowth infilling porosity. Some metals appeared to be incorporated in the silica-rich rims and others in the calcite rich matrix, suggesting precipitation of metal as both carbonates, silicates and complex double-salts. An examination of the kinetic of the carbonation reaction revealed that the reactivity of the different cements was different in the presence of carbon dioxide, and that when metal wastes were added the susceptibility of the paste to react with carbon dioxide increased. In general the results of this work indicate the potential of carbon dioxide for incorporation into the treatment of wastes during solidification. However, further work is necessary to establish the long-term performance of these carbonated waste forms as well as the behaviour of carbon dioxide upon different waste streams.
AuthorsLange, Lisete Celina
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