| dc.description.abstract | 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. | en_US |
