Environmental Engineering Reference
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identified by DTG. It occurs at a slightly higher temperature than smectite. The DSC
curves of the soil usually have a strong peak at temperatures less than 100ºC followed
by some weaker peaks at higher temperatures. The low-temperature peaks are asso-
ciated with desorption of water from the smectite surface. The higher temperature
peaks are due to dehydration from the interlayer.
Kaolinite and illite are identified from the FTIR patterns in most waste form
samples. The location of the molecular H
2
O vibration corresponds to that of smectite.
Quartz is also easily identified. A strong carbonate vibration band is present in the
FTIR pattern. The calcium carbonate phase is vaterite (μ-CaCO
3
). Vaterite is rare
geologically, but is often a carbonation product of portland cement. However, not
all waste form samples show an equal degree of carbonation, and not all cement-
containing soils show carbonation.
In the SEM, the clay minerals and quartz can be identified in the waste forms
by their morphology. The quartz grains are angular and often have smooth crystal-
lographic faces. In some cases the hexagonal plate-like structure of kaolinite can be
observed. Kaolinite typically occurs in nature as stacked hexagonal crystals and thus
can be differentiated from calcium hydroxide or calcium aluminates, the typical
cement hydration phases, which also have hexagonal shape. Some thicker hexagonal
plates are observed in the SEM; however, they are not stacked like kaolinite and
EDX indicates them to be calcium aluminate.
Some typical photomicrographs of the waste form are shown in Figure 9.15.
One of the products of cement hydration, calcium aluminate hydrate (Figure 9.15A),
can be identified in the photomicrograph. Illite from the soil can be identified (Figure
9.15B) by its layer structure and chemical composition. Ettringite is locally seen in
these waste forms (Figure 9.15C). When C-S-H is present, it is intimately mixed
with other phases of the soils. Foil-like morphologies corresponding to the C-S-H
phase in hydrated cement pastes are also often noted. Figure 9.15D shows the growth
of C-S-H next to the illite crystals.
In the SEM ettringite is sometimes seen, but not as commonly as XRD suggested.
Since XRD is a bulk technique and SEM can show spatial variation, the occurrence
of ettringite then must be localized. EDX spectrometry in the SEM shows a high
amount of calcium in these same samples. Clearly, after 10 years of ageing, these
samples show the presence of hydrated cement phases as well as products from the
carbonation of hydrated cement phases.
9.7
CONCLUDING REMARKS
Even a cursory literature search on the subject of S/S shows that multiple analytical
techniques are commonly used in the characterization of the waste forms. Since the
waste form is a multi-element, multi-phase, complex material, a better understanding
of its properties can be obtained only through the application of multiple techniques.
The total concentration of a contaminant has little correlation to its leachability,
which depends on the chemical species present. Very few techniques yield direct
evidence about the nature of the species of a contaminant, particularly at low
concentrations. XANES is one such technique.
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