Environmental Engineering Reference
In-Depth Information
FIGURE 9.14 XRD, DTG, DSC, and FTIR on field samples of contaminated soil 10 years
after remediation.
soil, in some cases two of the three. The basal reflection peaks for illite and kaolinite
are often strong while that of smectite is weak. XRD patterns of some soils showed
that two peaks were present in the 4° to 8° 2θ range. One of these corresponds to
smectite, and the other could be chlorite. The principal calcium carbonate phase is
vaterite. Calcite, another calcium carbonate mineral, was detected in only one or
two soils. Ettringite, a mineral characteristic of set portland cement, was detected
in many soils, and often more than one peak of ettringite is present. A peak from
unhydrated cement clinker, tricalcium silicate, is tentatively identified in some soils,
as the peaks are weak, and only one peak of tricalcium silicate is observed (many
peaks of C3S can be detected by XRD very easily; we are afraid that it is not evident
enough to confirm the presence of C3S with only one peak of C3S). However,
previous studies of cement hydration has indicated which peak of unhydrated tri-
calcium silicate is likely to remain. This peak is present only in those soils containing
ettringite. The presence of ettringite also allows us to infer the minimum pH (> 10.7)
of the waste form. 47,48
TG of the waste forms shows mass loss ranging from 5 to 12% (for example,
it is 8% for the sample shown in Figure 9.14C). In some instances the mass loss
occurs at less than 100ºC, which is due to loss from adsorbed moisture and ettringite
dehydration, if the latter is present. In some waste forms the dehydration events of
the individual minerals are separated, whereas in others, they are highly overlapping.
It is easier to identify kaolinite by DTG because of its well-defined dehydroxylation
characteristics. Dehydroxylation for smectite and illite occurs over a wider temper-
ature range and is somewhat diffuse. The peak location can be used to identify these
clay minerals. For smectite, desorption from surface and dehydration from interlayer
spaces can be observed by DTG. 49 Calcium carbonate decarbonation can be easily
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