Environmental Engineering Reference
In-Depth Information
sludge did not strongly retard the pozzolanic reactions between calcium hydroxide
and the Class C fly ash in the binder. The gradual disappearance and appearance of
some peaks suggest that zinc nitrate is strongly modifying the hydration process of
the binder. The very small grain size of the sludge precipitate would also make it
unstable.
The grain size is not very critical for qualitative phase identification. Strictly
speaking, however, the XRD pattern is not representative if the grain size is too
coarse (> 15 to 20 μm). For qualitative data, the grinding should be done at least
with an agate mortar and pestle. The sample powder should be further ground with
a mechanical mill for quantitative analysis. Quantitative XRD analysis requires
proper grain size (5 to 1 μm). The powder also needs to be mounted in the specimen
holder in such a way that preferred orientation is minimized. Sample spinning during
data collection reduces the effect of preferred orientation and coarser grain size.
Modern quantification software also allows correction for preferred orientation. The
data should be collected over a wide 2θ range and for a longer time so that statis-
tically valid proportions are obtained. If all the phases in a powder are crystalline
it is possible to quantify them without any internal standard. If amorphous phases
are present then an internal standard is needed. Quartz, which is usually present in
soil or fly ashes used as binders, can be used as an internal standard in simple
experiments. Quantification would require a better characterized standard with
proper grain size. NIST Standard Reference Material 676 Alumina (corundum) is
often used in such instances.
If more than one diffraction pattern is obtained, it is important that uniform
parameters are employed in data collection. Strict comparison is otherwise difficult.
Even for the same diffractometer, the conditions may subtly (for example, the
intensity of the X-ray tube) change over time. In particular, the widths of the slits
used before and after the specimen should be noted.
9.5.6
F OURIER T RANSFORM I NFRARED (FTIR) S PECTROSCOPY
Infrared (IR) spectroscopy, particularly Fourier transform infrared (FTIR) spec-
troscopy, is commonly used in the characterization of stabilized/solidified waste.
The technique is rapid, does not require extensive sample preparation, and can
provide an overall characterization of the specimen. In conjunction with other
techniques, such as XRD and TG, it can provide important constraints in sample
characterization. Since IR spectroscopy focuses on the molecular bonds in a phase,
both amorphous and crystalline phases can be studied by this method. In the
absence of more expensive, time-consuming, and difficult-to-access characteriza-
tion data, such as NMR, many qualitative but important constraints can be deter-
mined from IR spectroscopy.
IR has been widely used for characterization of clay minerals (sheet silicates);
other types of silicate structures can be easily identified by IR. Cement hydration
can be followed by IR. The nature of water in a mineral structure, as hydroxyl or
molecular water, can be easily resolved by IR. Standard spectra for minerals neces-
sary for comparison are not always easily available, but can be obtained from a
number of sources. 38-40 The standard spectra for organics are more easily found.
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