Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 207

Very few long-term studies are available. 11,12 The long-term stability of a waste form
can be monitored and, hopefully, predicted by studying its microchemistry.

9.5.1 X-R AY A NALYSIS

At present, an EDX spectrometer is an almost integral part of an electron microscope.
Coupled with the microscopic observations, it can vastly improve our understanding
of the S/S processes.
With a properly prepared specimen, local quantitative chemical analysis from a
μm-diameter volume is possible with this technique. Thus, location of contaminants
in a waste form can be easily identified. The detection limit is typically 0.1%. The
electron beam in the microscope can also be rastered across the surface of the
specimen, and an elemental distribution map at the μm scale can be obtained. The
distribution of several elements can be mapped simultaneously. The elemental dis-
tribution correlates to the microstructure, as both are obtained without moving the
specimen. The technique can show whether a waste is segregating at grain boundaries
or is distributed within a phase. The spatial correlation between different elements
can give information about the affinity between different elements and the mineral-
ogy at the μm scale. The morphology of a phase sometimes may be insufficient for
its proper identification. For cementitious waste forms, calcium hydroxide and AFm
have the same morphology. EDX can then be used in their identification. The X-ray
fluorescence (XRF) process is used in the TEM for local chemical analysis. The
spot size in that case is much smaller, as the specimen is very thin (only a few
hundreds of nanometers) and spreading of the electron beam does not occur. The
XRF phenomenon is also used for X-ray mapping with synchrotron sources, where
specimens can be mostly analyzed in air, analysis can be obtained from a few μm-
diameter spot size, and, in addition, the speciation of an element can be determined.

9.5.2 S OLID -S TATE NMR S PECTROSCOPY

Solid-state nuclear magnetic resonance (NMR) spectroscopy was applied to the
characterization of cementitious materials almost as soon as effective techniques for
examining solids were devised. 13-15 Since cement materials are hydrated silicates
and aluminates, three NMR-active nuclei — Al, H, and Si — have been utilized as
probes in most of the work. H and Si are both spin-1/2 nuclei yielding spectral data
that are easier to interpret than that of Al, which is quadrupolar. A significant
advantage of NMR spectroscopy is that it is non-destructive; hence, the same sample
can be examined at different times after initial mixing of the ingredients, allowing
kinetic studies to be carried out. A limitation is that some binders, such as ordinary
portland cement (OPC), contain enough paramagnetic iron to interfere with some
NMR experiments. Low-iron-content “white” cements are often used in NMR exper-
iments, but the bulk of the evidence available suggests that the conclusions reached
in the white cement systems apply equally well to OPC.
The most common application of solid-state magic angle spinning NMR (MAS
NMR) to cements and cement-based waste forms is to monitor the development of
the silicate matrix by following the hydration reactions over time. The solid-state