Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 206

If the process used to treat a waste is unknown, much can be learned by studying
its microstructure and chemistry. Different cementitious materials can be identified
by the morphology and chemistry of the reactants and the products. The microstruc-
ture of waste forms depends fundamentally on the processing temperature. The low-
temperature waste forms produced from portland cement-like material yields a
relatively porous material. However, it is possible to reduce the porosity by the
addition of pozzolans such as silica fume and fly ash, which will fill up the pore
spaces. The high-level radioactive wastes, where the tolerance for failure is low, are
mostly treated by high-temperature processing, leading to glass or ceramic forma-
tion. These products have much lower porosity and would have less interconnected
pores. The presence of radioactive waste in a waste form also poses another special
difficulty: radiation damage. Radiation damage can change the structure of glasses
and ceramics and ultimately the microstructure.
The information obtained from microscopy is mostly qualitative, but with image
processing it can be turned into a quantitative tool. Image processing techniques are
now commonly available with the electron microscope. Its application to waste forms
may be difficult because of the complexity of its microstructure. However, even
semi-quantitative to qualitative observations can provide enough constraints for an
understanding of the S/S process.

9.5 MICROCHEMISTRY

Ultimately, the stability of a waste form is controlled by its chemistry at the micro-
scopic or even molecular level. The availability or release of a contaminant to the
environment will depend on the speciation of the contaminants. For example, chro-
mium in the trivalent form is relatively insoluble and benign, whereas the hexavalent
form is readily soluble and very toxic. A wet chemical analysis of the total chromium
in a waste form is no indication of its availability to the environment. The answers
about the speciation of a contaminant in a waste form can be obtained by studying
its microchemistry.
The contaminants can be present as discrete crystalline phases, as amorphous
phases, in solid solution with other elements, or adsorbed onto the surface of certain
phases. The degree of crystallinity of a waste form can be highly variable. One of
the main hydration products of portland cement, calcium silicate hydrate, is practi-
cally amorphous and constitutes about 70% (by mass) of the hydration products.
Fly ash, a commonly added constituent to waste form binders, can contain as much
as 90% glass.
Typically, regulatory tests are conducted on cement-based waste forms mostly
28 days after the stabilization. Though a substantial amount of the hydration reactions
in portland cement are completed by this time, 1 the addition of a waste to a cemen-
titious binder often retards its hydration process. However, only about 20% fly ash
in a blended cement is reacted at 28 days. 10 This means that the microstructure and
performance of cement-based waste forms continue to develop after that time. The
glassy waste forms containing high-level nuclear wastes are definitely metastable.
However, they may continue in that state for millions of years, as nature suggests.