Environmental Engineering Reference
In-Depth Information
has proven to be enormously productive and was extended to investigations at the
atomic and molecular level in the 20 th century. There is thus an enormous literature
that is relevant to microstructural investigations of S/S waste products. This chapter
summarizes a wide range of experimental techniques and their applications for S/S
waste forms.
Microstructural studies of solidified wastes help explain the efficacy of the S/S
process by assessing the microstructure of the binder and its interactions with the
wastes, and predict its long-term stability. Among the kinds of information of interest
are
•E ficiency of mixing of the waste and binder
Macroscopic structure and the presence of cracks or micro-cracks
Microscopic pore structure including size and distribution
Mineralogy and morphology of hydration products of the binder
Interactions between the hydration products and wastes
Stability of the mineral phases
A common waste form is cement-stabilized soil, which illustrates very well the
complications involved in acquiring structural information. Both cement and soil
consist of several mineral phases in various concentrations, and while some minerals
are easy to identify because of their properties, others are not. For example, a few
percentages of quartz can be easily identified by X-ray diffractometry (XRD) because
of strong reflection of X-rays from its atomic planes. In contrast, clay minerals are
not as easy to identify because these reflections are weak, and the chemical com-
position is variable. Multiple analytical techniques are necessary for positive iden-
tification of clay minerals. It is highly desirable to be able to identify small quantities
of clays, because clay in even small amounts can have an inordinately large influence
on the chemical and physical properties of a soil.
9.2
MICROSTRUCTURAL CHARACTERISTICS OF
CEMENT PASTES AND CEMENT-BASED WASTE
FORMS
Chapters 4 and 5 discussed hydraulic cement systems and organic binders used for
S/S of hazardous, radioactive, and mixed wastes. A hardened cement paste is a
heterogeneous multi-phase system. At room temperature, a fully hydrated portland
cement paste consists of 50 to 60% calcium-silicate-hydrate (C-S-H) gel, 20 to 25%
Ca(OH) 2 , 15 to 20% AFt and AFm by volume. These minor hydration products,
such as Ca(OH) 2 , 3CaO•Al 2 O 3 •6H 2 O, and AFt, form in small quantities depending
on the composition of the cementing material and hydration conditions. AFt domi-
nates the early hydration matrix and initial set of portland cement as the gypsum
ground with the cement clinker hydrates and interacts with the hydrating cement
minerals. Ettringite (AFt) converts to monosulfate (AFm) as the sulfate anion
becomes depleted from continued hydration reactions and C-S-H replaces AFt as
the dominant phase in the hardened cement pastes.
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