Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 202

TABLE 9.1
Gel Pores and Capillary Pores in Hardened Cement Pastes 1

Paste Properties
Affected Designation Diameter Description Role of Water

Capillary Pores 10 μm - 50 nm Large capillaries Behaves as bulk

water

Strength, permeability

50 - 10 nm Medium

capillaries

Moderate surface
tension generated

Strength, permeability,
shrinkage at high
humidity

Gel Pores 10 - 2.5 nm Small (gel)

capillaries

Strong surface
tension generated

Shrinkage to RH = 50%

2.5 - 0.5 nm Micropores Strongly adsorbed

water, no menisci
form

Shrinkage, creep

< 0.5 nm Micropores

“interlayer”

Structural water
involved in bonding

Shrinkage, creep

conductive coatings are also necessary for insulating materials. The analytical tech-
niques differ in whether or not they allow in situ experimentation. For example,
many X-ray-based techniques can be conducted in air or in a wet state, and thus
“real world” conditions can be simulated. In contrast, electron beam-based tech-
niques usually require high vacuum, and consequently moist specimens cannot be
observed. However, new developments in instrumentation have often blurred these
boundaries. Environmental electron microscopes allow observation of moist samples
without any conductive coating.

9.4 MICROSCOPIC CHARACTERIZATION OF S/S

WASTE FORMS

The microstructure of a waste form can be defined as the spatial relationship between
its different constituents, including pores. This information often cannot be resolved
with the naked human eye and can be obtained only with the aid of a microscope.
Among the properties that need to be documented are the grain size, morphology
of the individual phases, relationship between the different phases, whether or not
it is surrounding another (encapsulation, for example), pore spaces, and pore-filling
phases.
At an elementary level, the overall color of the waste form can be an indicator
of the degree of mixing of the waste and the binder. If a waste is solid or paste,
mechanical mixing is not usually achieved at the microscopic level. The waste and
the binder can easily be identified as regions of different colors. At low magnifica-
tions up to 100 or so, a hand lens or an optical stereomicroscope may suffice. At
this stage no sample preparation is necessary. The microstructure can be studied at
magnifications up to 1000X with a transmission optical microscope. The color, an
important optical property, can still be observed with such a microscope. Thin