Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 201

C-S-H gel is the main binding component and responsible for the hardened
pastes. Its chemical composition varies with hydration conditions. C-S-H is a micro-
porous material and has a very high surface area of about 100 to 700 m 2 /g, depending
on the measurement technique used. The structure of C-S-H is regular within a
particular sheet and resembles a clay structure; 1 however, there is no long-range
order. The hydrated paste has a more massive inner structure (inner C-S-H) different
from the more porous outer rim, the outer C-S-H, which is somewhat fibrous. 1,2
Calcium hydroxide is crystalline and is easily identified in the scanning electron
microscopy (SEM) by its platy hexagonal outline. The crystals are usually a few
μm in diameter, but the size depends on many variables. In mature systems, “mas-
sive” calcium hydroxides are found where these crystals are much larger and the
crystalline outline is absent. Ettringite can be easily identified in the optical micros-
copy (OM) or SEM by its needle-like crystals. The morphology and dimensions of
AFm are similar to that of calcium hydroxide. Thus morphology alone cannot be
used to differentiate between AFm and calcium hydroxide; energy dispersive X-ray
spectrometry (EDX) is necessary. AFm is quite common in fly ash containing waste
forms. Some of the contaminants may also enter its crystal structure.
Hardened cement pastes are porous materials and contain three types of pores:
gel pores, capillary pores, and air voids. In portland cement pastes, gel pores con-
stitute about 28% of the total C-S-H gel volume and have a size of 1.5 to 2.0 nm,
which is the size of a water molecule and will not permit the flow of water. Gel
porosity cannot be resolved by SEM and would be included in the volume occupied
by C-S-H. Capillary pores are the portion of the space that was originally filled by
water in the fresh cement pastes, and has not been filled by cement hydration
products. Air voids are the result of incomplete consolidation or entrapped air, or
both. Capillary pores are usually tortuous and tubelike, while air voids are shorter
but much larger. Table 9.1 describes the classification of gel and capillary pores and
their effects on the properties of hardened cement pastes. 1 Physical encapsulation
plays a very important role in immobilizing contaminants within solidified waste
forms. Thus it is critical to reduce or eliminate the capillary pores to control the
movement of contaminants within solidified waste forms.
The inclusion of these wastes, in most cases, interferes with the hydration of
hydraulic cements as discussed in Chapter 7. Of course, it also affects the microstruc-
ture of waste forms. However, in most cases, these wastes have no effect on the
structural characteristics of organic binders, except they are physically encapsulated.

9.3 ANALYTICAL TECHNIQUES

A variety of analytical techniques have been developed and used for the microstruc-
tural characterization of materials. Mollah et al. 41 classified the techniques for char-
acterization of S/S waste forms into three categories: molecular information, struc-
tural information, and surface information, as summarized in Figure 9.1.
An analytical technique usually probes a certain narrow property of a material.
Applications of spectroscopy in materials characterization have been reviewed in a
recent publication. 3 Most techniques also require specimens in a certain specific
physical form, and a specimen for one can rarely be used for another. Sometimes