Interactions between Wastes and Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 165

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10

PLATING SLUDGE

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0 1 2 3
Industrial Waste

FIGURE 7.2 Encapsulated sludge particle. 65

furnace dust, may contain a mixture of soluble species, such as chlorides and sulfates,
and less soluble species, such as oxides. Depending also on reaction kinetics, the
former may dissolve during or soon after mixing, releasing contaminants for other
interaction with the cement hydration products, whereas the latter may dissolve more
gradually and participate in secondary reactions during cement hardening and curing.
The degree of reaction of precipitates from pretreatment of liquid wastes will also
depend on their solubility; sulfides can be expected to have low solubility unless
subjected to oxidizing influences, and many phosphates, carbonates, silicates, and
hydroxides also have low solubility. Figure 7.2 shows an example of encapsulation
of a hydroxide sludge floc in a cementitious matrix, 65 where little reaction with the
cement appears to have taken place. However, energy-dispersive x-ray analyses of
this sample did show the presence of contamination within the surrounding cement-
based matrix, suggesting that some of the sludge dissolved during mixing, or that
migration of contaminants from the sludge floc took place. Encapsulated sorbents
and ion exchange beads will also release contaminants into the pore solution, pos-
sibly leading to other interactions with the cement hydration products. Encapsulated
metals, such as shredded reactor fuel cladding, can hydrolyze over time to create
metal hydroxides and acid or hydrogen gas. Some soil components, such as clay,
may exhibit pozzolanic reactivity with cement over time, as discussed in Chapter 4.
Encapsulation of droplets of organic liquids that are not miscible with water has
also been observed. 48,66
Immobilization of encapsulated contaminants can be compromised by physical
deterioration of the matrix, as either cracking due to physical stresses or advanced
matrix dissolution (Chapter 10) can lead to exposure of encapsulated materials.
Formulations containing high concentrations of soluble salts or high-ettringite
cements 67 are particularly vulnerable to the latter.

7.3.3 P RECIPITATION

Whereas metal contaminants may have been precipitated as a pretreatment prior to
S/S with cement, a dissolved metal ion may also be precipitated during treatment
with cement, if its concentration in the pore solution of the resulting cement-based
product exceeds the saturation concentration with respect to a metal salt. Thus,