Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 138

hydroxide for portland cement. The HC process cooks the waste with an aqueous
solution containing high concentrations of silicates, aluminates, and hydroxides,
converting the reactive ions into poorly soluble minerals. The addition of a small
amount of sulfide or powdered vermiculite to the formulation further enhances
chemical fixation of certain key contaminants (e.g., cadmium, chromium, techne-
tium, and cesium).

6.5.2 H YDROCERAMICS VERSUS G EOPOLYMERS

HCs represent a subset of the “geopolymeric” (GP) concretes mentioned in Chapter
4, in that both are made by activating a powdered aluminosilicate substrate (poz-
zolan) with a basic solution. However, both the formulations and finished products
are different, as shown in Table 6.5.1.
Table 6.5.2 characterizes several aluminosilicate concretes, the raw materials
from which they were made, and a pelletized, commercial, sodium-form “Zeolite
A.” Two of the concrete specimens contained 50 wt% of a calcine produced by
“steam reforming” a slurry of an INEEL liquid waste simulant with kaolin 7 at 720°C
(the gross composition approximates nepheline). The data in Table 6.5.2 illustrate
the following points:

1. All aluminosilicate concretes exhibit a great deal more cation exchange

capacity (zeolitic behavior) than do the materials from which they are
made.

2. Sodium-based activators generate more ion exchange (IX) capacity than

do potassium-based activators.

3. Concretes made with sodium-based activators do a better job of retaining

their alkali when leached with water.

4. Metakaolin produces products with higher IX capacity and greater leach

resistance than does Class F fly ash.

5. There is little correlation between the concrete's physical strength and its

leach resistance.

The physical characteristics (strength, appearance, and porosity) of HC concretes
resemble those of portland cement-based radwaste concretes (grouts). The leach
behavior of constituents (most fission product elements, uranium, TRU, the alkaline
earths, fuel cladding metals, plus RCRA-characteristic metals) is also similar to
other waste forms that generate an alkaline buffer solution under leach test condi-
tions. 8 Again, HC concrete's outstanding characteristic is that it surpasses the per-
formance of DOE's benchmark glass using the current standard leach tests.

6.5.3 M ANUFACTURE OF H YDROCERAMIC C ONCRETE

6.5.3.1 Materials

Raw HC grout consists of a mixture of waste (typically 30 wt% dry-basis), metaka-
olin (calcined kaolin), ~5% powdered vermiculite (to enhance 137 Cs fixation), ~0.5
wt% sodium sulfide (redox buffer and RCRA metal precipitant), plus ~10% sodium