Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 137

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Sept. 19, 2003.

6.5 HYDROCERAMIC CONCRETES

Darryl D. Siemer

6.5.1 I NTRODUCTION

The U.S. federal government selected vitrification over 20 years ago as the baseline
technology to treat reprocessing waste for disposal. Vitrification has proven expen-
sive within the DOE Complex 1 such that the latest request for proposal from DOE-
ID encourages contractors to consider undefined alternative technologies. 2 Great
Britain applied C-S-H-based cements to their reprocessing waste from the Cold War
that was not too radioactive (< 300 watts/m 3 ) to damage concrete, 3 and this same
approach could be used in the U.S.
The U.S. erected institutional barriers to this approach, including QC protocols
that tacitly assume contaminants are released when the waste form matrix dissolves,
similar to glass. Consequently, current U.S. waste acceptance criteria (WAC) are
generally keyed to the corrosion (leach) rates of a candidate material's bulk constit-
uents (usually sodium), not to those of its toxic or radioactive components. Concretes
made with C-S-H, calcium aluminate, or magnesium phosphate-based cements will
not pass such tests because: (1) such waste is largely comprised of water-soluble
sodium salts with which conventional cements do not form insoluble minerals; (2)
with the exception of the calcined waste stored at INEEL, DOE has not used waste
pretreatment technologies (e.g., calcination) that make these high-sodium salty
wastes more compatible with conventional grouts; and (3) the leach tests are pre-
ceded by size-reducing operations (grinding, crushing, and so on) that deliberately
destroy the specimen's physical integrity and compromise physical encapsulation of
soluble constituents. This approach undercuts a chief advantage of any cementitious
technology: the ability to make large monolithic waste forms relatively simple and
cheap.
The hydroceramic (HC) process 4,5 addresses the problem of leachable salts by
trapping sodium salt molecules within an aluminosilicate “cage mineral” (zeolite,
sodalite, cancrinite, and so on) assemblage formed during the curing step. HC uses
the same approach developed for the Formed Under Elevated Temperature and
Pressure (FUETAP) cement waste form, 6 but substituting metakaolin and sodium