Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 133

A significant effort was initiated at LANL to find the cause of the phenomenon
and prevent future occurrences. 12 The following two hypothetical mechanisms for
producing free liquid caused by irradiation were investigated and eliminated: (1) a
water-producing condensation reaction between previously unpolymerized
melamine and formaldehyde components of the melamine-formaldehyde (MF) resin,
and (2) reversal of the calcium sulfate hydration reaction (CaSO 4 .1/2H 2 O + 3/2H 2 O
= CaSO 4 .2H 2 O), an event that occurs at relatively low temperatures. 17 Gamma
irradiation via a 60 Co source proved that Envirostone sample waste forms generated
free liquid after absorbing a dose of 2.5 Mrads, but portland cement sample waste
forms generated no free liquid after absorbing a dose of 81 Mrads. 12
The generation of free liquid was ultimately shown to be reoxidation of hydrogen
formed from radiolysis of interstitial water or organic polymer in the solidified waste
form. Radiolysis of interstitial water has been investigated by several workers. 18-21
Hanford workers showed that free-liquid accumulation as a result of hydrogen or
other gas generation due to radiolysis could be correlated to the waste form's
compressive strength and permeability. 22 A computer model used these correlations
to predict the occurrence of free liquid formed from reoxidation of hydrogen pro-
duced by radiolysis as a function of the waste form permeability, the gas generation
rate, and internal pressurization. 23 Work at LANL with actual waste showed that
increasing either the amount of Envirostone or mix time reduced the occurrence and
volume of free liquid, presumably because of a reduction in pore size and perme-
ability. 15

6.4.2.2 Leaching Performance of Envirostone

LANL continued to use Envirostone as a solidifying medium at TA-55 until 1992,
when the waste forms were determined to be hazardous for chromium toxicity by
TCLP per RCRA standards, resulting in all TA-55 solidified waste forms being
reclassified as mixed waste drums and complicating storage requirements. 24,25 Port-
land cement waste forms were not hazardous for chromium toxicity and had TCLP
extract chromium concentrations an order of magnitude lower than was measured
for Envirostone waste forms. The difference in chromium solubility at the high pH
of cement and the low pH of the waste solidified in Envirostone or permeability
differences in the waste forms may have caused this difference in TCLP performance.
Figure 6.4.2 compares the TCLP performance of using Envirostone and portland
cement waste forms. Solidification operations returned to using portland cement as
the stabilization agent. No instances of free-liquid generation have been found in
the portland cement solidified waste forms generated since that time.

6.4.3 E XPERIENCE WITH E NVIROSTONE AT R OCKY F LATS

Envirostone was used at Rocky Flats Environmental Technology Site (RFETS) for
the immobilization of TRU-contaminated oils and halogenated solvents. 25 The pro-
cess used in this operation was called the Organic and Sludge Immobilization System
(OASIS). In this operation Envirostone, water, emulsifier, and oil were mixed in a
glovebox with an in-drum mixer to stabilize the organic wastes and produce an