A Study of Alteration Phases on Glass-Bonded Zeolite and Sodaiate using the Vapor Hydration Test

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ABSTRACT This paper focuses on the reaction of glass-bonded zeolite and sodalite ceramic waste forms, naturally occurring sodalite, and their respective alteration phases that formed after reaction in a Vapor Hydration Test (VHT). Scanning electron microscope (SEM) analysis of the glass-bonded zeolite and sodalite samples indicate that NaCI is the major alteration phase present, along with KCI, Na-Al-Si, and Na-K-Ca-Al-Si phases. The order of precipitation of salt crystals from ceramic waste form occurred in the sequence NaCI => KC1 zz CaCI 2 and MgC12. We also found aluminum silicate phases containing Ca, Mg, K-Na-Ca, K-Mg, Na-K-Mg, K-CaNa-Mg, as well as CaCOM, Si0 2 . The ratio of Si/Al in the alteration phases was noted to increase from approximately 1.5 to 5 during the progressive alteration of the samples. This trend indicates a relative increase in the contribution of silicon from the relatively Si-rich glass regions of the samples over time. Phases enriched in Cs, Ba and rare earth elements (RE) were also observed on the altered samples. Alteration phases initially appeared along the micro-fractures during the early stages of alteration. The number of micro-fractures, density of alteration phase cover, and grain size of alteration phases increased with reaction time and temperature. INTRODUCTION Argonne National Laboratory is currently developing the glass-bonded zeolite and

sodalite ceramic waste forms to isolate and dispose of salt-rich alkali metal, alkaline earth, and RE fission product wastes [1, 2]. The corrosion behavior of these waste forms needs to be investigated to evaluate the potential long-term repository behavior of these materials. The VHT is an accelerated testing method that can be used to study corrosion rates, alteration phase paragenesis, and potential incorporation of fission products into alteration phases [1]. This testing protocol can thus be used to gain a further understanding of the corrosion behavior of the ceramic material and provide infornation about potential waste form alteration processes that may occur on geologic time-scales [3, 4]. EXPERIMENTAL All of the ceramic samples used in the present testing program were provided by Argonne National Laboratory. Two types of ceramic waste forms were examined in the present set of tests. The first is referred to as the baseline ceramic, and was produced by blending equal amounts (by weight) of Bayer 57 glass plus salt-loaded zeolite 5A (Table I). The second type, the reference ceramic, was produced by blending 25% Bayer 57 glass plus 75% salt-loaded zeolite 4A with a clay binder. Stable isotopes of Cs, Ba, Sr, La, Ce, Nd, Pr, and Y were also added as surrogates for radioactive isotopes to both varieties of salt loaded zeolite. A hotisostatic press of powdered glass, plus salt-loaded synthetic zeolite was used to produce the simulated ceramic waste fonrs used in these tests [I]. The time and temperatures used in the hot 189 Mat. Res. Soc. Symp. Proc. Vol. 556

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1999 Materials Research Society

Table I. Composition of Various Comp