Performance of Cement-Basedseal Systemcomponents Inawaste Disposal Environment
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ABSTRACT A grout based on portland cement, Class F fly ash, and bentonite clay was developed as part of the closure system of shallow subsurface structures for disposal of low-activity radioactive wastes. Heat output, volume change, and compressive strength of the sealing grout were monitored with time, at elevated temperature, and in physical models, to determine if this closure grout could maintain adequate volume stability and other required physical properties in the internal environment of the disposal structure. To determine if contact with an alkaline liquid waste would cause chemical deterioration of the sealing grout, cured specimens were immersed in a liquid waste simulant containing high concentrations of sodium and aluminum salts. Over a period of 21 days at 60 'C, specimens increased in mass without significant changes in volume. X-ray diffraction of reacted specimens revealed crystallization of sodium aluminum silicate hydrate. The new phase has an X-ray diffraction pattern similar to that of the commercial synthetic zeolite, Losod. Scanning electron microscopy used with X-ray fluorescence showed that clusters of this phase had formed in grout pores, to increase grout density and decrease its effective porosity. The testing was repeated at 100 'C for 5 days using a simulant containing sodium hydroxide and aluminum nitrate and the results were similar. Physical and chemical tests collectively indicate acceptable performance of this grout as a seal-system component. INTRODUCTION Tests of the effect of alkaline wastes on a portland-cement based grout were undertaken as part of a research program on development of sealing grouts for the DOE Hanford Grout Vault Program. 1-3 The grouts were developed to provide stable, cemented layers that fill the void between the top of the wasteform grout and the precast reinforced concrete panels that form the lid of the vault. This material was not designed specifically to serve in contact with the liquid waste. However, to establish what effect such an event would have on the performance of the void-filling grout, samples of cured grout were exposed to simulated liquid wastes under temperature conditions similar to those expected within the vaults. After initial tests showed that the grout interacted chemically with simulated 106-AN liquid waste (Table I) exposure tests were repeated using a solution containing only sodium hydroxide and aluminum nitrate. The second series of tests demonstrated the causes of reactions that alter grout properties. These investigations allow prediction of the effects on the containment system of free waste liquid within the sealed vault. METHODS AND MATERIALS Grout Exposed to Simulated 106-AN Waste A simulated waste liquid was developed by Oak Ridge National Laboratory to duplicate the chemical characteristics of the contents of Tank 106-AN at the DOE Hanford Reservation. Constituents of this simulated waste are listed in Table I. In preparing the simulant, all of the 191 Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials R
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