Analcime reactions at 25 90 8C in hyperalkaline fluids D. SAVAGE 1, *, C. ROCHELLE 2 , Y. MOORE 2 , A. MILODOWSKI 2 , K. BATEMAN 2 , D. BAILEY 2 AND M. MIHARA 3 1 Quintessa Ltd, 24 Trevor Road, West Bridgford, Nottingham NG2 6FS, UK 2 British Geological Survey, Keyworth, Nottinghamshir e NG12 5GG, UK 3 Japan Nuclear Cycle Development Institute (JNC), Waste Isolation Research Division, Waste Management and Fuel Cycle Research Center, Tokai Works, Muramatsu, Tokai-Mura, Naka-gun, Ibaraki 319-1194, Japan ABSTRACT Extensive use of cement and concrete is envisaged in the construction of geological disposal facilities for radioactive wastes. The hyperalkaline porefluids typical of groundwaters that have reacted with these materials have the potential to react chemically with other engineered barrier components such as bentonite, potentially degrading their performance. Analcime, NaAlSi 2 O 6 .H 2 O, has been identified from previous modelling and experimental studies as a potential alteration product of bentonite. Laboratory experiments to investigate the stability of analcime under hyperalkaline porefluid conditions have been performed. Experiments used both batch and fluidized bed equipment at 25, 50, 70 and 908C in K-based pH buffer solutions, both under- and over-saturated with respect to analcime. Results from dissolution experiments demonstrate that release of Na was greatly enhanced (by up to a factor of thirty) over that for Si and Al, particularly at pH 10 and 11. However, enhanced release of both Na and Al occurred in the batch experiments at pH 12 13. Near stoichiometric dissolution was observed in fluidized bed experiments under steady-state conditions at 708C. Sodium was removed from the analcime structure by ion exchange for K, without involving dissolution and re-precipitation of the analcime framework. Scanning electron microscopy of reacted analcime grains showed that some grains had pronounced cracks parallel to original cleavage traces. These cracks were a result of volume decrease due to the substitution of K for Na ions and water molecules in the analcime structure to form leucite, KAlSi 2 O 6 . Synthesis of the dissolution data shows that the rate of dissolution increased with increasing temperature in the range 25 708C and with pH at each temperature. Absolute rates of dissolution ranged from 10 10 mol m 2 s 1 at pH 9.5 at 258C to 10 7 mol m 2 s 1 at pH 12 at 70 and 908C. The rate of dissolution at any temperature was pH-dependent, such that the rate could be described by k (a H +) n , where k is the rate constant and n is 0.3 at 258C, 0.4 at 508C, 0.6 at 708C and 0.7 at 908C. Attempts to measure the growth rate of analcime in supersaturated solutions at 70 and 908C were unsuccessful, although a limiting rate at 708C, pH 10 was calculated to be 4610 11 mol m 2 s 1 , roughly 1006 less than the rate of dissolution under the same conditions. These results imply that any trace amounts of analcime in bentonite will be converted to leucite by reaction with cement fluids with a high K/Na ratio. In some instances, leucite may thus incorporate K + in preference to other phases (e.g. illite, K-feldspar) during alteration of bentonite by cement porefluids. KEYWORDS: analcime, leucite, dissolution kinetics, experiments, ion exchange. Introduction EXTENSIVE use of cement and concrete is envisaged in the construction of geological disposal facilities for low- and intermediate- level radioactive wastes, for structural, encapsula- tion, and backlling purposes (e.g. Savage, 1995; Francis et al., 1997; Glasser, 2001). Saturation of these materials with groundwater will occur in the post-closure period of disposal, producing a hyperalkaline poreuid with a pH in the range 10 13.5 (Berner, 1992). The relatively low solubility of cement and slow groundwater ow # 2001 The Mineralogical Society * E-mail: davidsavage@quintessa.or g Mineralogica l Magazine, October 2001, Vol. 65(5), pp. 571–587