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Mineralogical and geotechnical investigations on the possible use of compacted bentonite as a buffer material in nuclear waste repositories are reported. The swelling capacity is highly dependent on the density of the compacted bentonite. Swelling pressures >30 MPa were measured for dry densities of ~2.0 g/cm3. Added iron or magnetite powder up to 20 wt% had no influence on the swelling capacity. Compacted mixtures of 20 wt% ground set cement and bentonite showed higher swelling pressures but lower swelling strain capability than compacted bentonite alone. Steam lowered the swelling pressure of compacted bentonite to ~60% of the original value. The influence was, however, reversible by ultrasonic treatment. The thermal conductivity of saturated compacted bentonite at a density of 2.0-2.1 g/cm3is ~1.35-1.45 W/m°K The volumetric heat capacity ranges from 3.1 x 106to 3.4 x 106j/m3°C The saturated hydraulic conductivity of the compacted bentonite is <10-12m/s. The apparent diffusion coefficients for various ions in compacted bentonite for water contents in the range of 20 to 25 wt% are: K+: 5 x 10-11, Cs+: 6 x 10-12, Sr2+: 3 x 10-11, UO22+: <10-13, Th4+: <10-13, Fe2+: 4 x 10-11, Fe3+: 4 x 10-11, Cl-: 1 x 10-10and I-: 1 x 10-10m2/s. The 'breakthrough time' for an apparent diffusion coefficient of 10-11m2/s in compacted bentonite 1 m thick was estimated to be ~3000 years. The mineralogical longevity was investigated on natural K-bentonites from Kinnekulle, Sweden, and Montana, USA. Although these materials have undergone considerable changes during diagenesis and contain various amounts of mixed-layer illite-smectite, they still have a substantial swelling and adsorption capacity. The investigations demonstrate that although the properties of bentonite are negatively influenced to a certain extent by heat, hot steam, iron and cement, compacted bentonite is still the best choice to act as a buffer material in a nuclear waste repository.

Li +, Na +, Ca 2 § Sr 2+, CH 2+, or Zn2+-saturated samples of a cis-vacant montmorillonite from Linden, Bavaria, were heated to temperatures between 200-700~Half of each heated sample was subsequently autoclaved under steam at 200~ (--1.5 MPa) to promote rehydroxylation. The smectites were characterized by cation-exchange capacity (CEC), determination of exchangeable cations, infrared (IR) spectroscopy, and thermoanalytical investigations of evolved water in a thermobalance linked with a mass spectrometer.Changes in the montmorillonite structure and dehydroxylation behavior are related to three respective mechanisms: type of the interlayer cation, interlayer cation radius, and the movement of the interlayer cation. The migration of the smaller Li +, Cn 2+, and Zn 2+ ions after heating produces a strong reduction of CEC due to the Hofmann-Klemen effect before the initiation of dehydroxylation. Thereafter, the CEC of these smectites remains constant over a large temperature interval during dehydroxylation. After rehydroxylation, Cu 2+ and Zn2+-rich samples release 16-23 meq/100 g ofMg 2+ from the structure. No Mg z+ release is observed for the Li+-rich montmorillonite. Also the dehydroxylation behavior after rehydroxylation differs between the Cu 2+, Zn 2+, and Li+-rich samples. The mass curves of the evolved water during thermoanalysis of the rehydroxylated Cu 2+ and Zn2+-rich smectites show a peak doublet between 480-700~ For the Li +, Na +, Ca 2+, and Sr2+-rich montmorillonites, the second peak disappeared and a third peak at -760~ developed after rehydroxylation. The resulting structure after rehydroxylation of all samples is celadonite-like.

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