Sodium-saturated Wyoming bentonite was hydrothermally reacted at 150° and 250°C for 30 to 180 days to determine smectite alteration rates that might be applied to nuclear-waste repository design. Na-Ca solutions deficient in K were used to determine the role of interlayer cations in the creation of high layer-charge in the smectites. The results provide insight into the mechanism and timing of various steps in the diagenetic alteration of smectite to illite. X-ray powder diffraction (XRD) analyses of the reacted clay showed little effect on the character of the 17-Å reflection even after 180 days at 250°C. Potassium saturation of these reacted clays and re-examination by XRD indicated collapse of some smectite layers, leaving at most only 60% expandable layers. The development of layer charge sufficient to cause collapse on saturation with a low hydration energy exchange-cation does not require K in the reacting fluid. Rate constants for the illitization reactions as determined by K-saturated collapse are between 1.0 × 10−3 and 2.8 × 10−3/day with activation energies <3.5 kcal/mole. Ca in a Na-silicate-bicarbonate solution slightly reduced the illitization rate constants. These rate constants are higher than expected from extrapolation of studies of beidellite-composition glasses at higher temperatures, but lower than values obtained in studies of natural clays in artificial sea water. The release of Si, Al, and Mg in the 150°C experiments suggests congruent dissolution of the smectite. In contrast, at 250°C the release of Al was not stoichiometric with Si; as little as one half of the relative available Si was released. Rather than different mechanisms for dissolution at the two temperatures, the conclusion is that noncrystalline Al-rich phases formed at greater rates at higher temperatures. The cation-exchange capacities for several of these reacted smectites were significantly less than expected, suggesting a clogging of interlayer sites, perhaps by Al-complexes.

The alteration and transformation behaviour of Ceca smectite in two acid and saline solutions (NaCl and KCl) was studied in batch experiments. This type of smectite is proposed as potential backfill material for nuclear waste storage sites. The initial solution was enriched with 500 ppm of U, Th and Eu. The evolution of pH and solution concentrations were measured over a period of 25 months. The mineralogical and chemical evolution of the clays was also studied. X-ray diffraction revealed a significant loss of diffraction intensity and the clays became amorphous to Xrays with increasing reaction time. Deconvolution of the XRD data indicated a continuous collapse of the smectite layers but no illitization. Infrared spectroscopy revealed unchanged smectite spectra at the end of the experiment. Thorium precipitated as amorphous ThO2, but showed an incipient crystallization after 500 days. The Th, Eu and U were neither adsorbed onto the clays nor incorporated into a secondary phase.

The mineral stability of two bentonites was studied in the presence of pyrite concentrate to simulate the possible reactions between the bentonite barrier used in a high-level nuclear waste (HLW) repository and host rock containing up to 5 wt.% of admixed pyrite. Smectite was the only bentonite mineral affected by pyrite treatment under the experimental conditions used. Bentonites reacted differently with pyrite due to the different nature of the smectites. The distinct crystal chemistry of the smectites controlled by the composition of the parent rocks influenced the smectite surface properties (cation exchange capacity and layer charge distribution) which resulted in a different response of the bentonites to pyrite treatment. A partial transformation of the original smectites into H-smectites represented the initial stage of smectite destabilization on acid attack. Rising non-equivalent isomorphous substitution in the octahedral sheets of the smectites enhanced smectite reactivity and thus the reaction between bentonite and pyrite, causing lower stability of the bentonite containing high-charge smectite.