Stability of bentonites in salt solutions: II. Potassium chloride solution — Initial step of illitization?

Bentonites are supposed to be suitable materials for the production of geotechnical barriers in high level radioactive waste (HLRW) repositories if the bentonites' swelling capacity is maintained at the conditions expected; e.g. in contact with different solutions which may occur during the long term storage. Among all other common cations, K+ is believed to play a special role in contact with smectites because it can be adsorbed irreversibly and may cause collapse of the interlayer space.In the present study 36 different bentonites were reacted with KCl-solutions at 60 °C with and without wetting — drying cycles and extensive drying. The study was conducted mainly to study the mechanism of irreversible K+ fixation and the resulting loss of swelling capacity (LOS).The K+ exchange for the originally present cations was to some extend buffered by carbonates (minor components of some bentonites). Such buffer reactions are supposed to be relevant for real HLRW repository situations.The expected decrease of the swelling of the bentonites depended on the adsorption energy of the probe molecules (water, EG = ethyleneglycol, Cu-triene). The results of different methods (XRDEG, water adsorption, CECCu-triene, soda-soluble silica) indicated that the smectite fraction of the products (on average) was composed of approximately 50% swelling K+ smectite, 45% non-swelling collapsed K+ smectite (50% of which with fixed K+), and 5% illite which probably formed by dissolution and precipitation. This illite was detected by the increase of soda-soluble silica. In contrast to the solvation with ethyleneglycol, which only measures the collapsed layers rather than illite particles, the determination of soluble-silica is regarded as optimum for differentiating between collapsed layer domains and real illitization.The number of collapsed layers of the K-smectites depended on the layer charge density (LCD) but not on the tetrahedral charge. A model is proposed which explains the correlation of collapsed interlayers and LCD.