Long-term mineral stability of bentonites exposed to elevated temperature in neutral, alkaline and salty environments of natural analogues

Many experiments were performed to predict the long-term behavior of clay barriers at higher temperatures, different pH conditions, and salt concentrations. Nevertheless, the level of knowledge is far from the point where we are able to give a clear picture of the bentonite clay evolution in the long-term perspective. The only way how to tackle the problem of the time frame for the stability of bentonites is to find and study natural analogues which were exposed to the factors of different geochemical environments for a real time. We bring the results of natural analogues for long-term exposure of buried bentonite beds to the temperature between 20 and 300 C in neutral, alkaline and salty environments. All studied cases come from the East Slovak Basin Neogene basin with high recent thermal gradient and variable geological and geochemical environments during deposition of sediments and volcano-clastics. Buried bentonite beds are of volcanic origin (rhyolitic to andesitic volcanites) which creates ideal geological analogue to most of the bentonites tested as the potential barrier for the radioactive disposals. Our results are based on the detail study of mineral composition, mechanism of alteration, K-Ar dating, temperature exposure modeling and laboratory testing. They show that smectite as the main component of the bentonites is stable in neutral environment in large temperature interval for millions of years. Temperature up to 100 C is able to deteriorate only 10% of original bentonite composition. Higher temperatures up to 150 C still preserve 50% of bentonite composition for more than period necessary for radioactive waste disposals live-time. Clearly, smectites in bentonites of volcanic origin are much more temperature resistant than smectites in clay-stones of sedimentary origin. Mechanism of the alteration is continual and controlled by the temperature so it is possible to predict the process. Salty and alkaline environments (documented by the presence of halite and zeolites) along with an elevated temperature enhance the process of smectite alteration. Up to 80% of original smectite mineral in bentonites can be deteriorated at elevated temperature. The time frame and mechanism of smectite alteration is very complex and hardly predictable. It depends very much on the local geochemical conditions. In an extreme case, complete alteration of smectite was observed in geologically short period up to thousands of years at high temperature about 300 C. Results indicate the large risk of bentonite stability in a presence of salts and alkaline environment, however if bentonite is preserved from the influence of these factors it can ensure long time stability even at elevated temperature. (authors)