Stability of bentonites in salt solutionsⅢ - Calcium hydroxide

摘要

The prerequisite for the usage of bentonites as a geotechnical barrier in HLRW repositories is the stability of the bentonite under the conditions expected. In addition to high temperature and exposure to radiation, different types of aqueous solutions could also affect the bentonite. In this respect hyperalkaline solutions from Portland cements are considered, because aluminosilicates are known to be soluble under these conditions. The aim of the present study was to compare the 'alkaline-reactivity' of a set of different well characterized bentonites and to identify the reasons for the differences. Theoretically, this would allow for the selection of an optimum bentonite at least with respect to the stability at the cement-bentonite interface. The 'alkaline-reactivity' was characterized both by measuring the dissolved structural elements and analyzing changes of the solid material (e.g. the cation exchange capacity). However, the concentration of the dissolved structural elements could not be compared systematically probably because of the unknown nature and the different amounts of precipitates. As an example, the Si concentration in the alkaline solutions strongly varied from bentonite to bentonite because it depended on the type and amount of SiO_2 phases present in the different natural bentonites. Possibly, the associated SiO_2 phases governed the Si/(A1 + Mg + Fe) ratio in the alkaline solution. Amongst other parameters, this ratio determines which phases precipitate. This ratio obviously depends on the type and amount of SiO_2 accessories. Results obtained in the present study were in accordance with published literature. More pronounced dissolution/precipitation processes than found in the present study (60-90 °C, 3-5 months, saturated solution-excess Ca(OH)_2) are expected at even larger pH values, which - on the other hand - do not reflect conditions expected in a bentonite barrier in contact with cement. The most important conclusion from this study is that bentonites are surprisingly resistant against solutions at pH around 12 and up to 90 °C. Above this temperature and at larger pH values dissolution is expected to be much faster. Therefore, for a real HLRW repository it seems to be favorable to use low pH cements and keep temperature well below 100 °C.