Water redistribution between bentonite and salt at elevated temperature

Abstract

Bentonite added to crushed salt promotes the (self) compaction behavior of crushed salt as a kind of lubricant. The bentonite/crushed salt mixture is a candidate backfill material for HLRW repositories in salt formations. At least locally, the backfill material might be exposed to elevated temperature which could cause desorption of water from the bentonite surfaces. As a result the bentonite would lose its ability to act as lubricant for the (self) compaction and the salt would lose its mechanical stability owing to the possible uptake of water. The present study systematically investigates the behavior of bentonite/crushed salt mixtures assuming a closed system which represents either a closed pore or a sealed shaft or gallery. Salt (halite) is known to take up water at relative humidities >75%. This critical value is more or less independent from temperature at least in the range from 30 to 80 °C. Accordingly water redistribution (from bentonite to salt) would occur if the relative humidity in a closed bentonite/salt pore would exceed 75% r.h.. The investigation of a model pore (1 L bottle) with varying amounts of bentonite proved that the absolute humidity in the closed pore increased upon heating. The relative humidity, on the other hand, decreased which indicates that a water redistribution induced by heating will not occur at least until 80 °C. In contrast, water redistribution was clearly observed in the case of high initial water contents of the bentonite (>11 mass%) and low temperature. In conclusion, moderately dried bentonites should be used as lubricant for the improvement of the (self) compaction behavior of crushed salt.