Compacted bentonites are popularly being considered as buffer or backfill material in high level nuclear waste repositories around the world. These bentonites may undergo various conditions including hydration from around geo-environmental water and heating by the radiation of the used fuel during its period of operation. Water retention properties of as-compacted Czech bentonite B75 with three initial dry densities (1.27 g/cm3, 1.60 g/cm3 and 1.90 g/cm3) and bentonite powders were investigated within temperature 20–80 °C at unconfined conditions. Vapor equilibrium method was used to control constant relative humidity. The influence of temperature on water retention properties was analyzed and discussed. Results show that the temperature decreased water retention capacity for all cases. The water retention capacity is lower at high temperature especially at lower suction. The temperature has more significant effect on drying path than wetting path. Furthermore, the volume swelling decreased with the increased temperature upon saturation. The hysteretic behavior decreased with the increase of temperature for all studied materials.

Microstructure of compacted bentonite is an important aspect which controls its hydro-mechanical behavior, also is vital in constitutive modelling. In order to understand this mechanism, the wetting and drying behavior of compacted bentonite with initial dry densities of 1.9 g/cm3 was investigated by environmental scanning electron microscopy (ESEM). The attempt of quantitative interpretation was done by digital image analysis at aggregates level (in micro meters). The macro pores between aggregates were clearly sensitive to change of suction, while there were only small volume changes of the aggregates. Volumetric strain of the measured aggregates varied. After one wettingdrying cycle, some new macro pores were formed and the hysteretic behavior of inter-aggregate pores was observed.

Compacted bentonites were chosen as the backfill material and buffer in high level nuclear waste disposal due to its high swelling pressure, high ion adsorption capacity and low permeability. It is essential to estimate the swelling pressure in design and considering the safety of the nuclear repositories. The swelling pressure model of expansive clay colloids was developed based on Gouy-Chapman diffuse double layer theory. However, the diffuse double layer model is effective in predicting low compaction dry density (low swelling pressure) for certain bentonites, and invalidation in simulating high compaction dry density (high swelling pressure). In this paper, the new relationship between nondimensional midplane potential function, u, and nondimensional distance function, Kd, were established based on the Gouy-Chapman theory by considering the variation of void ratio. The new developed model was constructed based on the published literature data of compacted Nabentonite (MX80) and Ca-bentonite (FoCa) for sodium and calcium bentonite respectively. The proposed models were applied to re-compute swelling pressure of other compacted Na-bentonites (Kunigel-V1, Voclay, Neokunibond and GMZ) and Cabentonites (FEBEX, Bavaria bentonite, Bentonite S-2, Montigel bentonite) based on the reported experimental data. Results show that the predicted swelling pressure has a good agreement with the experimental swelling pressure in all cases.

In this paper, the microstructure of the Czech bentonite B75 was investigated by three methods: Water retention curve (WRC) measurements, mercury intrusion porosimetry (MIP) measurements, and environmental scanning electron microscopy (ESEM) investigation. The experiments were performed on samples of various compaction levels (between 1.27 g/cm3 and 1.90 g/cm3) and at various suctions (between 3.3 MPa and 290 MPa) along both drying and wetting hydraulic paths. In the ESEM observations, target relative humidities (and thus total suctions) were imposed directly in the ESEM chamber to directly observe the effect of the hydraulic path on the microstructure. Apart from the smallest pores (nanopores), which are not accessible to the adopted experimental techniques, two pore families were identified: micropores and macropores. The transition pore size between micropores and macropores was found to be suction dependent. The microporosity was practically insensitive to compaction and only largest micropores were sensitive to suction. Smaller macropores were sensitive to compaction only, whereas larger macropores were sensitive to both compaction and suction. We observed that during wetting from the as-compacted state the macroporosity remained completely dry up to very low values of suction, whereas microstructure was found to be unsaturated up to the suctions between 10 MPa to 60 MPa. Both macrostructure and microstructure contributed to sample volume changes during suction change. While the microstructural volume change appeared to be reversible, permanent deformation remained on the macropore level.