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 at various compaction levels (between 1·27 and 1·90 g/cm³) and at various suctions (between 3·3 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 observe the effect of the hydraulic path on the microstructure. Apart from the inter-lamellar pores, which are not accessible to the adopted experimental techniques, two pore families were identified: micropores and macropores. The transition pore size between the micropores and macropores was found to be suction dependent. The microporosity was practically insensitive to compaction and only the largest micropores were sensitive to suction. Smaller macropores were sensitive to compaction only, whereas larger macropores were sensitive to both compaction and suction. It was observed that, during wetting from the as-compacted state, the macropores remained completely dry up to very low values of suction, whereas the micropores were found to be unsaturated up to the suctions between 10 to 60 MPa. Both macropores and micropores contributed to sample volume changes during drying and wetting. Although the microstructural volume change appeared to be reversible, the macrostructure exhibited permanent deformation.

Bentonites are considered to be the most suitable materials for the multibarrier system of high-level radioactive waste repositories. Since BCV bentonite has been proved to be an ideal representative of Czech Ca-Mg bentonites in this respect, it has been included in the Czech Radioactive Waste Repository Authority (SÚRAO) buffer and backfill R&D programme. Detailed knowledge of processes in the material induced by thermal loading provides invaluable assistance regarding the evolution of the material under repository conditions. Samples of both original BCV 2017 bentonite and the same material thermally treated at 200 °C were characterised by means of chemical analysis, powder X-ray diffraction, infrared spectroscopy, thermal analysis, cation exchange capacity, specific surface area (BET) measurements, the determination of the swell index, the liquid limit, the swelling pressure and water retention curves. The smectite in BCV 2017 bentonite comprises Ca-Mg montmorillonite with a significant degree of Fe³⁺ substitution in the octahedral sheet. Two main transformation processes were observed following heating at 200 °C over 27 months, the first of which comprised the dehydration of the montmorillonite and the subsequent reduction of the 001 basal distance from 14.5 Å (the original BCV 2017) to 9.8 Å, thus indicating the absence of water molecules in the interlayer space. The second concerned the dehydration and partial dehydroxylation of goethite. With the exception of the dehydration of the interlayer space, the PXRD and FTIR study revealed the crystallochemical stability of the montmorillonite in BCV 2017 bentonite under the selected experimental conditions. The geotechnical tests indicated no major changes in the mechanical properties of the thermally treated BCV 2017 bentonite, as demonstrated by the similar swelling pressure values. However, the variation in the swell index and the gradual increase in the liquid limit with the wetting time indicated a lower hydration rate. The retention curves consistently showed the lower retention capacity of the thermally treated samples, thus indicating the incomplete re-hydration of the thermally treated BCV 2017 exposed to air humidity and the difference in its behaviour compared to the material exposed to liquid water.

The geological containment of high-level radioactive waste has become widely accepted among international organizations, and it has been adopted by many countries as part of their national nuclear waste disposal plan. The multi-barrier system, including the compacted bentonite blocks or pellets serving as human-made containment or buffer media, is the key component of high-level radioactive waste disposal, which contains a waste canister that isolates the nuclear waste from a human being geosphere for one million years. The bentonite clay surrounding the nuclear waste capsule is subjected to prolonged exposure to elevated temperatures because of the continuous decay of radioactivity. Long-term heating at high temperatures could change the buffers’ microstructural characteristics and physicochemical and hydromechanical properties, which can influence their self-sealing ability. This paper offers a comprehensive overview of the current understanding of thermal effects on bentonite-based buffer systems. The thermal impact on the microstructure, Atterberg limits, and swelling pressure of bentonite clay are intensely reviewed, and the findings are summarized. This review paper highlights new insights into the design of multi-layered containment approaches for high-level radioactive waste isolation.