At the same time, sulfate anions are found within a centimeter from the contact surface of bentonite and concrete, while chloride anions penetrate much deeper into concrete (\(\sim\)5 cm). Under the influence of waters filtered through OPC, the physical, mechanical and chemical properties change, including a decrease in the pH from 12.5-13.5 to 10-10.5, the dissolution of phases containing Na and K, and the dissolution of portlandite [25,26]. Previous studies have shown that bentoniteites are stable even in highly acidic environments when heated [27, 28, 29, 30]; however, concrete creates a highly alkaline environment (pH \(\approx\) 12) under the influence of storage facility groundwater, which can cause a change in its physical and chemical properties [31], and, accordingly, a transformation of the structure of the montmorillonite [32] found in immediate contact with concrete. As the main component of bentonite, smectite is exposed to the negative effects of highly alkaline solutions. Globally, studies of the stability of bentonite, and, in particular, smectites, in alkaline solutions have been carried out for more than 10 years. In the works of a large number of authors [33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47] and others, one can single out the main conclusion that pH has a key effect on the stability of smectite. As a result of various laboratory experiments, it has been shown that the reactivity of smectite increases significantly with increasing pH starting at 12 [33,36,41,46]. Moreover, the use of simulations by [PERSON] et al. [45,48,49] revealed that smectite dissolution processes are already significant at a pH of 12.5. In a study by [PERSON] et al. [47], bentonite were subjected to the thermochemical action of a highly alkaline K-Na-OH-type raster, simulating the early stage of OPC leaching. The in the experiments; three of them were montmorillonite bentonite and one was saponite. The results of the study showed that trioctahedral smectite showed greater resistance to dissolution than dioctahedral ones. There was also precipitation of secondary phases, such as zeolites and K-feldspar. Of the dioctahedral smectites, smectite with the highest charge of the tetrahedral network and the most heterogeneous composition showed the greatest stability. In studies [33; 39; 42; 50] on the reaction of bentonite with alkaline solutions, it has been noted that in solutions with a high content of K\({}^{+}\), the partial illitization of smectite can occur with the formation of mixed-layer minerals, which was also noted in the study by [PERSON] et al. [51] in the paper presented by the authors' summary table. In bentonite, the dissolution of smectite is accelerated, but so too is the dissolution of quartz and amorphous silica [37; 52]. In [PERSON] et al. [37], the dissolution of smectite occurred through the beidellization step, but K was not used in their experimental solutions. In experiments [52] carried out over 6 years with 5 M NaOH and 5 M KOH solutions, smectite dissolution was observed in parallel with the transformation of dioctahedral smectite into trioctahedral due to an increase in the proportion of magnesium in the octahedral networks. The formation of various zeolites was also observed. In studies [17; 53], an increase in the proportion of trioctahedral smectite has also been observed by the same mechanism. The processes of the interaction of concrete with groundwater, as well as the interaction in the concrete-bentonite system, need to be studied in detail, since this, in general, can lead to a decrease in the operational characteristics of the EBS. The purpose of this study was to identify the patterns of structural transformations of montmorillonite as the main component of the bentonite buffer in contact with cement/the concrete materials of EBS, which can be used in DGRs both as insulating materials and as matrices for IL/LL RW.