## 3 Results and Discussion
### Changes in the Composition and Structure of Bentonite during Experiments
In terms of the qualitative mineral composition, the original bentonite corresponded to the commercial sample of the bentonite studied in the article by [58]. The smectite content in the sample was 64.3 wt% (Table 6). In the course of interaction with the leached concrete, the contents of quartz, feldspars, anatase, kaolinite, siderite, and calcite decreased. The relative amount of smectite in the samples increased by 4.2 wt%. The decrease in the content of SiO\({}_{2}\) in the composition of bentonite indicates the dissolution of quartz, and the increase in Al\({}_{2}\)O\({}_{3}\) indicates a relative increase in montmorillonite in the composition of the solid component, which was also confirmed by the data of mineral analysis (Table 5). When studying the structure of smectites and, in general, 2:1 clay minerals, the shapes and positions of the profile in the region of reflections (060), which appear in the range of angles 61-63\({}^{\circ}\)2\(\Theta\) when using CuK\(\alpha\), acquire a special role in the interpretation of possible structural transformations. To clarify the position of the 060 reflection, the samples were taken using an internal standard. Si was used as a reference with the position of the reflection (311) at an angle of 56.119\({}^{\circ}\)2\(\Theta\). To better visualize the observed changes, the 55-64\({}^{\circ}\)2\(\Theta\) range was examined on a larger scale. In the decomposition and fitting of the intensity peaks in the 060 smectite region in the Fityk program (version 1.3.1) [67] the contribution of CuK\(\alpha_{2}\) was taken into account. According to the results of this analysis, it was concluded that the original sample only had a reflection at d = 1.499 A, which corresponds to the position of the 060 reflection for montmorillonite. The position of the 060 reflection for the samples after 1 and 3 months of treatment was preserved, while no additional peaks were observed in the region of smaller angles. This indicates the formation of trioctahedral smectite. The FTIR spectroscopic study of the natural sample of bentonite confirmed that it was mainly composed of dioctahedral Al-rich smectite montmorillonite (Figure 7b). This is evidenced by the combination of the following characteristics of the IR spectra: the most \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mineral** & **Initial Sample** & **1 Month** & **3 Month** \\ \hline Quartz & 13.3 & 12.3 & 10.8 \\ Albite & 5.3 & 6.5 & 6.7 \\ Microcline & 6.7 & 6.8 & 4.8 \\ Calcite & 1.4 & 1.2 & 1.1 \\ Siderite & 0.7 & - * & - * \\ Antatase & 0.7 & 0.5 & 0.9 \\ Kaolinite & 2 & 1 & 1.1 \\ Illite & 4.5 & 5.9 & 4.6 \\ Smectite & 64.3 & 64.8 & 68.5 \\ Chlorite & 1.1 & 0.9 & 1.3 \\ \hline \hline \end{tabular} \({}^{*}\)Trace amounts.  
\end{table} 
Table 6: Mineral composition of examined samples, determined by the Rietveld approach (wt%). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample** & **Na\({}_{2}\)O** & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **K\({}_{2}\)O** & **CaO** & **TiO\({}_{2}\)** & **Fe\({}_{2}\)O\({}_{3}\)** \\ \hline Initial sample & 1.13 & 3.2 & 19.66 & 67.1 & 1.18 & 2.4 & 0.86 & 4.26 \\
1 month & 1.04 & 3.05 & 19.81 & 66.77 & 1.45 & 2.72 & 0.81 & 4.13 \\
3 month & 1.2 & 3.03 & 19.56 & 66.4 & 1.49 & 2.72 & 0.84 & 4.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Chemical composition of rock-forming elements (wt%). intense band of Si-O stretching was relatively symmetrical and located at 1040 cm\({}^{-1}\), with a doublet at 522 cm\({}^{-1}\) (Al-O-Si deformation) and at 467 cm\({}^{-1}\) (Si-O-Si deformation); the OH stretching bands occurred at 3625 cm\({}^{-1}\). The absorption band near 619 cm\({}^{-1}\), attributed to the coupled Al-O and Si-O out-of-plane vibrations, shows the high octahedral Al content, while the bands at 881 and 850 cm\({}^{-1}\) indicate octahedral substitutions of Fe for Al, and of Mg for Al, respectively [68; 69; 70; 71].