## 3 Results
### Physical-Chemical Characterization
Considering the atomic concentrations obtained by EDX, structural formulas were calculated using the model of a di-trioctahedral chlorite (i.e., [62]) structure, assuming the possibility of hydroxide precipitation in a previous dicoctahedral smectite interlayer. They were calculated based on the anionic base (-O\({}_{10}\)(OH)\({}_{8}\)), distributing Al in the tetrahedral layer (T2 sites) [62; 63] and in the octahedral layer (M1, M2 sites). Likewise, Fe was assigned as Fe\({}^{2+}\) in the trioctahedral brutic layer (M3, M4); assuming that the charge compensating the sum of the charges generated in T2 and M1, M2 must be produced due to the existence of a certain proportion of Fe\({}^{3+}\) in the brutic layer. Mg and part of the Fe and Al, not assignable to T2, M3 and M4, were assigned to the octahedral layer (M1, M2). Figure 4: EDX chemical profiles in bentonite, expressed in wt.%: (**a**) Fe, Al and Si after 1, 6 and 22 months; (**b**) detail of (**a**) showing the first 6 mm from the steel interface; (**c**) Na, Mg and Ca after 1, 6 and 22 months; (**d**) detail of (**c**) showing the first 6 mm from the steel interface. Reference values represented by dash lines. Analysis 4: (Fe3+0.33 Fe2+0.67)(OH)2) 3(Al0.22 Mg0.40 Fe1.51)2(Si3 Al1)4O10(OH)2 Analysis 5: (Fe3+0.34 Fe2+0.66)(OH)2)3(Al0.25 Mg0.92 Fe1.09)2(Si3.12 Al0.88)4O10(OH)2 Analysis 6: (Fe3+0.37 Fe2+0.63(OH)2)3(Al0.8 Mg0.6 Fe0.6)2(Si3.5 Al0.5)4O10(OH)2 The calculation reasonably corresponds to the intercalation of iron hydroxide into a smectite structure, although the iron proportions slightly exceed those present in the original octahedral sheet of FEBEX montmorillonite (Ca+2 homoionised: (K0.04_0.01 Ca0.24_0.06 (Fe0.09 Al1.41 Mg0.50)Si3.94 Al0.05(O10(OH)2) [64]. The substitution of aluminum in the tetrahedral sheet significantly increased compared to the average montmorillonite, while octahedral Mg remained in similar proportions as in the reference FEBEX. Analyses on fresh fractured bentonite samples close to the interface after 22 months revealed a larger coating of smectite aggregates (Figure 5c), with a high content of Fe and Mg. The Si/Fe ratio calculated for analyses shown in Figure 5c exceeded 4, except for analysis 7, and differentiated by a single aggregate of brighter color that was 0.75. The structural formula calculated for analysis 7 was (Fe3+0.425 Fe2+0.575(OH)2)3(Al0.48 Mg0.61 Fe1.02)2 (Si3 Al1)4O10(OH)2, compatible again with the di-trioctahedral chlorite structure used to simulate an iron (II, III) hydroxide interlayer simulation. The iron proportion increased over the expected for intercalation. Some mixing of iron oxides and formation of new iron silicates could not be discarded. Analyses 8-11 showed phyllosilicates rich in Mg and Fe and Si/Fe ratios higher than 4, revealing the necessary presence of iron oxides in the mixture. Figure 6 shows the concentrations of Fe2+, Fe3+ and Fetotal along the bentonite column after 22 months. An increase in Fe2+, Fe3+ and Fetotal was observed in a thickness of 2 mm from the interface with steel, compared to the reference concentrations in FEBEX-Mg- bentonite. A similar trend was observed for Fe2+ and Fe3+ over the whole bentonite column. At the hydration zone, 12-21 mm from the interface with steel, the concentrations decreased slightly below the reference values. Most of the increase in iron was presumably related to the presence of iron (III) oxides as there was an increase in Fe2+ from 0.13 \(\pm\) 0.05 wt.% to 0.26 \(\pm\) 0.08 wt.% in the section closest to the heater (0-1 mm). Fe3+ also increased in contact with the steel, from 3.0 \(\pm\) 0.4 wt.% determined in the reference Mg-bentonite to 4.03 \(\pm\) 0.04 wt.%. In the following section (1-2 mm), Fe2+ and Fe3+ concentrations decreased by 0.17 \(\pm\) 0.01 wt.% and 3.85 \(\pm\) 0.6 wt.%, respectively. At the 2-3 mm section from the heater, the Fe2+ and Fe3+ values remained in the order of the reference Mg-bentonite.