## 4 Discussion  
The experimental design provided data after 1, 6 and 22 months, allowing for the study of the temporal evolution of steel-bentonite interactions. The temporal evolution described how the progressive passivation of the steel surface occurred, visibly decreasing the corrosion rate with time. The longest experiment, performed for 22 months, unusually for this type of laboratory experiment, permitted the formation of a well-defined cemented crust at the steel-bentonite interface due to the precipitation of iron corrosion products that could be analyzed from both faces. Due to the hydrothermal gradient, the bentonite's physical-chemical properties were modified. A rather homogeneous increase in water content was observed after 22 months (close to 30 wt.%) without apparent water content decrease as a function of distance to the heat contact. Similar experiments using natural FEBEX bentonite showed a slightly higher water content range (20-25 wt.%), achieved during a short time in the whole column (1 and 6 months) [53]. Since the pressure of water injection was relatively low (0.15 MPa) and the swelling properties of the Mg-saturated bentonite were homogeneous, hydration progressed very slow. The material was previously wetted and reprocessed to be Mg-saturated, so final mineral grains and grain size were artificially homogenized to <1 mm compared to natural FEBEX bentonite, which usually provided grains <5 mm with a visible heterogeneity of grain colors and different altered rock preserved grains (i.e., [66]). The great homogeneity, at small time lapses, could produce resistance to hydration at the swelling interface. There was no evidence of water loss associated with the thermal treatment or gradient in the short term that would have resulted in the dehydration of the bentonite near the heater, a phenomenon that was not detected, with values close to the initial moisture level being maintained. On the other hand, hydration was completed after 22 months, allowing for comparison of the short term (1 and 6 months) with the long-term (22 months), once the hydration front progressed. The bentonite's SSA remained in a range close to the initial reference values, except near the steel interface, due to the presence of corrosion products and modifications on the smectite, as observed by SEM-EDX and XRD, where minerals identified to be of the chlorite group and iron-rich were detected, mixed with the smectite matrix. Chlorite minerals were detected in a FEBEX in situ experiment by [PERSON] et al. [49] at a temperature of 50 \({}^{\circ}\)C and were synthesized at 300 \({}^{\circ}\)C in the presence of metallic Fe powder facing MX-80 bentonite [67], showing compositions and XRD powder patterns similar to those shown in this study. The composition of materials corresponding to iron silicates, seeking locations free of chloride and analyzed by SEM-EDX, is compatible with ferric chlorites. Their elemental  
Figure 9: Ionic concentrations of aqueous extracts in bentonite sections parallel to the steel interface. (a) Mg\({}^{2+}\) and Cl\({}^{-}\); (b) Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\). References values represented by dotted lines. proportions are consistent with the predominance of a di-octahedral character in the octahedral layer and tri-octahedral character with partially oxidized Fe\({}^{2+}\) in the brucitic layer. Assuming the already described distribution model, the presence of typical reflections of ferric chlorites outlining a main reflection at 7 A could be explained with a composition characteristic of ferri-sudoite (Figure 7). The CEC remained within a range similar to the reference bentonite, except in the section at 3-6 mm from the steel interface that decreased after 6 and 22 months. This phenomenon is typically connected to the dissolution of montmorillonite and the neoformation of other non-swelling minerals such as chlorite or iron oxides.