## 3 Results  
### Physical-Chemical Characterization  
The bentonite's water content after 1 and 6 months of reaction remained similar to the initial 15 wt.% (Figure 2a) but after 22 months increased to 31-33 wt.% along the whole column. A strong decrease in the bentonite's SSA (Figure 2b) was observed in the sections close to the heater, decreasing from 79 m\({}^{2}\)/g to 4.6-11.1 m\({}^{2}\)/g at the contact with the steel (a SSA reduction of 75-88%). The SSA trend in the three experiments was similar, decreasing towards both ends of the bentonite column but more intensely near the heater. The SSA values after 1 month were lower than those after 6 and 22 months, respectively, but considering the standard deviation of the measurements, this might not be relevant. Figure 2: _Cont._  
Independently of the reaction time, the bentonite's CEC remained close to the initial reference value (83 \(\pm\) 5 cmol(+)/kg) except in the section at 3-6 mm from the heater, where a relevant decrease was observed after 6 and 22 months, achieving values of 55 and 71 cmol(+)/kg, respectively (Figure 2c). The corrosion rate calculated by measurements of corrosion thickness at the steel discs by SEM-EDX (Figure 3) indicated certain passivation with time. The measurements of the corrosion thickness considered the gray compact rim formed at the bright steel surface (Figure 3). A corrosion rate of 39.6 \(\upmu\)m/year was calculated after 1 month of reaction, 20.0 \(\upmu\)m/year after 6 months and 4.8 \(\upmu\)m/year after 22 months (Figure 3). Figure 2: Evolution of physical–chemical properties along the bentonite blocks after 1, 6 and 22 months: (**a**) water content (wt.%); (**b**) specific surface area (m\({}^{2}\)/g); (**c**) cation exchange capacity (cmol(+)/kg). The concentration of the major elements in bentonite (Fe, Al and Si), determined by EDX analyses, remained stable over time, but a remarkable decrease in Al and Si, in agreement with an increase in Fe, was observed at 1-2 mm from the interface with the steel. Specifically, the increase in iron reached 1 mm from the interface after 1 month, 1.25 mm after 6 months and 1.5 mm after 22 months (Figure 4a,b). Quantitatively, Fe increased from an averaged 7.2 \(\pm\) 0.7 wt.% to values > in a range 10-30 wt.% in a thickness of 0-5-1.5 mm from the interface, and values >80 wt.% at a very short distance from the interface (<0.2 mm). Figure 3: Backscattered electron image of FEBEX-Mg after 22 months, distinguishing four altered zones from the steel left part) and confirmed by the EDX composition shown below the image. The atomic % of each zone is given to differentiate the observed materials. The evolution of Ca and Na, on the contrary, decreased over the first 1.5 mm from the interface, averaged values of 1.7 and 0.2%, respectively, from the original reference bentonite value. Inversely, Mg concentrated at this region and remained at relatively stable values beyond 1.5 mm from the interface (Figure 4c,d). SEM images and EDX analyses performed on fresh fractured bentonite samples close to the steel interface showed incorporation of Fe into the smectite. Figure 5 shows the aspect of clay aggregates presumably produced during bentonite alteration and oxides found attached to the steel plate after 1, 6 and 22 months, which had evidence of formation of new Fe-rich silicates. Secondary mineral phases, with different morphologies, formed on the smectite surface (Figure 5a). EDX analyses expressed in atomic percentage showed that laminar morphologies (analysis 1) contained approximately 79% Fe and 21% Cl, while small micrometric spheres contained 95% Fe and 5% Cl. Both phases were attributed to the precipitation of iron oxides with variable amounts of chloride. After 6 months, iron oxide with additional chloride was observed (analysis 3), nucleating on aggregates with multidirectional micrometer fibers (Figure 5b). The altered clay shown by the morphologies observed in analyses 4, 5 and 6 presented high Fe content (>40%) and a Si/Fe ratio close to 1.