## 3 Results
### Physical-Chemical Characterization
Figure 5: SEM images of details taken in fresh fractures close to the steel plate interface: (**a**) 1 month; (**b**) 6 months; (**c**) 22 months. (**a1,b1,c1**) include the composition of the zones marked in the respective subfigures. Data presented in atomic %. -: data below detection limits. The mineralogy of FEBEX-Mg-bentonite experienced minor changes over time (Figure S1, Supplementary Materials) considering the whole thickness of the bentonite disc. The main FEBEX-Mg-smectite XRD reflections were detected at 14.8 A (001), 4.45 A (020), 2.58 A (110), 1.70 A (210) and 1.50 A (060). The reference material also presented quartz (4.25, 3.34 A), cristobalite (4.04 A), calcite (3.0 A), and traces of magnetite (2.53 A) and maghemite (2.52 A), (Figure 7). The main mineralogical changes were observed near the steel interface (<3 mm). After 1 month, no minerals other than those present in the reference bentonite were detected, except ankerite, a Ca,FeII,Mg-carbonate (2.90 A) in the section <3 mm from the steel interface. After 6 months, also in the section <3 mm from the steel interface, low-intensity reflections of ankerite and maghemite were also detected. After 22 months, the presence of a 7.05 A reflection and the enhancement of a 4.8-4.7 A band supported some evidence of di-tri ferric chlorine presence, presumably ferri-sudolite-like (in agreement with the SEM-EDX data). It is known that the 14 A reflection of ferric (hydroxide layer) aluminous (octahedral layer) chlorite displayed a very small intensity compared to 7 and 4.74 A reflections (i.e., [65]). The 14.8-14.6 A basal spacing reflection predominant in most DRX patterns could contain smectite and the minor chlorite reflection. Ankerite (2.90 A) was detected close to the steel interface (<3 mm). In the section at 3-6 mm, lepidocrocite (\(\gamma\)-Fe3+O(OH)) (6.25 A) and hematite (Fe2O3) (2.70 A) were detected. Lepidocrocite was also detected in the section at 6-15 mm (Figure 7). Accessory minerals like muscovite (illite) and albite (plagioclases), present in the reference Mg-bentonite raw material, were randomly detected due to their heterogeneous distribution in bentonite. Figure 6: Concentrations of Fe2+, Fe3+ and Fetotal along the bentonite column after 22 months. Units expressed in wt.%, referred to dry mass of solid sample. The XRD patterns corresponding to the corrosion crusts that formed at the steel-bentonite interface were measured on both sides, the contact with steel (colored in fuchsia) and the contact with the bentonite (colored in orange) (Figure 8). Several intense reflections, observed at 6.40, 5.35, 4.84 and 3.52 A, identified after 1 month, were not related to any mineral alteration but attributed to the adhesive used to fix the corrosion crust on the airtight holder. In addition, hematite (2.70 A, 2.51 A) and maghemite (2.94 A, 2.52 A) were detected, and siderite at a low intensity (2.79 A). At the contact with the bentonite side, low-intensity reflections of ankerite (2.90 A) and hematite (2.70 A) were identified as corrosion products at the corrosion crust. The ferric chlorite phase (ferri-sudoite) (7 and 4.8 A) was barely identified with other compatible reflections at 2.65 A and 2.44 A. Low-intensity reflections of magnetite/maghemite were observed. After 6 months, reflections attributed to green rust (7.5 A, 3.75 A), maghemite and magnetite (2.96-95 A and 2.53-2.51 A) and siderite (low-intensity reflections at 2.79 A) were observed in the corrosion crust at the contact with steel. Smetic, albite and quartz, already present in the original bentonite were detected. Ferri-sudoite was detected by the reflections at 7.05 A, 4.80, 2.80 and 2.44. At the bentonite side, ferri-sudoite was detected too. Ankerite (2.90 A), hematite (2.70 A) and a maghemite/maghetite mix were identified as corrosion products. Siderite was not identified on this side. Figure 7: XRD on FEBEX-Mg sections from the steel interface after 22 months.