## 2 Materials and Methods
### Analytical Methods  
The water content was calculated by mass difference of samples before and after heating to a constant weight at 105 \({}^{\circ}\)C. The specific surface area (SSA) was determined by nitrogen adsorption using a Gemini V analyzer from Micrometrics Corporate (Norcross, GA, USA) and calculated by the BET method. The samples were ground and dried at 90 \({}^{\circ}\)C for 24 h. Subsequently, the samples were degassed at 90 \({}^{\circ}\)C for at least two hours using a nitrogen gas stream and analyzed using the 5-point isotherm method as in [PERSON] et al. [51]. The cation exchange capacity (CEC) was determined using the Cu-trien method [58], with modifications made for clays as in [PERSON] et al. [59]. The absorbance was measured at 577 nm using the UV-Visible Spectrophotometer Genesys 150 from Thermoscientific (Thermo Fisher Scientific, Waltham, MA, USA). Analysis of major ions including Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) was performed using a Metrohm 802\({}^{\text{TM}}\) compact IC plus (from Metrohm, Herisau, Switzerland) through ion chromatography. A 0.15 solid/liquid ratio was used to mix the samples with deionized water in order to obtain the aqueous extracts. The samples were then subjected to 24 h of interaction with water on a shaker, followed by centrifugation for 30 min at 10,000 rpm. Finally, the aqueous extracts were filtered using syringe filters with a pore size \(<\) 0.45 \(\upmu\)m.  
Longitudinal thin section samples were prepared for scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analyses. Each sample was dried by immersion in liquid nitrogen inside a vacuum chamber. After that, the samples were embedded and polished. The procedure is detailed in [PERSON] et al. [53]. A Q150T-S Quorum sputter coater system from Quorum Technologies (Lewes, UK) was used to coat the samples with Au. Surface and morphology images were captured using a Hitachi S-3000N SEM (Hitachi, Japan) that was coupled with an EDX XFIash(r) 6130 Bruker detector (Bruker Corporation, Billerica, MA, USA) for semi-quantification analyses of chemical composition. Internal standard quantitative analyses were used for EDX quantification. The equipment operated under high vacuum conditions with an accelerating voltage of 20 keV, a lifetime of 40 s, a working distance of 15.0 to 18.5 mm and a beam current of 300 mA. A study was conducted to determine the chemical composition in a longitudinal profile of bentonite, in the direction from the heater to the hydration zone. The images were largely magnified at the hottest zone where chemical variations were expected to be more significant. About 20 analyses were conducted over a thickness of 2 mm at the steel-bentonite interface, 15 analyses over a thickness of 3 mm at the hydration zone and 60 analyses at the intermedium zone over a thickness of 16 mm, although the number of analyses and thicknesses may vary slightly from sample to sample. EDX analyses were used to observe the diffusion of Fe along the bentonite and concentration changes of elements such as Mg, Ca, Na, K, Cl, S and Fe. The data were processed by removingC and O from the concentrations and recalculating the rest of the elements to achieve 100 wt.%. The mineralogy of bentonite was also analyzed on fresh fractured samples to observe the morphologies of the reaction products. Small samples that either fell apart or were manually separated were used for this study. To compare the redox state of Fe along the bentonite block in the longest experiment (after 22 months), the 1,10-phenanthroline photochemical method was used [60, 61]. This method allowed for discrimination between ferrous and total iron in mineral samples. The Fe\({}^{3+}\) was calculated by the difference between total Fe and Fe\({}^{2+}\).