## 1 Introduction
In addition, the water vapor produced in the hot part of the bentonite, close to the canister, could recondense in cooler areas, influencing the redistribution of aqueous species in bentonite's porewater [28]. The saturation of the bentonite barrier involves a swelling process that redistributes the mass, changing the dry density, and as a consequence, the porewater chemistry, that is in equilibrium with the mineralogy [29; 30]. Also, changes in the water content and chemistry mainly modify the exchangeable cation distribution and, in general, aqueous ion concentrations like Cl\({}^{-}\), SO\({}_{4}\)\({}^{2-}\), Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\) or Fe\({}^{2+/3+}\)[31; 32]. Due to the interaction between the steel and bentonite, redox reactions will occur on the surface of the steel and resulting soluble species and Eh/pH conditions will be transferred to the bentonite, generating potential mineral changes. Due to the variability of the existing candidate bentonite, with different mineralogs and exchangeable cation distributions [6; 32; 33; 34; 35], and the particular geochemical conditions that may occur depending on the design of the DGR, steel-bentonite interactions may produce different effects [36]. The interaction between steel and bentonite can lead to alteration processes like dissolution or re-precipitation of accessory minerals, i.e., short-term dissolution of CaCO\({}_{3}\) followed by precipitation of magnesium or iron carbonates due to cation exchange, where Mg or Fe ions replace Ca cations [37]. In addition, accessory silica minerals such as cristobalite and quartz, as well as montmorillonite, can undergo dissolution processes, contributing to the formation of iron silicates, such as nontronite or ferric chlorite [38]. [PERSON] et al. [39] reported the formation of iron-rich T-O phyllosilicates like greenalite and cornstedite, in addition to magnetite, illite and chlorite, in an experiment in which Callovo-Oxfordian clay was mixed with metallic iron and iron powder, kept at 90 \({}^{\circ}\)C for 6 months, and then the temperature gradually reduced by 10 \({}^{\circ}\)C monthly until reaching 40 \({}^{\circ}\)C. The variety of Fe-bearing minerals that could be formed depends on Fe/Si and Al/Si ratios and pH but also could be mediated by silica dissolution and carbonates [39; 40; 41]. Experimental investigations have shown the accumulation of Mg in the hotter zone of bentonite, near the canister surface, regarding FEBEX in situ experiments [18]. The same trend was also found in several bentonite analyses in the ABM-II experiment explained by the increase in exchangeable Mg near the hot steel-bentonite interface [41; 42] and by the precipitation of MgSO\({}_{4}\) in evaporation surfaces near this interface [36]. This accumulation, displacing cations such as Ca\({}^{2+}\), promotes the formation of 2:1 magnesium silicate mineral. [PERSON] et al. [43] reported the formation of trioctahedral saponite and chlorite phases or brucite-like sheet phases from montmorillonite modification mediated by high Mg and Fe content in porewater. [PERSON] [44] also observed saponite formation at the iron-bentonite interface in in situ experiments performed at the hard rock laboratory in Aspo, Sweden. A modeling simulation of the iron-bentonite interaction provided by [PERSON] et al. [45] suggested a sequence of predominance of Fe-bearing minerals over time, with magnetite prevailing at the first stage. The magnetite led to crostedtite (kaolinite-serpentine group) with a general formula (Fe\({}^{2+}\),Fe\({}^{3+}\))\({}_{3}\)(Si,Fe\({}^{3+}\))\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\) that dominated the corrosion products for the first 500 years. The next Fe-bearing minerals transformation was reported to form berthierine and, in some cases, chlorite. [PERSON] et al. [46] reported the formation of iron-rich saponite or 1:1 minerals at \(<\)150 \({}^{\circ}\)C through geochemical modeling.