Bentonite is a claystone formed by a complex mineralogical mixture, composed of montmorillonite, illite, and accessory minerals like quartz, cristobalite, feldspars, carbonates, and minor amounts of iron oxy-hydroxides. Bentonite presents complexity at various scales: (1): a single mineral may present different chemical composition within the same quarry (e.g., feldspars solid solutions); (2): montmorillonite presents variability in the cation-exchange distribution while illite may be presented as mixed-layer with smectite sheets; and (3): hardness and crystal size are larger in accessory minerals than in clay minerals, preventing uniform grinding of bentonite. The FEBEX bentonite used is originally from Almería (Spain), and it is a predominantly calcium, magnesium, and sodium bentonite. This Spanish FEBEX bentonite has been hydrothermally altered at laboratory scale for 7–14 years. A thermal gradient was generated by heating a disk of pressed iron powder, simulating the metal waste canister, in contact with the compacted bentonite sample. Hydration was forced from the opposite direction. XRD recorded patterns were very similar. In order to minimize the bias of XRD semi-quantitative determination methods, Rietveld refinement was performed using BGMN software and different structural models. Confidence in the quantification of the main phases allows us to convincingly detect other subtle changes such as the presence of calcite in the hydration front, right at the interface between the saturated and unsaturated bentonite, or the presence of goethite, and not hematite, in the saturated bentonite, near the source of hydration. Smectite component was 72 ± 3% and the refinement was consistent with the presence of ~10% illite, comparable with previous characterizations.

This study investigates the performance of two bentonite materials, specifically MX-80 (Na-bentonite) and FEBEX (Ca-Mg-Na-bentonite), employed as engineered barriers in deep geological disposal facilities for the isolation of high-level radioactive waste, contained in metallic canisters. Experiments conducted at the laboratory scale focused on the interaction of these bentonites with FeCl₂ powder, used as a soluble iron source, to observe enhanced alteration of the bentonite. The experiments were carried out under a hydrothermal gradient. A dominant Na-Cl-SO₄ saline solution was put in contact with the compacted bentonites from the top, while a constant temperature of 100 °C was maintained at the bottom using a heater in contact with the layer of FeCl₂. The experimental cells were examined after six months of interaction. Various changes in the physical and chemical properties of the bentonites were observed. An increase in the water content, a reduction in the specific surface area and cation exchange capacity, changes in the distribution of aqueous species, and the formation of secondary minerals were observed. Reaction products formed at the bentonite interface with FeCl₂, primarily comprising akaganeite, goethite, and hematite. The smectites showed evident structural modifications, with an enrichment in iron content, and a shift in the exchangeable ion distribution in the case of MX-80 bentonite. This work provides valuable insights into the complex interactions between bentonite barriers and materials that dissolve iron, serving as proxies for deep geological disposal environments and indicating the potential long-term behavior, taking into account higher concentrations of dissolved iron than those expected in a real repository.

Carbon steel and bentonite are materials selected as engineered barriers for high-level radioactive waste confinement. Their long-term interaction must be evaluated to confirm the barrier’s stability. Three laboratory experiments of the carbon steel—Mg-bentonite interaction were conducted for 1, 6, and 22 months under a hydrothermal gradient. Changes in bentonite’s water content, specific surface area, and cation exchange capacity were measured. Mineralogy was studied by X-ray diffraction and scanning electron microscopy. The redistribution of aqueous species and the redox state of iron were determined across the bentonite columns. Results indicated water saturation after 22 months. The specific surface area of bentonite was reduced near contact with the steel, while the cation exchange capacity mostly decreased at 3–6 mm from the steel interface. The corrosion rate decreased with time and bentonite enriched in Fe in the first 1.5 mm from the steel contact. The formation of new Fe-bearing minerals, such as di-tri ferri-sudoite, magnetite, hematite, maghemite, lepidocrocite, siderite and ankerite was observed. Aqueous species redistributed in the porewater of bentonite with decreasing concentrations of Fe and Cl as a function of time and increasing concentrations of Na, Ca and SO₄ after 22 months. This occurs under conditions where the bentonite is saturated with Mg, which conditioned the formation and nature of iron clay minerals with time.