Reactivity of Callovo-Oxfordian Claystone and its Clay Fraction With Metallic Iron: Role of Non-Clay Minerals in the Interaction Mechanism

International audienceIn order to better understand the possible interactions between steel canisters and a claystone host rock, in this case the Callovo-Oxfordian rock (COx), the present study investigated in detail, under conditions relevant to high-level radioactive waste repositories (anoxic conditions, temperature of 90°C), the reactions between metallic iron and: (1) COx; (2) the clay fraction extracted from COx (CF); and (3) mixtures of CF with quartz, calcite, or pyrite. Batch experiments were then carried out in the presence of NaCl-CaCl2 background electrolyte, for durations of 1, 3, and 9 months. Solid and liquid end-products were characterized by a combination of techniques including liquid analyses, transmission and scanning electron microscopies, X-ray diffraction, N2 adsorption at 77 K, and Mössbauer spectroscopy. The interaction between CF and metallic iron appeared to proceed by means of pathways similar to those illustrated in previous studies on interactions between metallic iron and purified clays. In spite of the many similarities with previous studies, significant differences were observed between the behavior of COx and CF, particularly in terms of pH and Eh evolution, iron consumption, chemical composition of the neoformed particles, and textural evolution. Such differences demonstrate the important role played by non-clay minerals in reaction pathways. The addition of carbonates or pyrite to CF did not lead to significant change in reactivity. In contrast, under the conditions used in the present study, i.e. for relatively low iron:clay ratios, the presence of quartz strongly influenced reaction pathways. In the presence of quartz, magnetite was observed only in trace abundances whereas the amounts of magnetite were significant in experiments without quartz. Furthermore, filamentous serpentine particles with a small Al:Si ratio appeared which could develop from an FeSiAl gel that only forms in the presence of quartz. Considering that most clay rocks currently being considered for radioactive waste disposal contain significant amounts of quartz, the results obtained in the present study may be of significant interest for predicting the long-term behavior of clay barriers in such sites