Physicochemical reactivity in clay-rich materials: tools for safety assessment

Natural or engineered clay-rich materials are ubiquitous when it comes to achieving sequestration of acid gas, confinement of pollutants or high level radioactive waste (HLRW), and trapping hydrocarbon oil and gas in geological settings. The sequestration, confinement, and trapping functions rely on properties such as low permeability, high sorption and ion exchange capacity, and, in some cases, on swelling abilities. Clay-rich materials contain specific clay minerals possessing these properties due to the small size and high tortuosity of the pores as well as the very high specific surface area and the surface charge of these minerals (especially smectites). For performance and safety purposes, the persistence of this initial sealing function has to be ensured over time, as the clay minerals of interest and the foreign anthropogenic materials (concrete, steel, and other clay materials in situ) will undergo physicochemical interactions and may lead to irreversible transformations. The clay minerals will also be subjected to perturbations due to the heat release of waste packages in the case of HLRW repository, and liquid water and vapour transfers. To tackle the complexity of these phenomena, we combine multi-scale and multi-technique characterisation (middle and far Fourier transform infrared, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) on samples coming from laboratory experiments and natural analogues, and integrate the results through reactive transport modelling. As an example, the characterisation methodology is used to establish the illitisation of clay-stones due to a basaltic dyke intrusion. The approach is compared with classical ones and the application to diagenetic clay sequences for petroleum exploration is discussed. We also explore the high sensitivity of smectic (gel phase)/smectite properties as a function of water content/composition and temperature by investigating the interactions between metallic iron and smectitic clays. This comprehensive study reveals an iron/clay mass ratio threshold above which the smectites tend to be altered into 7 Angstroms Fe-rich clay minerals with much lower swelling and cation exchange capacity. With a comprehensive description and understanding, the prediction of the long-term evolution of such systems seems to be at hand. However, modelling the overall behaviour of clay-rich materials remains a difficult task because of the strong, multi-scale coupling between chemical, mechanical and transport phenomena, potentially mediated by a smectitic gel phase. (authors)