When man-made stones meet natural rocks – Shedding light on Mg-rich phases appearing at the interface between concrete and clay

Claystones and cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposal of radioactive waste in deep geological repositories. An approximately 180 million year old marine clay formation, the Opalinus Clay, was selected as the first-priority host- rock for the final disposal of radioactive waste in Switzerland while cement-based materials are used as structural elements, backfill or to condition the waste matrix in deep geological repositories. Interaction between the two contrasting materials is driven by chemical gradients in the porewaters causing diffusive fluxes of dissolved species across their interfaces. At the Mont Terri rock laboratory, a long-term, up to 20 years lasting, passive diffusion–reaction field experiment is performed under realistic boundary conditions to explore the mineralogical alteration at clay-cement interfaces. SEM/EDX studies of the first samples retrieved after 2.2 and 4.9 years by core drilling across the interface of cementitious materials and Opalinus Clay showed strong Mg enrichment adjacent to the interface due to formation of magnesium-silicate-hydrates (M-S-H) phases. Complementary micro-XRD and ²⁹Si NMR studies showed that M-S-H phase are not well crystalline, but rather disordered, which hampers analysis by classical methods. Synchrotron based X-ray Absorption Spectroscopy (XAS) is one of the few techniques to provide structural information for amorphous materials. Here we use Mg K-edge XANES combined with ab initio calculations using the FDMNES code to explore the atomic structure of different amorphous Mg-containing phases. The main result is that all phases have a TOT phyllosilicate structure with silica defects creating end of silica chains and an interlayer composed of Mg bound to hydroxide groups. Significant differences were found between the M-S-H and the Ca-containing M-C-S-H phases. The latter phases have an enlarged structure compared to the M-S-H phases due to structural incorporation of the slightly larger Ca atom. The results also show that at least two geometrically distinct Mg octahedral sites are present in M-S-H and M-C-S-H phases implying that the Mg octahedra are attached to both Si tetrahedra in a manner similar to that in talc and to hydroxide groups in a manner similar to that in brucite. This study demonstrates that soft X-ray microspectroscopy in combination with ab initio modelling is a powerful approach to explore the structure of unknown and amorphous to poorly crystalline phases formed at the interface between man-made cement and a million years old natural rock.