Bentonite/CEM-II cement mortar INTERFACE EXPERIMENTS: A proxy to in situ deep geological repository engineered barrier system surface reactivity.

The present study focuses on the interaction between cement mortar (OPC-based CEM-II) and the FEBEX-bentonite; this interaction takes place at a small spatial scale (~1 cm/~1 cm; compacted cement mortar/compacted bentonite thickness) within a timeline of 6 and 18 months. This work was designed to determine the early interaction processes and compare them with large-scale FEBEX in situ underground research laboratory experiments. The study aimed at the primary reactions that occurred at the interface in a small spatial scale (nm-μm scale). The experimental device consisted of a composite column containing the cement mortar/bentonite materials. A granitic groundwater solution was injected through the cement mortar/bentonite system and collected out of the column in sequential syringes for analysis of the chemical composition evolution. For the study of the post-mortem samples, an innovative use of grazing incidence X-ray diffraction was performed to determine the phases produced at the interface. Scanning electron microscopy coupled to energy dispersive X-rays and local specific surface area measurements were also applied. The main results showed the initial development of a Mg perturbation in FEBEX-bentonite at the interface related to the formation of 7 Å precursors of Mg-clay 2:1 sheet silicates as the main neogenic phases expected in the long term. Additionally, a Ca-carbonation skin (calcite) occurred in cement mortar at the interface. The specifications of the reaction products observed at small scales of time and space (μm) are highly promising for the development of reaction concepts and support modelling in the future, which could offer a useful perspective for advancement in the upscaling of concrete/bentonite interface perturbation. • Cement mortar/bentonite interface reactions occur at short time and small spatial scale. • Mg perturbations are visible in bentonite due to the formation of Mg silicates hydrates. • Calcite crusts at the interface occur from decalcification in the cement matrix. • Al migration from bentonite promotes the formation of ettringite and C-A-S-H into the cement. • The specific surface area increases next to the interface. [ABSTRACT FROM AUTHOR]