Suchergebnisse

The present study focuses on the interaction between cement mortar (OPC-based CEM-II) and the FEBEXbentonite; 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. ; This work was supported by funding from the European Union's Horizon 2020 Research and Training programme from EUROATOM [H2020-NFRP 2014, 2015] under grant agreement nº662147; CEBAMA

Bentonite is a key barrier for the isolation of high-level radioactive waste within Deep Geological Repository. However, bentonite may be altered by contact with cementitious materials and their alkaline pore fluids. This study offers an extensive morphological and semi-quantitative characterization of the bentonite surface exposed to three types of alkaline pore fluids released by different cement-based materials. The bentonite surfaces were studied using a thorough scanning electron microscopy exploration and analysed using an energy-dispersive ꭕ-ray detector (SEM-EDX). In addition, statistical, element mappings, ꭕ-ray diffraction and infrared spectroscopy analyses were performed. The aim was to have a picture of the morphological and chemical alterations of bentonite at very early stages in accordance with the integrated approach necessary to address bentonite stability in the long-term. As a consequence of the reactivity, two types of morphologies stood out in the matrix of bentonite: platelets and coatings-like crusts characterized by their high Mg and Ca content. These alterations presented a different scope depending on the type of alkaline pore solution involved and suggested the precipitation of authigenic magnesium silicate hydrates (M-S-H) and/or trioctahedral clay minerals and Ca‑carbonates. The knowledge of the performance of bentonite subjected to these alkaline solutions can help in the evaluation of the most suitable cement-based materials to be used next to bentonite ; The experimental work was supported by funding from the European Union's Horizon 2020 Research and Training programme from EURATOM [H2020-NFRP 2014, 2015] under grant agreement n◦662147; CEBAMA