Evidence of (Ca, Mg) smectite formation at Clay/cement interfaces

Abstract : Several experimental and in situ studies have investigated cementitious materials and clay host rock interactions in the ANDRA Underground Laboratory at Bure, France (Gaboreau et al., 2012) and in the Mont Terri channel (Jenni et al., 2014; Mäder et al., 2017). As it has been observed at interfaces between the both materials, the extent of detectable perturbations is highly dependent on the nature of cementitious materials. Adapting the concrete recipe has therefore been a common approach to limit the interactions. Although Ordinary Portland cements (OPC) are considered to be relevant materials for nuclear waste repository condition, " low pH " (~11-12) cement recipes have been developed by adding pozzolan and blastfurnace slag to reduce the alkali content and to limit interactions (Lothenbach et al., 2012). An interface between low alkali cement (LAC)-based concrete (CEM III/B containing 66% slag and 10% nano-silica) and Opalinus Clay (OPA) was characterized for mineralogy and chemistry in the framework of the CI project (Mont Terri Consortium, Switzerland). Volumes of materials impacted by interactions in term of mineralogy and chemistry do not seem very different from those observed at OPC/Clay interfaces. The first millimeter of the LAC concrete in contact with OPA is a white crust formed of more than 25 % of a Mg-bearing cement phase (also called M-S-H in Dauzères et al., 2016), ~11 weight % of calcite, associated with low Ca/Si ratio calcium silicate hydrates (C-S-H) and other cement phases. The M-S-H is distributed relatively homogeneously along the contact with OPA. A specific mineralogical and chemical focus on M-S-H including electron diffraction combined with transmission electron microscopy and electron microprobe demonstrates that M-S-H is a ditrioctahedral (Ca, Mg) smectite with the following average chemical composition: (Ca 0.5±0.2) (Mg 2.0±0.4 , Fe 0.2±0.1 , Al 0.5±0.3 , □ 0.3±0.3) (Al 0.9±0.2 , Si 3.1±0.2) O 10 (OH) 2 , □ representing vacancies in the octahedral site. Some analyses near the LAC concrete/OPA interface show a charge balance highly positive due to very high Mg content. It may be due to (i) a mixing of M-S-H with another Mg-bearing phase, or (ii) the presence of Mg as brucite
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Poster
Clay Conference 2017, Sep 2017, Davos, Switzerland
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Soumis le : mardi 16 mai 2017 - 09:48:40
Dernière modification le : mardi 31 juillet 2018 - 10:14:03

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Catherine Lerouge, Stéphane Gaboreau, Sylvain Grangeon, Francis Claret, Fabienne Warmont, et al.. Evidence of (Ca, Mg) smectite formation at Clay/cement interfaces. Clay Conference 2017, Sep 2017, Davos, Switzerland. 〈hal-01523112〉

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