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Hydratation des argiles gonflantes: séquence d'hydratation multi-échelle et détermination des énergies macroscopiques à partir des propriétés microscopiques

Fabrice Salles 1
1 LMTE - Laboratoire de Modélisation des Transferts dans l'Environnement
SMTA - Service Mesures et modélisation des Transferts et des Accidents graves : DEN/DTN
Abstract : Smectites have interesting properties which make them potential candidates for engineered barriers in deep geological nuclear waste repository : low permeability, swelling and cations retention. The subject of this thesis consists in the determination of the relationship between hydration properties, swelling properties and cations mobility in relation
with confinment properties of clayey materials.
The aim is to understand and to predict the behaviour of water in smectites, following two research orientations: the mechanistic aspects and the energetic aspects of the hydration of smectites. We worked on the Na-Ca montmorillonite contained in the MX80 bentonite, with the exchanged homoionic structure (saturated with alkaline cations and calcium cations).
The approach crosses the various scales (microscopic, mesoscopic and macroscopic) and
implied the study of the various components of the system (layer-cation-water), by using original experimental methods (thermoporometry and electric conductivity for various relative humidities (RH) and electrostatic calculations.
Initially, the dry state is defined by SCTA (scanning calorimetry thermal analysis).
Then a classical characterization of the smectite porosity for the dry state is carried out
using mercury intrusion and nitrogen adsorption. We evidenced the existence of a mesoporosity which radius varies from 2 to 10 nm depending on the compensating cation. The thermoporometry and conductivity experiments performed at various hydration states made it possible to follow the increase in the pore sizes and the cations mobility as a function of the hydration state. We highlight in particular the existence of an osmotic mesoscopic
swelling for low RH (approximately 50-60%RH for Li and Na). By combining the results of thermoporometry, X-ray diffraction and electric conductivity, we are able to propose a complete hydration sequence for each cation, showing the crucial role of the compensating cation in the hydration of smectites : it is responsible for the structure of porosity in a dry state and of the evolution of the pore sizes as a function of the RH and it modifies the hydration sequence by its mobility inside the interlayer space.
The distinction between various types of water in the smectite structure is also achieved by thermoporometry at different RH : water bound to the cations and surfaces, water structured by porosity and free water. This distinction is important to understand the behaviour of smectite and in particular the diffusion properties in clayey materials.
The importance of the cation nature is also highlighted by the energetic model. Electrostatic calculations using the PACHA formalism (Electronegativities Equalization method) show that, for the small cations, the hydration energy of the layers is predominant. To obtain these results, we determine the surface enthalpies for the dry state, which show a coherent evolution as a function of the cation partial charge with the increase of pore sizes
and thus with particle sizes. Then, using a theoretical model, we calculated swelling energies, surface hydration energies and cation hydration energies.
The behaviour of mixed purified clay displays a behaviour closer to that of a calcic clay for the experiments carried out, in contradiction with the fact that the Na cation is majoritary. This observation implies results on the clay properties, different from that expected for hydration properties, swelling and interlayer cation mobility within the framework of the radioactive waste.
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Fabrice Salles. Hydratation des argiles gonflantes: séquence d'hydratation multi-échelle et détermination des énergies macroscopiques à partir des propriétés microscopiques. Matériaux. Université Pierre et Marie Curie - Paris VI, 2006. Français. ⟨tel-00129414⟩

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