Thermodynamics of hydration of MX80-Na: an experimental study of the hydration energies

Hélène Gailhanou (1) , Philippe Blanc (1) , Arnault Lassin (1) , Philippe Vieillard (2) , R. Denoyel (3) , E. Bloch (3) , Stéphane Gaboreau (1) , Eric Giffaut (4)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 HydrASA - Hydrogéologie, Argiles, Sols, Altérations (E2)
3 MADIREL - Matériaux divisés, interfaces, réactivité, électrochimie
4 ANDRA - Agence Nationale pour la Gestion des Déchets Radioactifs

Hydration properties of swelling clay minerals may be very variable depending on the chemical composition of the clay, on the nature of the interlayer cations and on the interlayer charge (Berend et al., 1995; Vieillard et al., 2011). The Wyoming smectite has been largely studied, notably for assessing its hydration behavior as a function of the interlayer cations, in connection with its structural characteristics (Ferrage et al., 2005; Salles et al., 2007). In the present work, carried out as part of a collaborative Andra/BRGM/HydrASA research program for ThermoChimie project, we propose an original experimental study, based on adsorption and desorption isotherms performed on MX80 clay samples. The goal is to determine energetic contributions to the reactions of hydration, which have been revealed to be non-negligible with respect to the stability of the clay minerals (Gailhanou et al., submitted). In particular, the present work addresses the problems of the hysteresis loop between adsorption and desorption isotherms and of the irreversibility of hydration reactions. This is directly related to the application of classical thermodynamics to the hydration reactions of clay minerals. In a first stage, an experimental study is dedicated to better understand the origin of the hysteresis loop which is systematically observed for the adsorption-desorption isotherms at 25°C. The development of the hysteresis loop has been studied by considering several kinetically related parameters: stabilization periods, temperatures (from 25°C to 60°C) and hydration steps (Figure 1). No sensible change was observed in the hysteresis loop. Therefore, the amount of adsorbed water depends on the followed reaction pathway (adsorption or desorption). The variations in microstructures and in the distribution of hydration layers (0/1/2 water layers; Ferrage et al., 2005) as a function of relative humidity (RH) could provide a possible explanation for this phenomenon.

Domains: Environmental and Society

Hélène Gailhanou, Philippe Blanc, Arnault Lassin, Philippe Vieillard, R. Denoyel, et al.. Thermodynamics of hydration of MX80-Na: an experimental study of the hydration energies. International meeting "Clays in Natural and Engineered Barriers for Radioactive Waste Confinement", Oct 2012, Montpellier, France. ⟨hal-00701810⟩