Benoit Carrier

ESEM study of the humidity-induced swelling of clay film.

We measured the humidity-induced swelling of thin self-standing films of montmorillonite clay by a combination of environmental scanning electron microscopy (ESEM) and digital image correlation (DIC). The films were about 40 μm thick. They were prepared by depositing and evaporating a suspension of clay and peeling off the highly oriented deposits. The rationale for creating such original samples was to obtain mesoscopic samples that could be used to bridge experimentally the gap between the scale of the clay layer and the engineering scale of a macroscopic clay sample. Several montmorillonite samples were used: the reference clay Swy-2, the same clay homoionized with sodium or calcium ions, and a sodium-exchanged Cloisite. The edges of the clay films were observed by ESEM at various relative humidity values between 14% and 95%. The ESEM images were then analyzed by DIC to measure the swelling or the shrinkage of the films. We also measured the adsorption/desorption isotherms by weighing the film samples in a humidity-controlled environment. In order to analyze our results, we compared our swelling/shrinkage and adsorption/desorption data with previously published data on the interlayer spacing obtained by X-ray diffraction and with numerical estimates of the interlayer water obtained by molecular dynamics simulation. The swelling and the hysteresis of this swelling were found to be comparable for the overall macroscopic films and for the interlayer space. The same correspondence between film and interlayer space was observed for the amount of adsorbed water. This suggests that, in the range of relative humidities values explored, the films behave like freely swelling oriented stacks of clay layers, without any significant contribution from the mesoporosity. The relevance of this result for the behavior of clayey sedimentary rocks and the differences with the behavior of nonoriented samples (powders or compacted powders) are briefly discussed.

Effect of Water on Elastic and Creep Properties of Self-Standing Clay Films

We characterized experimentally the elastic and creep properties of thin self-standing clay films, and how their mechanical properties evolved with relative humidity and water content. The films were made of clay montmorillonite SWy-2, obtained by evaporation of a clay suspension. Three types of films were manufactured, which differed by their interlayer cation: sodium, calcium, or a mixture of sodium with calcium. The orientational order of the films was characterized by X-ray diffractometry. The films were mechanically solicited in tension, the resulting strains being measured by digital image correlation. We measured the Youngs modulus and the creep over a variety of relative humidities, on a full cycle of adsorption-desorption for what concerns the Youngs modulus. Increasing relative humidity made the films less stiff and made them creep more. Both the elastic and creep properties depended significantly on the interlayer cation. For the Youngs modulus, this dependence must originate from a scale greater than the scale of the clay layer. Also, hysteresis disappeared when plotting the Youngs modulus versus water content instead of relative humidity. Independent of interlayer cation and of relative humidity greater than 60%, after a transient period, the creep of the films was always a logarithmic function of time. The experimental data gathered on these mesoscale systems can be of value for modelers who aim at predicting the mechanical behavior of clay-based materials (e.g., shales) at the engineering macroscopic scale from the one at the atomistic scale, for them to validate the first steps of their upscaling scheme. They provide also valuable reference data for bioinspired clay-based hybrid materials.

Measurement of Mechanical Properties of Thin Clay Films and Comparison with Molecular Simulations

Here, we focus on the hydromechanical behavior of self-standing clay films with a thickness of a few dozen microns. We measure their elastic and creep properties and how those properties depend on the interlayer cation and on the relative humidity (or water content). Those experimental results are compared with the elastic and creep behavior of nanometric clay particles, which we characterize by molecular simulations. Significant qualitative differences between the behavior of the clay films and that of the clay particles are observed, which suggests that the hydromechanical behavior of the clay films is significantly impacted by their mesostructure (i.e., by how the clay particles or tactoids are arranged in space). Upscaling the hydromechanical behavior of the clay films from that of the clay particles may be challenging.

Creep of Clay: Numerical Results at the Scale of a Layer and Experimental Results at the Scale of Thin Self-Standing Films

This work focuses on the creep of clay-based materials, which exhibit significant analogies with cement-based materials. Here, the authors studied the creep of clay at two scales and with two techniques: numerically (with molecular simulations) at the scale of a clay layer (nm), and experimentally at the scale of thin self-standing clay films (few dozen μm). At the scale of the clay layer, numerical simulations showed that the shear rate was constant over time and an affine function of the shear stress. Creep experiments showed that, after a transient period, the creep function of the authors thin self-standing clay films was a logarithmic function of time. A comparison of the results obtained at the two scales shows that the origin of the logarithmic feature of clay creep must at least partly originate from a scale greater than that of an individual clay layer. By analogy, such result is likely to hold for cementitious materials, which are also known to creep logarithmically with respect to time in the long term: the origin of this logarithmic feature is likely to stem at least partly from a scale greater than the scale of an individual calcium silicate hydrate (C-S-H) layer.

Effect of water content on the mechanical behavior of thin clay films

In this work we measure the effect of water on the elastic and swelling properties of thin smectite clay films. The thin films exhibit a well organized structure with parallel clay particles and are about 40 m thick. We measure the swelling potential of the films by analyzing Environmental Scanning Electron Microscopy pictures. We carry out tensile tests at various relative humidities to evaluate the impact of the water content on the elastic modulus of the films. In particular, the effect of the nature of the counter-ions is investigated. In parallel, we perform molecular dynamics simulations of clay layers with various water contents. At each water content, the thickness of the platelet and its elastic stiffness tensor are computed. We compare those numerical results at the scale of a clay layer with the experimental results at the scale of a clay film and discuss their relevance.