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Smectites are swelling clay materials with pronounced colloidal properties that are widely used in industry. These properties originate in the electrokinetic properties of the smectite layers and their linkage capacities. Thin layers may be dispersed or aggregated according to many parameters, such as concentration, particle size and morphology, exchangeable cation nature and chemical environment (pH, ionic strength). The literature usually provides general rules, like the sodium dispersion contains a lot of small units whereas the calcium dispersion contains a few large units. A volume of water molecules bound to the clay surface is considered as the immobile water phase that behaves like the solid phase obstructing the flow. The water immobilized around layers and trapped inside aggregates cannot participate to the flow. In this study, we evaluated the volume occupied by calcium and sodium units inside the dispersion containing the immobile water phase. First, the smectite was cautiously extracted from a raw bentonite and its physicochemical properties were determined. A large quantity of extracted and saturated smectite (Na-smectite and Ca-smectite) was obtained. Second, the unit size and a shape factor for each sample were evaluated using granulometry and scanning transmission electron microscopy on wet samples (Wet STEM) and some flow curves. Na-smectite dispersions contain 0.13 microm(2) surface units with a shape factor of 50. Ca-smectite dispersions contain 0.32 microm(2) surface units with a shape factor of 3.3. Finally, rheometry allowed us to evaluate the unit occupancy using an adaptation of the Krieger-Dougherty law. We used shape factors and evaluated the concentration from which the entire immobile volume was connected (6.4% for Na-smectite and 11.9% for Ca-smectite). This study explains the evolution of flow properties with increasing concentrations by the evolution of layer interactions at the microscopic scale for homoionic smectite particles in diluted dispersions.

The mechanical behaviors of smectite materials in water are very sensitive to many parameters, such as concentration, particle size and morphology, exchangeable cation nature, and chemical environment (pH, ionic strength). The literature usually provides general rules for sodium and calcium bentonites. However, it appears that the quantitative results present a large range of values. This study has established the relationships between the smectite nature (mineralogy, morphology, reactivity) and its rheological behavior according to the exchangeable cation. To develop a complete rheological study, we need 800 g of montmorillonite purified and saturated with Na + or Ca 2+ . Huge production necessitates optimized process with accurate control of the powder quality and the chemical environment. Smectite particles have been extracted from a natural bentonite and then saturated with sodium or calcium. Then, each sample was characterized with classical methods like X-ray diffractometry (XRD), and laser granulometry but also cutting-edge technologies like scanning transmission electron microscopy on wet samples (Wet STEM). The Na-smectite dispersion contains a lot of small units whereas the Ca-smectite dispersion contains a few large units. Rheological study consists in flow curves obtained with a high-resolution rheometer. Logarithmic representation of the rheograms permits detecting a yield stress, it separates solid and fluid behavior. The rheological datas (viscosity, consistency, and yield stress) allow defining the limits depending on the concentration and on the saturation state. A Newtonian law could be applied to around 4% for Na-and Casmectites. Seaward of these concentrations, the dispersions are shear thinning and modelled by Ostwald law. Finally a yield stress appears at 6% for Na-smectite and 11% for Ca-smectite, the flow curves could be modelled by Hershel-Bukley law. Results are in accordance with the bibliography.