Water absorption and swelling A fundamental property of bentonite is to absorb water and expand. However, not all bentonites have the same absorption capacity. Its level of hydration and swelling depends on the type of exchangeable ions contained, with different hydrophilic and solvating power. Swelling is mainly due to two factors: 1) water absorption at platelet surface level, and 2) osmotic repulsive forces, forcing platelets to detach and open up like a “stack of cards”. Sodium bentonite, with sodium cation prevalence (Na+) allows water to penetrate through the platelets, forcing them apart, thus leading to swelling. Conversely, calcium bentonite, with calcium cation prevalence (Ca2+), while getting hydrated in much the same way, due to its strong positive charge, has lower absorption properties, not permitting water to penetrate through the platelets. In this latter case, platelets flake off rather than swell. Viscosity and thixotropy of aqueous suspensions when bentonite is dispersed in water, highly stable colloidal suspensions are formed with high viscosity and thixotropy. At high enough concentrations, these suspensions begin to take on the characteristics of a gel. Suspensions are formed when water molecules penetrate into platelet interlayers. Here, hydrogen bridge bonds are formed by the hydrogen atoms contained in the water molecules. Platelets become isolated from each other, while bonded through interposition water. When left still, a mesh is formed which, by incorporating water, jellifies. Conversely, under mechanical stress, these bonds partially break, thus allowing platelets to move more freely. Viscosity under these conditions is lower than at rest. This reversible sol-gel-sol process is known as thixotropy. These properties shown by bentonite aqueous suspensions are mainly exploited in drilling slurries. Colloidal and waterproofing properties when water is absorbed by bentonite, a semisolid gel is formed with strong hydrostatic pressure resistance. A montmorillonite platelet can be figured out as a thin packet of negatively charged layers. Due to their negative charge, they repel each other while letting water through. In this way, while the packet swells, a stable shell is formed around the platelet. When saturated, this shell will repel water, even under pressure. For all these properties, bentonite is employed in ponds and docks, to seal off soil infiltrations, and line the base of landfills. Binding property This bentonite property is mainly exploited to produce green molding sand. In this application, bentonite with a suitable moisture content covers quartz sand grains and acts as a connective tissue to the entire mass. Under this homogenous coating, even at maximum compression, water will remain in a highly “rigid” state, binding the sand grains and lending maximum resistance to the sand mould. Bentonite vitrification temperature is higher than other clays. Therefore, when used as an additive, it makes green sand more durable, and, in particular, more resistant to heat stress. Surface properties (coagulation– absorption – adsorption) Bentonite absorption – adsorption properties are determined by the high specific surface and free charges present on each micelle. Coagulation occurs through the adsorption of ions of opposite charge to that of colloidal particles.