Water content swelling bentonite

Bentonite and the other clay minerals have considerable ion exchange capability, and their physical characteristics change dramatically depending upon the content of the salt solution in their pore space, and whether the clay is in the calcium or sodium form. As examples of publications considering the influence of calcium chloride on clays Sjoblom et al. (1999) noted the slowness of water to penetrate compacted bentonite (Na-montmorillonite is its major constituent), but that a calcium chloride solution could rapidly penetrate the bentonite and allow it to be washed away. Fresh water causes compacted bentonite to swell and to produce free-surface particles by exfoliation. These particles form a gel which further closes the pores to water uptake, while the CaCl2 solution causes the exfoliated material to shrink (or at least swell less). This allows more solution to enter the pores, causing differential expansion and a lower gel strength so that the clay may be more easily washed away. 

MattsotI , who measured swelling pressures of bentonite, found a linear relation between the logarithm of the water content and the Ic arithm of the swelling pressure. At high water content there is a deviation from linearity, pointing to a limited swelling caused by attractive forces. 

Bentonite rocks have many uses in the chemical and oil industries and also in agriculture and environmental protection. The usefulness of bentonite for each of these applications is based on its interfacial properties. These properties are determined by geological origin, chemical and mineral composition (especially montmorillonite content), and particle size distribution, and they include the specific surface area (internal and external), cation-exchange capacity (CEC), acid-base properties of the edge sites, viscosity, swelling, water permeability, adsorption of different substances, and migration rate of soluble substances in bentonite clay. 

The swelling of bentonites in water and, as will be discussed in Section 3.2.2, the migration of a nonadsorbing ion show no direct relationship to the montmoril-lonite content or other geological characteristics in this narrow range (35%-48% montmorillonite content, Table 3.1). However, they are influenced by several other factors, for example, the quality of the exchangeable cation (Chapter 2, Section 2.1), particle size distribution, aggregation, density, free pore size, other minerals, etc. 

Operational data were obtained from a Dutch foimdry. When drying in ambient air, the air velocity is the main factor (rather than temperature or humidity). In order to allow sufficient air velocity over the drying trajectory, additional fans and ventilators were installed in the foundry. In order to reach a good and constant quality, the fines content of the (regenerated) core-making sand was reduced. Water-based coatings can cause a swelling of the fine residual (bentonite and coal dust) particles, which then causes core defects. 

Smectite - The smectites are water swellable cl s having a sheet or platelet structure. Smectite is the mineralogical term for this class of clays, which includes montmorillonite, hectorite, and saponite. Montmorillonite clays derive their name from the Montmorillon section of France where this material was first observed and later classified. Most smectites are more commonly known under the geological term bentonite. By convention, a bentonite is understood to be an ore or product with a substantial smectite content. The name bentonite derives from Fort Benton, Wyoming, the site of an important deposit. Lattice substitutions within the smectite clays creates a charge imbalance which is compensated by exchangeable alkali and alkaline earth cations. This contributes to the ability of these clay to swell and impart considerable plasticity in ceramic formulations. When the exchangeable cations are predominately sodium, the individual platelets can separate to produce a colloidal structure in water. 

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