Swelling of bentonites

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. 

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 the bentonite samples (Table 3.2) is usually linked to the mont-morillonite content (Table 3.1), except for the B-II.a. lower sample in which the fraction of particles of <10 pm is low (Table 3.2). The presence of the large aggregates prohibits swelling. 

The method of preparation significantly affects the ability of bentonite to remove wine or juice proteins. Bentonite is made up of small platelets that are separated by a layer of water molecules. During hydration, the charged platelets repel each other and pop apart. As this occurs, swelling begins. Water molecules partially neutralize... 

The required quantity of bentonite is pre-swelled with about 5-10 times as much water. When cold water is used, the bentonite must be left longer in the water (about 8-10 h) than when warm water is used (3-4 h). After swelling is completed, the supernatant water can be poured off and the bentonite suspended with some of the juice. Preswelling has also been found to be beneficial in the case of preparations that are already granulated. This suspension is then mixed into the juice, stirring constantly. The juice must be kept in motion for at least a 15 min. The fining has no effect without suitable mixing and a minimum juice temperature of 12°C. 

A very small and specialized use of chemical grouts is for sealing piezometers. These devices, which are placed in drill holes to measure hydrostatic pressures in the formation, must be isolated in the hole to keep them unaffected by pressures in other zones penetrated by the drill hole. Usual procedures make use of bentonite balls or pellets, which swell in the presence of water to seal the drill hole with an impervious mass. The swelling takes time, however—as much as 36 h. The use of chemical grouts with a very rapid setting time can create a seal in seconds. 

Platelets are held together by cations. They impart a positive charge to the edge of the particles. These interlayer cations play a key role in the physicochemical properties of bentonite and in the stability of aqueous dispersions. Normally calcium is predominant and the clay swells to a moderate extent when dispersed in water. When Ca ions are replaced by Na, e.g., by reacting with Na2CC>3, the bentonite is said to be activated. This activation makes the clay much more swellable. 

Despite great progress in characterization and parameterization of bentonite, models and knowledge of the physical behavior of partially saturated swelling clays still need improvement in areas such as the effective stress behavior, vapor flow, and water retention. 

The evolution of stress in the bentonite barrier at FEBEX was affected by the existence of gaps between the pre-fabricated bentonite blocks. The swelling pressure did not develop until moisture swelling of the bentoninte blocks had closed the gaps completely. 

Figure 8. Swelling during saturation of bentonite under a vertical stress of I MPa.

The swelling mechanism of bentonite results from the unique microstructure and its net negative charge (Low 1987). 

This example is to test the swelling effects under capillary pressures up to 10 Pa occurring in extremely low-permeable bentonite materials. For this purpose, a simple 1-D case is set up. A one meter long bentonite column is heated on the left hand side. Element discretization length is 0.01m. The initial conditions of the system are atmospheric gas pressure, full liquid saturation and a temperature of 12°C. The heater has a constant temperature of 1(X) C. Flow boundary conditions on the left side are gas pressure of 10 Pa and 15% liquid saturation. On the right side we have atmospheric pressure, full liquid saturation and no diffusive heat flux. As a consequence, a typical desaturation process of bentonite is triggered. The complete set of initial and boundary conditions and the material properties for this example was described in detail by Kolditz De Jonge (2003). 

The buffer in this experiment is made of bentonite blocks. Because there are gaps of up to 30 mm at the upper contact to the host rock the swelling of the blocks is not hindered at these blocks. The differences in back pressure at each block leads to different changes of the pore structure and permeability at each block. 

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