Transformation product of volcanic origin (Fig. 1), whose name derives from one of the places of discovery near Ford Benton, Wyoming, USA. In Germany, there are significant deposits in Bavaria, where they are extracted by surface mining (Figures 2 and 3).
This product is used as mold material binder and consists of the three-layer mineralmontmorillonite by more than 75 % by mass. Each individual multi-layer package develops from a combination of one octahedron with two tetrahedrons (Fig. 4). The large variety of clay minerals in this group results from the fact that in these kinds of three-layer minerals it is possible to replace the Si4+ ion of the tetrahedron layer by Al3+ ions and/or the Al3+ ion of the octahedron layer by Mg2+, i.e. by lower-valence ions. This results in generation of negative excess charge in the layer packages, which is balanced out by cations (e.g. Na+, K+). This ability is the basis for the technically relevant properties of this kind of clay minerals, such as swelling, thixotropy, and ion exchange capacities.
Within the group of three-layer clay minerals, montmorillonites (smectite clays) assume great importance. The free bonding forces within the multi-layer packages allow for deposition of water molecules, which causes expansion in the direction of the c-axis (intra-crystalline swelling) (Fig. 5). By ionization of the octahedron coordination with differently charged ions, the electroneutrality of the overall lattice is disturbed. For compensation of charge differences, ions are accumulated in the lattice and/or between the individual layers. These “exchangeable” ions have great impact on the practical behavior (activation) of bentonite materials with regard to their application in the casting industry.
Due to their layered architecture, montmorillonite crystals have a flaky structure. The thickness of these crystalline flakes is one nanometer, i.e. one millionth of a millimeter. The diameter of the flakes may vary between 100 and 800 nm. The low thickness of montmorillonite crystals, their excellent bending capacity and their large surface area are remarkable characteristics (Fig. 6).
As a general rule, two types of bentonite are available. Natural sodium bentonite, which has a sodium ion coverage on its clay surface due to mineral formation (large deposits in the USA, North Africa, Russia), and calcium bentonite, which can mainly be found in Europe in natural deposits. Due to their good ion exchange capacities, these types of Ca bentonites can be chemically activated by sodium salts (Na2CO3) (s. Active bentonite). Bentonite materials activated in this way do not only provide better swelling capacities (s. Swelling) but also higher thermal resistance; these are influential factors in terms of conditioning behavior and efficiency of bentonites. Today, the following types of bentonite are mainly used in Europe: American bentonite (Wyoming bentonite), Greek bentonite, Sardinian bentonite and Bavarian (South German) bentonite.
Active bentonite, Hard bentonite, Dead bentonite