Mineralogy, Structure, and Green Processing of Clay Minerals
This chapter deals with the formation, structure, and properties of important clay minerals, the nomenclature of clay minerals, the green processing of clays (forming, drying), and the interaction of clay particles with water. Particular emphasis is devoted to modeling the rheological behavior of clay -water systems, the infl uence of pH, size and charge of electrolyte ions on clay particle dispersion and aggregation, and the origin and signifi cance of the zeta potential, ΞΆ .
The natural raw materials utilized to produce silicate -based ceramics can be divided into: (i) highly plastic materials , such as clays comprising the minerals kaolinite, illite, or montmorillonite; (ii) sparingly plastic minerals for special (electro)ceramic applications, such as pyrophyllite and talc; and (iii) nonplastic materials , such as tempering additives (quartz, chamotte) and fl uxes (feldspar, apatite, nepheline, calcite, dolomite, etc.) that are added to clays to alter the chemistry, workability and sintering behavior of the ceramic masses. Synthetic raw materials include precursors of glazes (lead oxide, barium carbonate, tin oxide) and special ceramic masses (alumina, zirconia, magnesia), as well as hydrothermally synthesized wollastonite and diopside, and synthetic kaolinite with a very narrow grain size distribution and high plasticity for high -performance electroceramics. Wet chemical techniques such as coprecipitation, freeze -and spraydrying, and sol -gel synthesis are also applied to produce raw materials for special applications.
Plastic natural ceramic raw materials, consisting predominately of kaolinite, illite and/or montmorillonite, are accompanied by residual quartz, feldspar, mica, and calcite as well as organic residues. In particular, the limestone content has been used to distinguish between clay ( < 4 mass% lime), marly clay (4 -10 mass% lime), clayey marl (10 -40 mass% lime), marl (40 -75 mass% lime), calcareous marl (75 -90 mass% lime), marly limestone (90 -96 mass% line), and limestone ( > 96 mass% lime). Kaolinitic raw materials formed in situ (autochthoneous) are called kaoline , while kaolinitic raw materials found in secondary deposits (allochthoneous) are called clays . Marl and marly limestones are important raw materials for Portland cement production (see Section 5.2.1 ).