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According to Velde “the contact of rocks and water produces clays, either at or near the surface of the earth”1. Clay particles can be smaller than 2 microns and are in the form of needles or plates.
Clay minerals, in terms of their atomic structure, are based on two basic units - a tetrahedral sheet and an octahedral sheet. They can be classified into two main groups1:
- Regular mixed-layer
The structure and composition of clay minerals can vary and therefore affect their physical and chemical properties. There are many useful techniques used to identify clay minerals, including scanning electron microscopy, X-ray diffraction, differential thermal analysis, and differential scanning calorimetry.
Thermal Analysis for Clays
The term thermal analysis was initially defined as a research method where the physical properties of a sample as a function of temperature are examined. Currently, the term is understood as the determination of changes in the properties of a sample as a result of an imposed temperature program.
Thermal analysis is thought to have started at the end of the nineteenth century, with one of the first researchers to use the technique being Le Chatelier in 1887. Clays were one of the main samples studied by this method. The initial experiments on clay samples provided a comprehensive description of types of water that can be found in clay minerals, such as molecular water as interlayer water or hydroxyl groups as a part of the clay mineral structure.
Currently, thermal transformation of clay minerals is a subject of much interest and is at the center of a large number of investigations. It has been found that the heating parameters, such as temperature, heating rate and time, and cooling settings significantly influence the occurring processes (e.g. dihydroxylation). In general, if the phase transition of the sample occurs, exothermic or endothermic reactions take place.
Like differential thermal analysis (DTA), thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) allow researchers to study the thermal effects accompanying the heating of the clay sample. These include endothermic or exothermic chemical reactions (decomposition, oxidation and reduction) and phase transitions (recrystallization and melting).
The DTA method is based on the temperature difference between the sample and a reference sample, while the TGA method is based on the weight gain/loss upon heating as an identification factor. The thermal behavior of the clay sample was investigated using an analyzer where the sample was heated from 20 to 1000 °C at a constant rate of 10 °C/min in air.
Conventional thermal analysis (DTA, TG, and simultaneous TG and DTA) is the most common and accurate to study clays and clay minerals, in particular for the understanding of the mechanism of the thermal transformation. The most promising technique for future studies on clay minerals is sample-controlled technique (SCTA) where careful control of the experimental conditions is based on microprocessors and microcomputers4. The SCTA method is particularly powerful for the kinetic study of the thermolysis of clays and clay minerals.
- B. Velde, Origin, and Mineralogy of Clays. Clays and the Environment, Springer-Verlag (ed.) 1995.
- H. H. Murray, Applied Clay Mineralogy - Occurrences, Processing, and Application of Kaolins, Bentonites, Palygorskite-Sepiolite, and Common Clays, Chapter 2: Structure and Composition of the Clay Minerals and their Physical and Chemical Properties, 7–31, 2006.
- B. R. Ilić, A. A. Mitrović, L. R. Miličić, Thermal Treatment of Kaolin Clay to obtain Metakaolin, Hem. ind. 64 (4) 351–356, 2010.
- F. Rouquerol, J. Rouquerol, P. Llewellyn, Handbook of Clay Science Vol. 5B, Thermal Analysis, 2013.