Clay products

Ball clays are composed almost entirely of kaolinite and. as between 70 and 90% of the individual particles are below 0.01 mm in size, these clays have a high plasticity. Their plasticity at times is enhanced by the presence of montmorillonite. They contain a low percentage of iron oxide and, consequently, give a light cream colour when burnt. They are used for the manufacture of sanitary ware and refractories. 

If a clay or shale can be used to manufacture refractory bricks, then it is termed a fireclay. Such material should not fuse below 1600°C and should be capable of taking a glaze. 

The presence of quartz in significant amounts gives strength and durability to a brick. This is because, during the vitrification period, quartz combines with the basic oxides of the fluxes released from the clay minerals on firing to form glass, which improves the strength. However, as the proportion of quartz increases, the plasticity of the raw material decreases. 

Sulphate minerals in mudrocks are detrimental to brick making. For instance, calcium sulphate does not decompose within the range of firing temperature of bricks. It is soluble and, if present in trace amounts in the finished brick, causes effluorescence when the brick is exposed to the atmosphere. Soluble sulphates dissolve in the water used to mix the clay. During drying and firing, they often form a white scum on the surface of a brick. Barium carbonate may be added to render such salts insoluble and to prevent scumming. 

Pyrite, and other iron-bearing minerals such as hematite and llmonite, provide the iron that primarily is responsible for the colour of bricks. The presence of other constituents, notably calcium, magnesium or aluminium oxides, tends to reduce the colouring effect of iron oxide, whereas the presence of titanium oxide enhances it. High original carbonate content tends to produce yellow bricks. 

One of the most widely used ceramic raw materials is clay. This inexpensive ingredient, found naturally in great abundance, often is used as mined without any upgrading of quality. Another reason for its popularity lies in the ease with which clay products may be formed when mixed in the proper proportions, clay and water form a plastic mass that is very amenable to shaping. TTie formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength. 

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Brownell W E 1976 Structural Clay Products, Applied Mineralogy vol 9 (New York Springer) pp 43-60... 

In 1989, 30% of the barium carbonate produced was used in glass manufacturing, 30% in brick and clay products, 20% in the manufacture of barium chemicals, 5% in the manufacture of barium ferrites, 5% in the production of photograpliic papers, and 10% in miscellaneous uses. 

Beneficiation (2,11,12,21—27) iavolves a process or series of processes whereby the chemical and/or physical properties and characteristics of raw materials are modified to render the raw material more processible. The extent of beneficiation is determined by a combination of the starting raw materials, the processiag scheme, the desired properties of the product, and economics. Powder cost iacreases with iacreased beneficiation consequently, low value-added clay raw materials used to produce iaexpensive stmctural clay products typically undergo a minimum of beneficiation, whereas higher value-added alumina powders undergo more extensive beneficiation. 

Clays or shales that may be utilized ia the manufacture of bricks, tiles, and other heavy clay products exist ia every state ia the United States. (See Building materials, survey). Some blending of materials is often necessary to control shrinkage of the product, and the economics of manufacture are governed by the demands of fuel, labor, transportation costs, and the market. 

Adsorbents. Acid activated clays have been widely used to treat mineral, vegetable, and animal oils. The primary objective of such treatment is decolorization and, at least in the case of edible oil, to remove components that contribute to off-tastes. Typically the oil is filtered through a granular clay product or treated with finely ground clay and subsequendy filtered. 

In the United States, kaolin is the principal clay product and about 9 million metric tons were reported mined and processed during 1991 (25). Bentonite production was reported as being 2 million tons for the same year. World production data for clays, often as a function of geographical location, use, or specific producer, are available (26). 

Any product that depends on aesthetics for consideration for purchase and use will be improved by the use of color. Hence, many ceramic products, such as tile, sanitary ware, porcelain enameled appHances, tableware, and some stmctural clay products and glasses, contain colorants. 

Low-grade ceramics - stone, and certain refractories - are simply mined and shaped. We are concerned here not with these, but with the production and shaping of high-performance engineering ceramics, clay products and glasses. Cement and concrete are discussed separately in Chapter 20. We start with engineering ceramics. 

Stone and clay products (except cement) 15 years... 

The morphology of weathered feldspar surfaces, and the nature of the clay products, contradicts the protective-surface-layer hypothesis. The presence of etch pits implies a surface-controlled reaction, rather than a diffusion (transport) controlled reaction. Furthermore, the clay coating could not be "protective" in the sense of limiting diffusion. Finally, Holdren and Berner (11) demonstrated that so-called "parabolic kinetics" of feldspar dissolution were largely due to enhanced dissolution of fine particles. None of these findings, however, addressed the question of the apparent non-stoichiometric release of alkalis, alkaline earths, silica, and aluminum. This question has been approached both directly (e.g., XPS) and indirectly (e.g., material balance from solution data). 

Clay minerals, oxides, and humic substances are the major natural subsurface adsorbents of contaminants. Under natural conditions, when humic substances are present, humate-mineral complexes are formed with surface properties different from those of their constituents. Natural clays may serve also as a basic material for engineering novel organo-clay products with an increased adsorption capacity, which can be used for various reclamation purposes. 

Compatibility The particles must be compatible with the polymeric substrate. Since natural clays are alumino-silicates, they must be prepared or functionalized so that they will be compatible with a polymer. For natural clay products, this process can be quite complex, given the wide chemical variability of different samples, even from the same mine. 

With the aid of color illustrations Harrell and Russell (I) have shown some of the color changes that can occur in several clay bodies when the furnace atmosphere is varied from oxidizing to reducing and when its moisture content is controlled. Brownell (2) has summarized the roles of several factors influencing the development of scummed surfaces on brick and other structural clay products. Matson (3) has discussed the relationship of the firing temperatures of ancient Near Eastern pottery and the colors developed together with other physical properties of the wares. 

Brownell, W. E., Scum and Its Development on Structural Clay Products, ... 

Structural Clay Products Institute, Chicago, 111., Research Report No. 4 (1955). 

AMERICAN CERAMIC SOCIETY (ACerS). Founded 1899. It has 12,000 members. A professional society of scientists, engineers, and plant operators interested in glass, ceramics-iiielal systems, cements, refractories, nuclear ceramics, white wares, electronics, and structural clay products. It is located at 65 Ceramic Dr., Columbus, OH 43214. http //www.acers.org/... 

CERAMICS. Derived from the Creek word ketamos ("burnt stuff t, ceramics comprise a wide variety of materials that constitute a major industry. The principal facets of the ceramic industry, in order of increasing value of annual production, are (I) abrasives (2i porcelain enamel coalings (3) refractories (4) whilewares l5) structural clay products (6) electronic and technical ceramic products and (7) glass. Class accuunls for about 43 1 of all ceramics produced. Sec also Glass... 

The Dutch magazine KGK (Klei Glas Keramiek = Clay Glass Ceramics) reports the results of an extensive physical and chemical research of five clay deposits in the area between the rivers Rhine and river Meuse in the Netherlands. The chemical part involved both main and trace elements. The analysis of trace elements was carried out within the framework of an Act on the requirements for the environmental quality of soil materials in relation to, for example, the chemical composition of baked clay products. One of the things which have to be investigated in the baked product is the degree in which incorporated pollutions leach out (are extracted) under the influence of the weather, and especially under the influence of water. The analysis of one of the five deposits is represented in table 8.2. 

I have worked out this technique. However, when I discovered that it was not used in ceramic applications, I refrained from incorporating it in this book. Yet I still think that this technique has educational value, both when applied on clay and on baked clay products. 

Ferrario JB, Byrne CJ, Cleverly DH (2000) 2.8-Dibenzo-p-dioxins in Mined Clay Products from the United States Evidence for Possible Natural Origin. Environ Sci Technol 34 4524... 

Ferrario J, Byrne C, Cleverly D (2000) Summary of Evidence for the Possible Natural Formation of Dioxins in Mined Clay Products. Organohalogen Compds 46 23... 

FIGURE 11.17 XRD patterns of two organoclays (Cloisite 10A, from Southern Clay Products, and VB166a) and two PSCNs with intercalated (PS/10A) or exfoliated (PS/VB16) morphology. 

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