Clay-water mixtures

Three main properties render clay suitable for making ceramic materials its plasticity when wet, its hardness when dry, and the toughness, increased hardness, and stability that it acquires when fired. The addition of water to dry clay produces a clay-water mixture that, within a narrow range of water content, has plastic properties it is deformed, without breaking or cracking, by the application of an external stress, and it retains the acquired shape when the deforming stress is removed. Wet clay mixtures can, therefore, be modeled, molded, or otherwise made to acquire a shape that will be retained after the forming operations. Water-poor mixtures are not plastic, however, and excess water results in mixtures, known as slips, that are too fluid to retain a shape, as shown in Table 56. 

The plasticity of clay-water mixtures is due principally to two factors the flat platelet shape of the clay particles and their small size. When clay is wet, the water, which under such conditions is known as water of plasticity, envelops each particle, acts as a lubricant between the particles, and allows them, when an external force is applied to the mixture, to readily slide along each other, as shown in Figure 51. 

As for the drying properties of clay-water mixtures when wet clay dries at ambient temperature, the water of plasticity that surrounds the particles... 

The view that the clay surface perturbs water molecules at distances well in excess of 10 A has been largely based on measurements of thermodynamic properties of the adsorbed water as a function of the water content of the clay-water mixture. There is an extensive literature on this subject which has been summarized by Low (6.). The properties examined are, among others, the apparent specific heat capacity, the partial specific volume, and the apparent specific expansibility (6.). These measurements were made on samples prepared by mixing predetermined amounts of water and smectite to achieve the desired number of adsorbed water layers. The number of water layers adsorbed on the clay is derived from the amount of water added to the clay and the surface area of the clay. 

The assumption that the water is adsorbed in uniform layers on all the clay surfaces for a wide range of mixtures has been criticized (2, 20). The argument is that the individual clay particles in the clay-water mixture do not expand beyond a certain distance regardless of the quantity of water which is added. The clay layers group themselves into tactoids resulting in two populations of water those molecules which are found between the tactoids and those directly perturbed by the clay layers. If true, this would invalidate the procedure used to calculate the thermodynamic properties of the adsorbed water. However, other workers have reported complete delamination of certain smectites (21., 22). It is not clear under what conditions tactoids will form, or not, and this uncertainty is underlined in (21) (see remarks by Nadeau and Fripiat, pages 146-147). 

Soil-Water Mixtures Figure 2 presents the dielectric spectra of clay-water mixtures (kaolinite and montmorillonite) after the influence of dc conductivity is removed. In addition to the orientational polarization of bulk water at 20GHz and adsorbed water at 10MHz, a low frequency polarization is observed at kHz to MHz frequencies. It should be noted that this low-frequency relaxation process cannot be generalized for all mineral-water... 

Figure 2. Dielectric spectral responses of clay-water mixtures at 298 K. Parameters used in the Debye-type relaxation spectral functions are complied from Ishida et al. (2000).

The potter shown in Figure 1 uses water to help sculpt the sides of a clay pot. As he dips his clay-covered fingers into a container of water, the water turns brown. The clay-water mixture appears uniform. However, if the container sits overnight, the potter will see a layer of mud on the bottom and clear water on top. The clay does not dissolve in water. The clay breaks up into small pieces that are of such low mass that, for a while, they remain suspended in the water. This type of mixture, in which the different parts spontaneously separate over time, is called a suspension. In a suspension, the particles may remain mixed with the liquid while the liquid is being stirred, but later they settle to the bottom. 

For clay-water mixtures with a particle size in the 0.5-1 jam range, assuming 6 = 0, Ylv = 0.07 J/m, and the pore size equal to half the particle size gives 0.5 MPa. Unfired ceramics containing little binder are very weak, and this stress is high enough to cause cracking. 

Forming techniques used for clay-based ceramics require control of water content in the batch. Water content, in turn, affects the response of the clay during forming [27], As the water content of the batch increases, the yield point of the clay-water mixture, and thus the force required to form the desired shape, generally decreases [26], However, the relationship is complex and depends on the composition of the clay, its structure, additives to the batch, and other factors [14], One method for quantifying the behavior of clay-water pastes is to measure the plastic yield point as a function of water content [14], The water contents and maximum yield points in torsion are compared for several clays in Table 9. Kaolins and plastic fire clays require the least amount of water to develop their maximum plasticity, ball clays require an intermediate amount, and bentonite requires the most. 

In 1809, Reuss observed the electrokinetic phenomena when a direct current (DC) was applied to a clay-water mixture. Water moved through the capillary toward the cathode under the electric field. When the electric potential was removed, the flow of water immediately stopped. In 1861, Quincke found that the electric potential difference across a membrane resulted from streaming potential. Helmholtz first treated electroosmotic phenomena analytically in 1879, and provided a mathematical basis. Smoluchowski (1914) later modified it to also apply to electrophoretic velocity, also known as the Helmholtz-Smoluchowski (H-S) theory. The H-S theory describes under an apphed electric potential the migration velocity of one phase of material dispersed in another phase. The electroosmotic velocity of a fluid of certain viscosity and dielectric constant through a surface-charged porous medium of zeta or electrokinetic potential (0, under an electric gradient, E, is given by the H-S equation as follows ... 

The importance of clay-water mixtures in the ceramic industry has already been referred to. The next step is to determine how our knowledge of colloidal suspensions can be applied to industrial processes. 

The first method really provides a measure of the cohesiveness or bonding power of the clay. To a stiff clay-water mixture a non-plastic material, such as graded sand, is added a little at a time, until the clay-sand-water mixture falls apart. The percentage of sand (based on the dry weight of the clay) necessary to cause this is the plasticity index or bonding power of the clay. 

General characteristics of sedimentation of a clay-water mixture. (Modified from Imai, G., Soil and Found., 21, 7-20,1981.)... 

Ceramic materials have been made since well before the dawn of recorded history. They are generally fashioned from clay or other natural earths at room temperature and then permanently hardened by heat. Silicate ceramics include objects made from clays, such as pottery, bricks, and table china. The three major ingredients of common pottery are clay (from weathering of feldspar as described previously), sand (silica), and feldspar (aluminosilicates). Clays mixed with water form a moldable paste because they consist of many tiny silicate sheets that can easily slide past one another. When the clay-water mixture is heated, the water is driven off, and new Si—O—Si bonds are formed so that the mass of platelets becomes permanently rigid. 

Some early oven doors were of the guillotine type, which could be raised or lowered vertically with a simple hoist. These were replaced when machinery was developed to install and remove doors horizontally with the door in a vertical position. All the early doors were sealed with a clay-water mixture known as luting material. Most oven doors are self-sealing a thin metal strip on the door is pressed against the metal doorjamb by screw or spring pressure exerted against door latches. 

Plastic deformation of a moldable powder-additive mixture is employed in several forming methods for ceramics. Extrusion of a moist clay-water mixture is used extensively in the traditional ceramics sector for forming components with a regular cross section (e.g., solid and hollow cylinders, tiles, and bricks). The method is also used to form some oxide ceramics for advanced applications (e.g., catalyst supports, capacitor tubes and electrical insulators). A recent development is the repeated co-extrusion of a particle-filled thermoplastic polymer to produce textured microstructures or fine-scale structures. Injection molding of a ceramic-polymer mixture is a potentially useful method for the mass production of small ceramic articles with complex shapes. However, the method has not yet materialized into a significant forming process for ceramics mainly because of two factors ... 

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