Clay-water mixtures plasticity

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... 

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. 

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. 

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 ... 

Although plasticity is a recognizable property, it is difficult to define scientifically. A substance is said to be plastic if it is cohesive (i.e. holds together), is mouldable to any shape, and retains that shape. If we restrict ourselves to solid-water mixtures, we find that there are few substances other than the clay minerals that when mixed with water develop plasticity, and they all have at least one thing in common—their ultimate particles are very small—of colloidal dimensions. 

The idea that, if a single available material cannot fulfill a set of desired properties, then a mixture or a compound of that material with another one might be satisfactory is likely as old as mankind. Adobe, likely the oldest building material, is made by blending sand, clay, water and some kind of fibrous material like straw or sticks, then molding the mixture into bricks and drying in the sun. It is surely one of the oldest examples of reinforcement of a "plastic" material, moist clay, with natural fibers that was already in use in the Late Bronze Age, nearly everywhere in the Middle East, North Africa, South Europe and southwestern North America. In a sense, the basic principle of reinforcement, i.e., to have a stiffer dispersed material to support the load transmitted by a softer matrix, is already in the adobe brick. Therefore, the "discovery" of natural rubber reinforcement by fine powdered materials, namely carbon black, in the dawn of the twentieth century surely proceeded from the same idea. 

Plastic Refractories and Ramming Mixes. Plastic refractories are mixtures of refractory grains and plastic clays or plasticizers with water. Ramming mixes may or may not contain clay and are generally used with forms or steel work. The amount of water used in these products varies but is held to a minimum to faciUtate drying. Plastics are generally suppHed in a ready-to-use state. 

Most masonry cements are finely iaterground mixtures where Portland cement is a principal constituent. These cements also iaclude finely grouad limestones, hydrated lime, aatural cement, po22olans, clays, or air-entraining ageats. Secoadary materials are used to impart the required water reteatioa and plasticity to mortars. 

Fireclays can be dividea into plastic clays and hard flint clays they may also be classified as to alumina content. Firebricks are usually made of a blended mixture of flint clays and plastic clays which is formed, after mixing with water, to the requirea shape. Some or all of the flint clay may be replaced by highly burned or calcined clay, called... 

Building mud, a composite material, is easily prepared by mixing clay or clayey soil with fibrous matter, such as straw or dung of herbivorous animals, and sufficient water to obtain a plastic, pliable mass. In ancient Egypt, for example, mud was made by mixing clayey soil with sand, chopped straw, and sufficient water so as to make the mixture pliable and suitable either for use as mortar or stucco or for making bricks. 

Plastic clay is dried to a moisture content of less than 5% and stemming material is made from it by mixing four parts by weight of the clay with one part by weight of water. The mixture is stirred carefully until a uniform paste is obtained. 

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