Types of Plasters

Gypsum plaster It is produced by mixing water with calcium sulphate hemihydrate, with the help of heat of about 300°F (150°C). When the dry plaster powder is mixed with water, it re-forms into gypsum. If plaster or gypsum is heated above 200°C, anhydrite is formed, which will also re-form as gypsum if mixed with water. 

Gypsum plaster is often being used to simulate the appearance of surfaces of wood, stone, or metal. 

Lime plaster Lime plaster is a mixture of calcium hydroxide and sand. With the presence of carbon dioxide, calcium hydroxide change into calcium carbonate (limestone). To make lime plaster, limestone is heated to produce 

Cement plaster It is a mixture of suitable plaster, sand, Portland cement and water which is normally applied to masonry interiors and exteriors to achieve a smooth smface. Walls constructed with stock bricks are normally plastered while face brick walls are not plastered. Various cement-based plasters are also used as proprietary spray fireproofing products. These usually use vermiculite as lightweight aggregate. Heavy versions of such plasters are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges and vessel skirts. 

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The use of mineral ligands (types of plaster) that confer fragility to the stationary phase can be used when one wants to recover the compounds after their separation. This can be done by removing the zones of interest from the support and extracting them using a solvent. 

F or centuries man has attempted to devise an effective process by which combustible materials could be rendered noncombustible or at least fire-resistant, but until about 10 years ago progress in this field was very limited and the effectiveness of the processes was questionable. Back in the days of the Roman Empire, efforts were made to reduce the fire hazard in props, curtains, and decorative-effect materials used in stage and theatrical plays by impregnating the fabrics with fine clay, and at about the same time clay, gypsum, and other types of plaster were used to coat wood, in an effort to make it resistant to fire. Since that time there has been steady progress in the field of fire-resistive and fire-retardant treatment processes, but it has been slow and the field of activity has been rather limited. 

Type of plaster Lime + Cement Cement (+lime ) Lime... 

The next example will demonstrate making and using molding plaster of paris molds for vacuum forming. For this type of work the product must be labeled molding plaster of paris. Other types of plaster such as is used for patching walls will not work. You can find molding plaster of paris at most hobby stores. 

Treatments used for various types of fractures are cast immobilization, traction, and internal fixation. A plaster or fiber glass cast is the most commonly used device for fracture treatment. Most broken bones heal successfiiUy once properly repositioned, ie, fixed in place via a cast. This type of cast or brace is known as an orthosis. It allows limited or controlled movement of nearby joints. This treatment is desirable for certain fractures. 

Plaster is the rehydrated calcined gypsum. The American Dental Association classifies five types of dental plaster according to the physical properties type 1, impression plaster type 11, model plaster type 111, dental stone type IV, high-strength dental stone and type V, high-strength. 

Although plaster has been a very successful and serviceable material, it is seriously lacking in hardness, edge strength, chip resistance, abrasion resistance, and strength to fulfiU many needs of dentistry. Some of these requirements have been partially filled by the development of the type III and type IV plasters. Table 3 Hsts the compression strength of dental plasters. 

Impression Plasters. Impression plasters are prepared by mixing with water. Types I and II plasters are weaker than dental stone (types III and IV) because of particle morphology and void content. There are two factors that contribute to the weakness of plaster compared to that of dental stone. First, the porosity of the particles makes it necessary to use more water for a mix, and second, the irregular shapes of the particles prevent them from fitting together tightly. Thus, for equally pourable consistencies, less gypsum per unit volume is present in plaster than in dental stone, and the plaster is considerably weaker. 

Type III dental stones are used for casts requiring higher compressive strength and abrasion resistance than casts formed using the type II plaster. These dental casts are used for the processing of denture-base materials. 

Absorption - Processes water can be removed from a material by the capillary action of porous bodies. An example is the cream of clay and water used for casting pottery, which is deprived of the greater part of its water by placing it in molds of plaster of Paris. The capillary character of this mold withdraws the water from the liquid clay mixture and deposits upon itself a layer of solid clay, the thickness of which is controlled by the time of standing. Certain types of candies, such as gumdrops, are dried mainly by contact with the starch molds in which they are cast. The drying effect of sponges, towels and materials of this kind is due to this same action. 

The two other main types of human-made building cements, lime cement and gypsum cement, have been and still are used in many areas of the world. Both these cements require quite elaborate thermal procedures for producing their main components, which are slaked lime in lime cement and plaster of Paris in gypsum cement. Making them involves the calcination of an appropriate type of stone, a process that has been practiced since prehistoric times. Slaked lime is made by the calcination of limestone plaster of Paris, by the calcination of gypsum (see Textbox 33) (Cobum et al. 1990 Lea 1962). 

P.Y.108 is applied in various types of industrial finishes, especially in original automotive (O.E.M) and in automotive refinishes. It is also recommended for metallic finishes, although it is much less weatherfast in such systems. The pigment tends to seed, i.e., it forms specks upon storage. The mechanism behind this phenomenon remains to be elucidated. Besides, P.Y.108 also lends color to emulsion paints, in which it is durable enough to satisfy the requirements for exterior paints based on synthetic resin dispersions. It is also fast to acids, alkali, and plaster. 

Several different materials and installation systems are available and include various types of proprietary materials, plasters containing perlite or ver-miculite, concrete mixtures, or lightweight concretes. The selected material and installation system, encasement, or surface application should provide protection for the expected fire duration. For more information refer to Chapter 7, Section 7.3.2. 

The intensity of seismic motion that can be tolerated by various kinds of structures must be established before acceptable charge weights at various distances can be determined. Obviously, the level of motion required to damage a structure depends upon its construction. For example, a steel-framed warehouse can tolerate a more. intense seismic wave than a residential structure with plaster walls. Because plaster is the weakest of the most commonly used materials of construction, and because of the prevalence of such structures, most damage criteria are based oh this type of structure... 

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