Refractory aggregates

G. MacZura, "Production Processes, Properties, and Appbcafions for Tabular Alumina Refractory Aggregates," in L. D. Hart, ed.. Alumina Chemicals Science and Technology Handbook, The American Ceramic Society, Columbus, Ohio, 1990. 

Bauxitic Kaolins andMullites. Deposits of bauxitic kaolins, kaolins having aluminous minerals, have been discovered that have alumina contents between 50 and 70%. These materials are made into refractory aggregates called calcines, grog, clinker, or grain. In addition to sdectivdy mined deposits, synthetic compositions can be prepared from kaolin and alumina and other minerals to produce compositions of desired alumina and mineralogical content. These synthetic mullites are readily available in the form of sintered and fused aggregates. 

Used with appropriate refractory aggregates, calcium aluminate cements may be used to make refractory castables having applications in the steel and other heat-using industries. Cements made from bauxite may thus be used at hot face temperatures up to 1350 C. White calcium aluminate cements, which are low in iron and higher in alumina, can be used with appropriate aggregates to make castables similarly resistant up to 2000°C. 

Tabi 9-7C. Characteristics of Hiqh-temperature Insulation. Classification op Refractory Castables (Refractory aggregates plus hydraulic cement)... 

Mouldable Refractory. A mixture of graded refractory aggregate (or aggregates) and a plasticizer, usually clay, supplied mixed with water so that it is already workable chemical bonding agents may also be incorporated the workability is such that the material can readily be placed by hand malleting. 

Ramming Material. Graded refractory aggregate, or a mixture of aggregates, with or without the addition of a plasticizer ramming materials are usually supplied at a consistency that requires a mechanical method of application chemical bond(s) may be incorporated. The rammed mass becomes strong and monolithic by vitrification or sintering in situ. 

Ramming Mix. A mixture containing graded refractory aggregate and other bonding materials which will harden when heated, by the formation of a chemical and/or ceramic bond. Such a mixture may be supplied either dry or mixed with water or other liquids and, in either case, requires to be installed by the use of mechanical or pneumatic tools or vibration. (B.S. 1902 Pt. 7). ASTM C71 defines a ramming mix as a refractory material, usually tempered with water, that cannot be extruded, but can be rammed in place to form a monolithic structure. 

Coatings Coatings are mixtures of fine refractory aggregates and bonds. They contain higher water content than mortars and are supplied in a ready-to-use form. The bonds may be ceramic, hydraulic, chemical, or organic. They can be applied manually, pneumatically, by mechanical projection, or by spraying. Manual application involves the use of a brush or trowel. 

Tap Hole Mixes Tap hole mixes are specifically used for filling and sealing of blast furnace tap holes. They are supplied as extruded preformed blocks. They are moldable because of their plastic consistency. They contain refractory aggregates and organic and ceramic bonds. Tap hole mixes develop a carbon bond on heating. 

Because corrosive fluids can penetrate the refractory, such penetration usually results in disruption and even destruction of the matrix of the refractory. The matrix is the area of sintered lines that hold together bonded bricks and monolithic refractories. Usually, the matrix contains more impurities and more porosity than aggregate particles. Therefore, corrosion affects the weakest component of the refractory at a higher rate than denser or higher-purity particles. In extreme cases of matrix attack, rapid erosion of coarse refractory aggregate can occur. 

As mentioned, refractory castables are combinations of refractory aggregates, matrix components, bonding agents, and admixtures. The proportions of each component used vary in each castable composition to achieve the desired physical and chemical properties and characteristics for the intended castable application. The general range of component quantities in a refractory castable is shown in Table 2. 

Coatings. These products are mixtures of fine refractory aggregates and bond(s). They are usually supplied ready-for-use with a higher water or liquid content than is required for mortars. They are often designated as coatings and may have a ceramic, hydraulic, chemical, or organic bond. The mixmres are applied manually (with a brush or trowel), by pneumatic or mechanical projection, or also by spraying. 

The multitude of refractory aggregates (for example, calcined clay, flint clay, andalusite, mulhte, bauxite, fused white or brown alumina, sintered alumina, etc.) and the different grain sizes of these materials—in addition to the big variety of bonding agents, additives, and mixing liquids—enable endless combination possibihties in development work on unshaped refractory products. Therefore, a qualitative standardization is not really possible as it is the case for refractory bricks. As a result of the ENV 1402-1 standard, which now provides a more precise definition of the products, there will probably be adjustments on the market in the long run. If also considering economic aspects, suitable products will obviously be developed on the basis of all the available constituents. 

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