Refractory grade cement contains a proportion of alumina. The higher the alumina content, the faster the cement hardens when applied and the better its refractory properties. Because of the improved properties of cement after addition of alumina, an extra development in the design of refractory grade cement came about. This development led to "CAC," or high-purity Calcium Aluminate Cement, a specialist limestone-based cement containing calcined alumina rather than the standard bauxite.
Aluminous cement, or cement that contains alumina, differs from standard Portland cement because it has a different chemical composition and different end uses or applications. Rather than bauxite and limestone, used to create high-grade cement, standard Portland cement is made from aluminous clay or shale instead of bauxite. This means that refractory grade cement contains calcium aluminates (the calcium comes from limestone) and Portland cement contains aluminosilicates (the silicate component arises when clay or shale are used).
In commercial settings, the amount of alumina used in a given cement is classified according to a simple grading system. Cement that contains a "low" quantity of alumina is graded CAC40, cement that contains a "medium" quantity of alumina is graded CAC50, and cement that contains a "high" quantity of alumina is graded CAC70 or above. The cements also have different shades or colors, ranging from grey (generally low alumina content) to white (generally high proportion of alumina). Portland cements are classified according to the type I, II and III system, where type III represents the highest quality.
The highest alumina content cements, and refractory grade cements, have the quickest drying times. The cement dries in six hours and structural reliance is possible within 24 hours. Also, the bending strength after one day is more than eight megapascal, and the crushing strength after one day is more than 70 megapascal, a significant improvement on standard products. The crystals inside white alumina refractory grade cement are hexagonal in shape, which leads to a strong macrostructure. The true density of the basic material is more than 3.95 grams per centimeter cubed, making it tightly packed compared to standard refractory and abrasive materials.
Refractory grade cement has a variety of high-end uses such as in bridge construction, large building developments and airport building. Importantly, refractory grade materials can withstand very high temperatures and pressures, and so also have applications in furnace building, production of temperature resistant oven slabs and in the design and manufacture of rotary kilns, for firing other objects. Other application areas include the petrochemicals industry, steel making and ventilation system manufacture.
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