Calcium Aluminate Cements - History, Manufacture & Grades

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Ordinary Portland cements (OPCs), mainly based on lime-silica mineral phases, are often used in the construction industry. On the contrary, calcium aluminate cements (CAC) have lime-alumina compounds as the core reactive phases.

Also known as aluminous cement or high alumina cement (HAC), CAC was developed after a drive to create cements that are impervious to sulfate.

Some of the notations in this article use cement chemistry nomenclature, as listed in Table 1.

Table 1. Cement Chemistry Nomenclature

History

Isolated calcium aluminate species were first documented in 1848, when Ebelman observed the reaction of alumina with marble. Before this, Vicat had produced Portland cements in 1846 that contained increased amounts of alumina to resist sulfate; however, it was only later that the real industrial importance of these compounds was recognized. Bied won the grants for both French and English patents for production of calcium aluminate cement in 1908 and 1909, respectively. He achieved this by using bauxite and lime as raw materials, and burning them together in a cupola furnace.

Published by Rankin and Wright in 1915, the lime-alumina phase diagram demonstrates the presence of five calcium aluminate compounds—CA, CA₂, C₃A, CA₆, and C1₂A₇. The reactivity of calcium aluminates generally increases with the lime content, making C₃A the most reactive among all these compounds. The reason is that compounds with higher lime content exhibit relatively faster dissolution re-crystallization reactions. However, alumina content is directly proportional to the refractory nature of the calcium aluminate compound.

Manufacturing Calcium Aluminate Cements

Two major methods are available for creating calcium aluminate cement on an industrial level. The first method is by fusion, which involves liquefying a raw feedstock of bauxite and limestone together, in a reverberatory furnace. In the second method, high-purity limestone and high-purity alumina are sintered together in a rotary kiln. The kiln produces a relatively purer grade of cement, namely the HAC.

Grades of Calcium Aluminate Cement

Most often, calcium aluminate cements and high-purity calcium aluminate cements contain over 35% and 70% alumina (Al₂O₃), respectively. But cements containing 80% Al₂O₃ and more are developed from 70% Al₂O₃ grade materials mixed with fine alumina and a dispersive package. This is because if a material is directly produced from a mixture of reagents in that ratio, it will mostly result in the formation of grossite (CA₂) and CA₆ phases. Such phases will hamper the setting of any resulting cement.