| Portland cements (OPC), which are commonly used in the construction industry, are based mainly upon lime-silica mineral phases, whereas in calcium aluminate cements the main reactive phases are lime-alumina compounds. Calcium aluminates (CAC) may go under the other names of aluminous cement or high alumina cement (HAC). Calcium aluminate cements evolved from a drive to develop sulphate resistant cements. Some of the notation used later in this article uses cement chemistry nomenclature, which is documented in table 1. Table 1. Cement Chemistry Nomenclature | | | | A | Al2O3 Alumina | | C | CaO Lime | | S | SiO2 Silica | History The first documentation of isolated calcium aluminate species dates back to 1848 when Ebelman reacted alumina with marble. Earlier than this, Vicat in 1846 had developed Portland cements with increased alumina contents for sulphate resistance, though the real industrial importance of these compounds was not recognised till some time later. Both French and English patents were granted to Bied in 1908 and 1909 for the manufacture of calcium aluminate cement. Bied used bauxite and lime as raw materials fired together in a cupola furnace. The lime-alumina phase diagram, which was published by Rankin and Wright in 1915 (figure 1), shows the presence of five calcium aluminate compounds C3A, C12A7, CA, CA2 and CA6. As a rule of thumb, the reactivity of calcium aluminates increases as lime content increases, making C3A the most reactive compound of these compounds. This is because the dissolution re-crystallisation reactions are much quicker for compounds of increasing lime content. However, as can be seen from the phase diagram, the higher the alumina content the more refractory the calcium aluminate compound (phases on the right having higher melting points). (Figure 1.). | 
| | Figure 1. The lime-alumina binary phase diagram. | Manufacture of Calcium Aluminate Cements There are two main methods of manufacturing calcium aluminate cement on an industrial scale. The first is by fusion in which a raw feed stock of bauxite and limestone is melted together in a reverberatory furnace. In the second, high purity limestone and high purity alumina are sintered together in a rotary kiln, which produces a much purer grade of cement known as high alumina cement. Grades of Calcium Aluminate Cement Generally calcium aluminate cements are over 35% in alumina (Al2O3) content and high purity calcium aluminate cements are over 70% in alumina. Cements of 80% Al2O3 and over however are manufactured from 70% Al2O3 grade materials mixed with fine alumina and a dispersive package. (As producing a material directly from a mixture of reagents in that ratio would result in forming predominately the grossite (CA2) and CA6 phases, which would prevent any resulting cement from setting.) | 
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