Limestone Bauxite

Fig. 3.3 Manufacture of calcium aluminate (high alumina) cement

the concrete, in which changes in the crystal structure, accelerated by high temperatures and humidity, have caused serious loss of strength, increased porosity and subsequent chemical attack. Depending upon the degree of conversion, calcium aluminate cement becomes friable and a deeper brown in colour; the exact degree of conversion can only be determined by chemical analysis of a core sample. It is now recognised that such failures can be prevented by using a minimum cement content of 400 kg/m3, limiting the water/cement ratio to a maximum of 0.4, and by ensuring controlled curing during the 6- to 24-hour initial hardening stage. The concrete should be covered or sprayed to prevent excessive water loss, particularly where substantial increases in temperature may occur.

Additionally, in order to prevent alkaline hydrolysis of the concrete, aggregates containing soluble alkalis should not be used; hard limestone is generally considered to be the best aggregate. Coloured calcium aluminate cement concrete has the advantage that it is free from calcium hydroxide, which causes efflorescence in Portland cements. The BRE Special Digest SD3 (2002) gives methods for assessing existing calcium aluminate cement concrete [high alumina cement concrete (HACC)] constructions and suggests appropriate remedial actions. In some cases where the depth conversion of HACC structural members is significant, with time there is an increasing risk of reinforcement corrosion.

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