Calcium aluminate cements

Calcium aluminate cements^[1] are cements consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement", "high-alumina cement" and "ciment fondu". They are used in a number of small-scale, specialist applications.

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History

The method of making cement from limestone and low-silica bauxite was patented in France in 1908 by Bied of the Pavin de Lafarge Company. The initial development was as a result of the search for a cement offering sulfate resistance. The cement was called "Ciment Fondu". Subsequently, its other special properties were discovered, and these guaranteed its future in niche applications.

Composition

The main active constituent of calcium aluminate cements is monocalcium aluminate (CaAl₂O₄). It usually contains other calcium aluminates as well as a number of less reactive phases deriving from impurities in the raw materials. Rather a wide range of compositions is encountered, depending on the application and the purity of aluminium source used^[2]. Constituents of some typical formulations include:

The mineral phases all take the form of solid solutions with somewhat variable compositions

Manufacture

The cement is made by fusing together a mixture of a calcium-bearing material (normally limestone) and an aluminium-bearing material (normally bauxite for general purposes, or refined alumina for white and refractory cements). The liquified mixture cools to a basalt-like clinker which is ground alone to produce the finished product. Because complete melting usually takes place, raw materials in lump-form can be used. A typical kiln arrangement comprises a reverberatory furnace provided with a shaft preheater in which the hot exhaust gases pass upward as the lump raw material mix passes downward. The preheater recuperates most of the heat in the combustion gases, dehydrates and de-hydroxylates the bauxite and de-carbonates the limestone. The calcined material drops into the "cool end" of the melt bath. The melt overflows the hot end of the furnace into moulds in which it cools and solidifies. The system is fired with pulverzed coal or oil. The cooled clinker ingots are crushed and ground in a ball-mill. In the case of high-alumina refractory cements, where the mix only sinters, a rotary kiln can be used.

Reaction with water

The hydration reactions of calcium aluminate cements are very complex. The strength-developing phases are monocalcium aluminate, dodecacalcium hepta-aluminate and belite. Calcium aluminoferrite, monocalcium dialuminate, gehlenite and pleochroite contribute little to strength. The reactive aluminates react with water initially to form a mixture of CaO.Al₂O₃.10H₂O, 2CaO.Al₂O₃.8H₂O, 3CaO.Al₂O₃.6H₂O and Al(OH)₃ gel, the amounts of each depending upon the curing temperature. The first two hydrates subsequently decompose to a mixture of 3CaO.Al₂O₃.6H₂O, Al(OH)₃ gel and water, this process being called "conversion". Because of the loss of water, conversion causes an increase in porosity, which can be accompanied by a decrease in strength. This need not be a problem in structural concrete provided that a sufficiently high cement content and a sufficiently low water/cement ratio are employed^[3].

Applications

Because of their relatively high cost, calcium aluminate cements are used in a number of restricted applications:

• in construction concretes, rapid strength development is achieved, even at low temperatures.
• in construction concretes, high chemical resistance is possible.
• in refractory concretes, strength is maintained at high temperatures.
• as a component in blended cement formulations, various properties such as ultra-rapid strength development and controlled expansion can be obtained.

References

1. ^ P. C. Hewlett (Ed)Lea's Chemistry of Cement and Concrete: 4th Ed, Arnold, 1998, ISBN 0-340-56589-6, Chapter 13
2. ^ H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, p 317
3. ^ Taylor ibid p 330