Strength development calcium aluminate cements

Table 10.1 gives typical chemical compositions of commercially produced calcium aluminate cements. The essential compound in all of them, because it develops the main hydraulic activity and is consequently responsible for the strength development, is monocaicium aluminate, CA. In white calcium aluminate cements, it can occur with various combinations of the other binary phases of the CaO-Al20j system and corundum. The crystal structures of these compounds and the phase equilibria relating to their formation were considered in Chapter 2. In the sintering process by which these cements are made, the reaction conditions are of the utmost importance, as... 

Fig. 9.4 Schematic strength development curves for calcium aluminate cement concrete at w/c = 0.4 and cement content of 400 kg/m. (according to [20])...

Pozzolanic materials or pozzolanas do not exhibit cementing properties if mixed with plain water. However, they possess the capacity to react at ambient temperatures with calcium hydroxide, in the presence of water, to yield strength-developing calcium silicate/aluminate hydrates. They include a variety of materials of natural and artificial origin, such as fly ash, microsilica, burnt clays, and diatomaceous earths. 

The development of calcium aluminate cement was spurred by efforts to overcome the problems associated with sulfate attack on Portland cement based concrete used in the construction of railway tunnels in gypsiferous grounds. The first patent relating to this type of binder was filed in 1908 by Bied in France. The cement was introdueed into production in 1913, and became known as Ciment Fondu. After it was recognized that calcium aluminate cement gains strength much faster than Portland cement, the binder was used in World War I by the French military in the constraction of gun emplacements and shelters, where this property was of paramount importance. After the war, the cement became widely used in other structural applications however, its use in this area became limited, after failures of structures built with this cement were reported from different countries. Nowadays calcium aluminate cement is being used in a variety of special applications. 

In mixes with Portland cement as the main constituent, the setting is due to a rapid formation of ettringite and the hydration of calcium aluminate cement. The hydration of the calcium silicates has little influence on the setting process, but contributes to the subsequent strength development (Gu et al., 1994). In addition to the phases formed in the hydration of pure Portland and calcium aluminate cements, stratlingite (gehlenite hydrate, C2SAHg) may also be formed in the hydration process. 

Legal restrictions, high price, and concerns about long-term performance limit the widespread use of calcium aluminate cement for constraction purposes. Its use may be attractive in applications where rapid strength development is required, as in the production of prefabricated concrete elements. In apphcations in which only a limited service time is planned, calcium aluminate cement may be used without concern. 

By combining calcium aluminate cement with 5-20% of Portland cement or Portland cement with 5-20% of calcium aluminate cement a binder may be obtained that exhibits a very short setting time and rapid strength development. Of these two alternatives the latter is more widely used, not least for economic reasons. 

Fentiman, C.H. et al. (1990) The effect of curing conditions on the hydration and strength development inFondu slag, m. Proceedings International Symposium on Calcium Aluminate Cements, London, p. 272. 

Sugama, T., and Carciello, N.R. (1991) Strength development in phosphate bonded calcium aluminate cement. Journal of the American Ceramic Society 74, 1023-1030. 

Gu, P., Beaudoin, J. J., Quinn, E., Myers, R., Early Strength Development and Hydration of Ordinary Portland Cement/Calcium Aluminate Cement Pastes,Cem. Based Mater., 6 53-56 (1997)... 

In mixes in which calcinm aluminate cement is the main constituent, the reaction mainly responsible for fast setting and strength development is a very fast hydration of the calcium alirminate phases. The accelerated hydration responsible for fast setting and strength development is dne to the increase of the pH value of the mix, brought about by... 

Phosphate-bonded high-alumina cement is reported to be useful for special purposes [28]. Increase in strength and heat resistance is claimed with P2O5 addition [29]. Special cements based on calcium aluminate and sodium polyphosphate, (NaP03> , which set and harden at 50 300°C, have recently been developed for use in geothermal wells [30]. 

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