Hydration of Calcium Aluminates

The hydration of calcium aluminate is an excellent example of the Le Chatelier model that is the through solution reactioa Obviously, as the results of other authors have shown [6,123,124], the process is composed of dissolution, followed by nucleation and crystal growth of new phase. This can be well followed on the example of CA hydration. 

The following calcium aluminate hydrate phases may be formed in the hydration of calcium aluminates. 

Ca2Al(0H)g.2H20].0H. In C4AHjc, an additional layer of H2O molecules is present between the principal layers. Tetracalcium aluminate hydrate is not among the hydrate phases formed in the hydration of calcium aluminates constituting calcium aluminate cement however, it may be formed in the hydration of tricalcium aluminate, a constituent of Portland cement. 

The total heat of hydration of calcium aluminate cement is in the range 450-500 J/g, and is similar to that of Portland cement. However, 70-90% of it is liberated within the first 24 hours (at 20°C), making dissipation of the heat into the environment more difficult than in situations where Portland cement is employed, where the liberation of hydration heat takes place much more slowly. This may be critical, especially in the erection of massive stmctures, where a significant rise of temperature may take place shortly after mixing. Maximum temperatures of up to 80°C may be reached in the bulk of the concrete stracture. This in turn may accelerate the conversion of the calcium aluminate hydrates formed. Wet curing has to be employed to prevent superficial dehydration and dusting of the hardened concrete. 

The chemical shrinkage associated with the complete hydration of calcium aluminate cement amounts to about 10-12 ml per 100 g of cement. This value is greater than that common in Portland cement (about 5 ml/100 g). 

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. 

Cong, S., and Kirkpatrick, R.J. (1993) Hydration of calcium aluminate cements a sohd state AJp NMR study. Journal of the American Ceramic Society 76,409—416. 

Muller D. et al (1992) Progress in the Al MAS NMR spectroscopy monitoring the hydration of calcium aluminate cements, in Proceedings 9th ICCC, New Delhi, Vol. 6,... 

At this point, it is important to distinguish admixtures from additives, the latter being materials that are added during the manufacture of a cement. Additives include gypsum as a grinding aid and set controllers that function by inhibiting the hydration of calcium aluminate (see Section 5.2.4.2). 

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