Hydration reactions and products

The individual hydration products and relevant phase equilibria were described in Chapter 6. The major initial products arc CAHjg at low temperatures, CjAHg and AH, at intermediate temperatures and CjAH, and AH, at higher temperatures (Fig. 10.1). With the passage of time, the other products are replaced by C,AH(, and AH, at rates that depend on the temperature and other factors this process is called conversion. An amorphous or gel phase containing Ca has been reported to form simultaneously with the crystalline products, in a proportion relative to the latter that increases with temperature (E5,E6). 

The subsequent course of the reaction, essentially as described by Capmas and Menetrier-Sorrentino (C48), is as follows. An induction period occurs, during which the concentrations remain near their maximum values, the 

Within minutes after mixing, the solution is supersaturated in AH, CAHjo, CjAHg and C,AH. As noted earlier, AH, and CjAHg begin to form while the concentrations are still rising. For a cement in which CA is the only active constituent, CAH,q subsequently becomes a more important product below about 15°C (Fig. 10.1). For cements containing Cj A, C2AHg is a major product at lower temperatures. 

In the conversion reaction, CAH,q and C2AH8 redissolve and C,AH is formed, together with additional AH the structure of which then approxi-mates to that of gibbsite. At 5°C, this process may take many years, but at 50°C or above, the formation of C,AHg and gibbsite is virtually immediate. The only phases in the pure Ca0-Al203-H20 system stable in contact with aqueous solutions up to well above 100°C are gibbsite, CjAH and CH. 

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Skibsted, J. 1988. High-speed Si and Al MAS NMR studies of Portland and high alumina cements Effects of microsilica on hydration reactions and products. In Geopolymer 88 Proceedings of the First European Conferenoe on Soft Mineralurgy, Davidovits, J., and J. Orlinski (eds.), 2, 179-196. Universite de Technologic de Compiegne. 

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