Hydrogamet phases compositions

Mixes of C4A3S with calcium hydroxide, but without calcium sulfate, yield the hydrogamet phase C3AHg and an AFm phase of the approximate composition... 

Just like the strength development, the release of the heat of hydration also depends on the composition of the cement. In cements yielding mainly the hydrogamet phase an intensive exothermic maximum is apparent within the first hour of hydration. Such a maximum is markedly delayed and the overall amount of released heat is significantly reduced if straflingite is formed (Hormnertgen and Odler, 1991). 

One such Ca-aluminate hydrate mineral was observed in fly ash-water pastes (Tishmack 1999). The fly ash analysed in the study was washed with deionized water and Na-EDTA to selectively remove the soluble S-bearing minerals prior to being blended with additional CaO and water. The XRD analysis of the hydrated pastes showed peaks that closely matched the pattern of hydrogamet with one notable exception, a strong peak at 32.1° 2-theta. The phase may be a tetracalcium aluminate hydrate prototype AFm mineral noted to form in systems that have a low S C3A composition (Highway Research Board 1972). It may also be possible that the phase had a composition close to that of hydrogamet with some substitution of Si for Al. 

Taylor and Newbury [24] presented the results of analysis in the system of coordinates (A1+Fe)/Ca-Si/Ca and obtained a triangle which is shown in Fig. 4.7. Near the apexes, where there is most of points, the chemical composition corresponds to the following products CH, C-S-H and hydrogamets. Taylor and Newbury are of the opinion that the points in middle position, not near the apexes, correspond to the mixtures of phases. In the pastes matured for a short time the AFm phase was found [25]. The differences between the inner and outer C-S-H was not observed, perhaps except of some minor components their percentage in onter product is a tittle lower [25, 26]. The Ca/Si ratio higher than 2, results presumably from the presence of C-S-H—AFm mixtures in nanometric scale. This is possible because of the layer structure of both phases this perfect mixture of gels can be thns explained [9]. On the other side the monophase mixtures of CH with C-S-H are observed under the microscope only in the pastes with very low w/c [9]. 

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