Hydrogamet formation

Klimesch, D.S., and Ray, A. (1998a) Hydrogamet formation during autoclaving at 180°C in imstirred metakaohn-hme-quartz slurries. Cement and Concrete Research 28,1109-1117. 

The effect of aluminum ions on the transformation of C-S-H into 1.1 nm tobermorite was investigated by Mitsuda and Taylor [184], The solid solution of aluminum ions in tobermorite enlarges also the stability field of this phase in relation to xonothte [185], Diamond [186] found the maximum Al/(Si+Al) ratio in tobermorite to be 0.15 above this value the hydrogamet phase is appearing. Aluminum is tetrahedrally coordinated and accelerates the formation of 1.1 nm tobermorite. Finally, Mitsuda and Taylor [184] revealed that alkalis (l%Na20+1.7%K20) are activating the tobermorite formation from zeolite and calcium hydroxide. 

In the mixtures of lime and natural pozzolana Sersale [27] found the hydro-gehlenite C2ASHg. It was formed, apart from hydrogamet at later age, from 70 to 150 days, when the amount of combined calcium hydroxide changed from 46 to 60% [28], The formation of hydrogamets was found also by Takemoto and Uchikawa [23]. 

According to Ding and Beaudoin (1996), the addition of small amounts of sodium silicate promotes the formation of strathngite (C2SAHg) in calcium aluminate cement pastes. At the same time the hydration of the cement is strongly retarded. The additive delays, but does not prevent completely, the formation of the hydrogamet phase in the cement paste. 

It has also been reported that the second derivative DTA is more useful to examine the products formed during the autoclaving of cement-quartz-metakaolin mixtures. Klimesch and Rayt ] subjected a mixture of quartz (38.5%) and cement (61.5%) containing different amounts of metakaolin and autoclaved them for 8 hrs at 180°C. It was found that the second derivative differential thermal curve provided a more detailed information, particularly in temperatures of 800-1000°C. In Fig. 35, DTA and second derivative curves for cement-quartz-metakaolin pastes are compared. The exotherms occur at 840, 903, and 960°C due to the formation of wollastonite from C-S-H, aluminum-substituted tobermorite, and anorthite from the hydrogamet residue respectively. The small endot-herm at 828°C preceding the first exotherm is probably caused by well crystallized calcite. 

The last two reactions showing the formation of hydrogamet (C3 AHg) are known as conversion reactions. Direct formation ofhydrogamet from CA can occur in the presence of water at elevated temperatures ... 

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