Calcium Aluminates Plus Gypsum

Thermogravimetric technique has also been applied to follow the formation of sulfoaluminate products hydrated in a mixture containing tricalcium aluminate and g q)sum.[ l 

In the conduction calorimetric curves of the aluminate-gypsum system,the first stage is characterized by a peak at 30 minutes for the formation of ettringite. In the second stage, the peak occurs at 24-48 hours when the monosulfate phase forms by the reaction between ettringite and aluminate. 

Kalousek, et al., in an extensive work on the thermal analysis of cements concluded that both low and high aluminate sulfate forms ultimately disappear and are incorporated into the C-S-H gel. 

Attempts have been made to apply an evolved gas analysis technique (EGA) to investigate cement systems. In the EGA technique, a 

In Fig. 29, the thermal curves of C4AF-CaS04 2H20 mixes hydrated at aw/s ratio of 0.5 or 1.0 are given. Most samples hydrated for 2 days at 25°C contain mainly the low sulfoaluminate.The interconversions 

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The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO and the tricalcium aluminate keeps the SO level near zero concentration for the first 10-20 min causing rapid stiffening and quick set (approximately 15 min). It is also reported that chemical... 

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