Page 488 - Handbook of Thermal Analysis of Construction Materials

462 Chapter 11 - Gypsum and Gypsum Products

The results of the quasi-sealed method are illustrated in Fig. 12. A
quasi-sealed condition was achieved by inserting a thin tungsten wire
(12–250 µm in diameter) between the lid and sample cell. The two steps are
easily separated using the quasi-sealed conditions. This is clearly seen for
the curves obtained with 30 and 18.7 µm tungsten wires. The onset
temperatures for the reaction are increased to 125 and 127°C. In the curve
of the quasi-sealed system, dehydration first occurs after some minor
pressure of water vapor has been reacted. The change in slope of the TG
curve is then easily recognizable. It is concluded that two separate steps of
CaSO •2H O can be readily determined by using a quasi-sealed condition
4
2
obtained by the application of 12–30 µm tungsten wires in the sealing
mechanism of the apparatus.

Figure 12. TG results of gypsum in a quasi-sealed, open and completely sealed system. [10]

6.0 CONVERSION REACTIONS

6.1 Dihydrate to ββ ββ β-Anhydrite

The influence of the partial pressure of water vapor during calcium
sulfate dihydrate decomposition was explored by Lehmann and Rieke. [11]
This work provided confirmation of the direct conversion of the dihydrate