Page 492 - Handbook of Thermal Analysis of Construction Materials

466 Chapter 11 - Gypsum and Gypsum Products

The influence of the experimental parameters on the shape of the
peak doublet in the DTA curves of the liberation of the non-stoichiometric
water of β-hemihydrate is indicated in Fig. 15. The influence of the partial
pressure of the water vapor or the speed of liberation of the non-stoichio-
metric water can be discerned from the curves. Curve (1) was obtained
without a crucible lid. The water can be driven off much faster and a low
partial pressure of water vapor can be obtained. A higher partial pressure
results in a displacement of the peak to higher temperature and produces
improved resolution of the second peak. The areas of curves (2) and (3) are
approximately the same if the decreased heating rate and reduced chart
speed are taken into account. The sensitivity of the measurements was held
constant. Curve (3) illustrates that a lower heating rate results in a lower
partial pressure of water vapor, lower peak temperatures, and a decrease in
the intensity of the second peak.

Figure 15. The influence of experimental parameters on the shape of the peak doublet
exhibited by the DTA curves of β-hemihydrate. [11]