Page 475 - Handbook of Thermal Analysis of Construction Materials

450 Chapter 11 - Gypsum and Gypsum Products

Gypsum has useful fire-resistant properties due to its water of
crystallization (20.9 %). Gypsum plaster is widely used as an insulating
material for protecting columns and beams of wooden materials from the
high temperatures that develop during a fire.
It is apparent that rapid methods of analysis for gypsum, hemihy-
drate, and anhydrite are of significant practical interest. Thermal methods
of analysis appear to be particularly suited for this task. [3]
This chapter describes various aspects of thermal studies con-
ducted on CaSO •2H O and α and β forms of CaSO •½H O, using
4
2
4
2
differential thermal analysis, differential scanning calorimetry, and
thermogravimetry. The effect of environmental conditions on the quantita-
tive deterioration of the various calcium sulfate compounds is also exam-
ined. The development of more recent techniques such as controlled
reaction thermal analysis is also presented. [4]

2.0 DIFFERENTIAL THERMAL ANALYSIS
(DTA) AND DIFFERENTIAL SCANNING
CALORIMETRY (DSC)

Typical DTA curves of CaSO •2H O in a N atmosphere are
4
2
2
presented in Figs. 1 and 2 at cell pressures of 760, 590, 380, 150, and 197
[5]
torr. The two endotherms in Fig. 1 occurring at 150 and 190°C are typical
of numerous thermograms appearing in the literature, but with less overlap
possibly due to sample size effects. A small endothermic dent observed by
some investigators, immediately after the second endotherm, is not indi-
cated in Fig. 1. An exothermic peak observed at 375°C is characteristic of
hemihydrate.
Three transformations ascribed to the peak temperatures are de-
scribed by the following three reactions occurring at 760 torr:

3
Eq. (1) CaSO •2H O (150°C) ↔ β-CaSO •½H O + / H O
2
4
2
4
2
2
Eq. (2) β-CaSO •½H O (197°C) ↔ γ-CaSO + ½H O
2
4
2
4
Eq. (3) γ-CaSO (375°C) → β-CaSO 4
4