Page 480 - Handbook of Thermal Analysis of Construction Materials

Section 4.0 - Dehydration of Gypsum 455

4.0 DEHYDRATION OF GYPSUM

Calcining gypsum at temperatures below 100°C for various periods
(1 hour to 22 days) gives rise to a limiting value of mass loss around that
caused by the evolution of 1.5 molecules of water, irrespective of the time
[6]
of heating (15 days at 70°C, 2 days at 80°C, and 12 hours at 90°C). DTA
curves for isothermally heated gypsum are presented in Figs. 7 and 8. At
relatively low temperatures (up to 90°C), dehydration of gypsum does not
proceed beyond the hemihydrate state for any heating period (Fig. 7).
Above 90°C, a continuous loss of mass with time is prolonged, resulting in
the formation of γ-anhydrite (soluble) as a final product (Fig. 8). Trace
amounts of residual water are reached towards the end of the dehydration
process. The residual water is driven off completely below the conversion
temperature of the soluble (γ-form) to the insoluble anhydrite (β-form)
probably because of the comparatively long periods of heating used in these
experiments. In other work, the residual water is considered attached to the
γ-CaSO and cannot be expelled below the temperature of conversion of γ
4
to β-CaSO . [7]
4
Additional study on the effect of prolonged heating was reported by
[8]
Khalil, et al., who noted that hemihydrate forms between 100 and 220°C
when gypsum is heated at a constant rate (10°C/min). However, complete
formation was confirmed to occur below 100°C with long heating times.
The disappearance of the hemihydrate and the formation of the soluble
anhydrite (γ-CaSO ) occurs with additional heating at 250°C for a few
4
minutes or between 100 and 130°C for a few hours. On further heating
(around 360°C), insoluble anhydrite (β-CaSO ) appears and transforms to
4
the α-form at 1230°C (Fig. 9).
The presence of CaSO persists up to 1300°C where it begins to
4
partially decompose at 1350°C. It was reconfirmed that the heating at 120°C
and 130°C causes a marked decrease in the hemihydrate content, and the
simultaneous increase of the γ-anhydrite is suggested from the notable
change in the magnitude of the endotherm. Heating at 110°C, 120°C, and
130°C for more than 5 hours causes the complete disappearance of gypsum
(the product being mainly γ-anhydrite as the heating time is prolonged). It
is noted that re-heating of previously heated gypsum samples results in
markedly reduced DTA peaks compared to those for previously unheated
gypsum. This could be related to disorder in the lattice of previously
[9]
calcined gypsum samples. A transformation from the monoclinic to the
orthorhombic form could also be responsible for the phenomenon of lower
dehydration of the previously calcined samples.