Page 485 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Simultaneous TG - DTG - DTA 459

[8]
Khalil and Gad’s results indicate that heating gypsum continu-
ously at a constant rate of 10°C/min results in a loss of one and a half
molecules of its water of crystallization or about 15.3%. This leads to
hemihydrate formation between 180 and 220°C. The γ-anhydrite formed
between 220 and 300°C with a total loss of nearly all the combined water
of crystallization amounting to about 20.4%. Complete hemihydrate forma-
tion occurs when gypsum is heated at relatively lower temperatures for
longer periods. Heating at temperatures ranging between 100 and 130°C
causes the loss of nearly all of the two molecules of water forming the γ-
anhydrite with varying amounts of hemihydrate. Longer periods at these
temperatures favor the formation of γ-anhydrite. Heating at higher tempera-
tures causes the conversion of γ-anhydrite to the β-form at 360°C. The latter
changes to the α- modification at about 1230°C. Further heating up to
1300°C shows insignificant loss in mass other than that due to the loss of
the water of crystallization.

5.0 SIMULTANEOUS TG-DTG-DTA

Isa and Oruno describe a method that enables identification of
intermediates (in the gypsum dehydration process) more easily. [10] The
method involves the use of simultaneous TG-DTG-DTA under various
sealed atmospheres corresponding to three systems—open completely,
sealed, and quasi-sealed. Endothermic DTA peaks appear earlier (129 and
133°C) than the point of decreasing TG. This technique, resembling the
quasi-isothermal and isobaric thermogravimetry (Q-TG), is superior to the
latter in that it needs less of the sample.
Typical results for the open system are shown in Fig. 10. The first
endothermic peak from the DTA, due to dehydration, begins at 102°C. The
TG and DTG curves begin at 107°C and 102°C respectively (for the
simultaneous technique) indicating the coincidence of the thermal and
thermogravimetric behaviors of hydration. This temperature is, however,
dependent on the sample amount, heating rate, the sensitivity of the TG,
DTG, and DTA, and the signal to noise ratio. The DTA exhibits two
endotherms (at 122 and 130°C), and there are considered to be two reaction
steps.