Page 493 - Handbook of Thermal Analysis of Construction Materials

Section 7.0 - Controlled Transformation Rate (CRTA) 467

It should be noted that other workers, e.g., Holdridge and Walker,
indicate that for prolonged storage, e.g., 18 months, the first peak is larger
than the second one. [13] At shorter times of storage, the second peak was of
larger intensity. Comparison of the data is rendered difficult unless the
experiments are carried out under identical conditions.

6.2 Conversion of Soluble to Insoluble Anhydrite

The soluble form of anhydrite (the descriptor soluble or insoluble
will be used as the soluble form has previously been referred to as the γ or
β form and sometimes to the III form) has a hexagonal-trapezohedric
lattice, a lower specific gravity (2.587) than the insoluble II form (2.985),
and is permeated by interstitial cavities. The insoluble form has a rhombic
pyramidal lattice (cone packed). The CaSO chains are deposited in the
4
interstitial cavities during the conversion. As previously discussed, a small
partial pressure of water vapor during the DTA measurements of the α-
hemihydrate influences the exothermic lamination peaks (Figs. 16 and 17).
Crystals of various shapes and sizes dehydrate at different rates and form
insoluble anhydrite lamellae at different temperatures. The curve in Fig.
16 shows that the exothermic peak disappears at a low rate of heating
(2°C/min) in open atmosphere. The concept of “concealing” the exotherm
describes the possibility that partial conversion of dehydrated crystals has
occurred while larger crystals are still dehydrating, masking the overall
effect. The curve in Fig. 17 demonstrates that low heating rates are not the
only factor governing the shape and appearance of the exothermic peak. The
sample in Fig. 17 was heated at a rate of 1°C/min with the use of a pinhole
in the crucible lid. The use of a lid and a higher sample mass resulted in a
higher partial pressure of the water vapor and consequently, a large and
sharp α-peak. It is apparent that contradictions in the literature may arise
from differences in material parameters and experimental details.

7.0 CONTROLLED TRANSFORMATION RATE
THERMAL ANALYSIS (CRTA)

Conventional thermal analysis requires that the temperature of the
sample follows some predetermined program as a function of time. Con-
trolled transformation rate thermal analysis (CRTA), referred to as the