Effect of Water Vapor on the Formation of Modifications

Abstract

The two recognized forrns of calcium sulfate heniihydrate can be identified by the position of a relatively small exothermic peak in their differential thermograms. Hemihydrates prepared a t various water vapor pressures up to 760 mln Hg were found to produce this esotherniic peak in a position which is characteristic for the 0-for~il. These results indicate that a-hemihydrate cannot be made a t atmospheric pressure, as was previously suggested on the basis of heat solution measurements. The typical differential thern~ogram of a-hemihydrate is only obtained with material made by dehydration in solution or by autoclaving. The effect of ambient water vapor pressure 011 the position of the three peaks that occur in the differential thermogram of CaS0.1.2H?O has also been studied. I t was found that the incipient ten~perature of the second endothermic peak, corresponding to the transition of hemihydrate to soluble anhydrite, is displaced independent of the rate of heating fro111 145 "C to 187 OC with increasing water vapor pressures up to 760 mm Hg. This indicates that, for each temperature, a threshold water vapor pressure exists, above which soluble anhydrite cannot be formed.Calcium sulfate heinihydrate can be prepared froin dihydrate (CaS04.2H20) by three types of dehydration n~ethods, i.e., calcination a t atmospheric pressures, digesting in salt solutions, and autoclaving in lump forin followed by grinding. The heinihydrates obtained differ in a variety of properties, such as density ( I ) , gauging water requirement (2), heat of solution (3, 4), differential thermal analysis (d.t.a.) exotherms ( 5 , G), and possibly X-ray diffraction pattern (7). There is general agreement that these differences, lvhich reflect the degree of crystal lattice perfection and surface characteristics, indicate the existence of a t least two hemihydrate n~odifications. This has led to the developn~ent of the a-and P-hemihydrate theory (4, 8). Based on limited heat of solution data, the theory states that a-hemihydrate is not only produced by solution processes or autoclaving but also a t atmospheric pressure, if the atmosphere is saturated with water vapor. In the absence of water vapor the resulting hemihydrate is of the p-form and a t interinediate water vapor pressures the heats of solution indicate mixtures of the two modifications. For example, approxin~ately 75% a-hemihydrate and 25% p-hemihydrate are obtained by dehydration in an atmosphere "nearly saturated with water vapor" (4).A number of objections have been raised against the part of the theory which deals with dehydrations a t atmospheric pressures (9, 10). Hence, a study has been made of the effect of the ambient water vapor pressure present during dehydration on the formation of heinihydrate modifications. For this purpose, differential thermal analysis has been used under controlled atmospheric conditions, such as dry nitrogen, steam, and a number of intermediate partial water vapor pressures. This method has the advantage that it permits the preparation of ...

Thermal properties and microstructure of gypsum board and its dehydration products: A theoretical and experimental investigation

“…Gypsum, or CaSO 4 ·2H 2 O, contains about 20.9% by mass chemically combined water, which is readily lost at an elevated temperature. This phenomenon is called dehydration, and the dehydration mechanism of gypsum has been intensively investigated so far . Groves reported that first CaSO 4 ·0.5H 2 O and then CaSO 4 is formed during gypsum dehydration, which was also confirmed by Kuntze and Paulik et al .…”

Thermal Behavior of the Gypsum Binder in Dental Casting Investments

“…They found that the two modifications were different in thermodynamic properties such as specific heat and heats of hydration. The difference noted by Powell in his DTA study (1958) is also a thermodynamic property, and this method has gained increased support for the identification of wet-and dry-calcined hemihydrate (Fleck et al, 1960;Kuntze, 1965;Miyazaki, 1966;Ball and Urie, 1970). The present study indicates that while the DTA method is valid for the identification of wet-and drycalcined hemihydrate, the second exothermic reaction at higher temperature ranges cannot be overlooked in wet-calcined hemihydrates.…”

“…The exotherm did not appear in the range observed in the hemihydrate powders (about 2200C), but it did appear at a higher temperature range of about 350 to 4500C. It is reasonable to expect the exothermic reaction to occur in this high-temperature range, since the dehydration conditions produced in the DTA apparatus are considered to be similar to those prevailing in many dry calcination methods (Fleck et al, 1960;Kuntze, 1965;Miyazaki, 1966;Holdridge and Walker, 1967); therefore, a dry-calcined hemihydrate, which is characterized by an exotherm at a high-temperature range (Powell, 1958), was formed during the dehydration of the DTA-TG specimens. In the specimens containing additives, the exothermic reaction was the one most affected by the addition of both H3BO3 and NaCI; the peak was shifted to higher temperatures by about 50°C with H3BO3 and to lower temperatures by about 150°C with NaCl.…”

Thermal Behaviour and Crystal Structure of Sodium-Containing Hemihydrates of Calcium Sulfate

“…The differences observed in the exothermic effects are possibly associated with the dehydration kinetics dependent on surface area, porosity, crystallinity, etc. [11,15,16]. Opposite to these observations, Budnikov and Kosyreva [17] have claimed that the thermogram of the -form does not exhibit an exothermic effect.…”