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 ...
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