Calcium sulfate dihydrate, dehydration

The dehydration and rehydration reactions of calcium sulfate dihydrate (gypsiun) are of considerable technological importance and have been the subject of many studies. On heating, CaS04.2H20 may yield the hemihydrate or the anhydrous salt and both the product formed and the kinetics of the reaction are markedly dependent upon the temperature and the water vapour pressure. At low temperatures (i.e. < 383 K) the process fits the Avrami-Erofeev equation (n = 2) [75]. The apparent activation energy for nucleation varies between 250 and 140 kJ mol in 4.6 and 17.0 Torr water v our pressure, respectively. Reactions yielding the anhydrous salt (< 10 Torr) and the hemihydrate ( (HjO) >17 Torr) proceeded by an interface mechanism, for which the values of E, were 80 to 90 kJ mol. At temperatures > 383 K the reaction was controlled by diffusion with E, = 40 to 50 kJ mol. ... 

Plaster of Paris is produced by a dry dehydration (calcination) of calcium sulfate dihydrate, which is available either as natural gypsum or as a by-product of the chemical industry. Increasing amounts of dihydrate are also produced as flue gas gypsum in the desulfurization of flue gas in power plants that use sulfur-containing fossil fuels as source of energy. A number of technologies are available to produce plaster of Paris. 

A third possibility of converting calcium sulfate dihydrate into a-hemihydrate consists in dehydrating the dihydrate by sulfuric acid (Nogishi et al., 1981 Zirrz et al, 1991). At an appropriate combiration of acid concentration, temperature, and reaction time the dihydrate suspended in the acid converts quantitatively to a-hemihydrate. Uner more drastic conditions (that is, at a higher acid concentration, higher temperature, and/or longer reaction time) the dehydration does not stop at the hemihydrate level, and anhydrous calcium sulfate is formed instead. 

A typical TG curve of a commercial plaster (Fig. 29) shows two steps for mass loss. The first step is caused by the dehydration of calcium sulfate dihydrate. The second step is due to the dehydration of calcium sulfate hemihydrate. The second step is attributed to the hemihydrate present in the original sample and also to that formed during the first step of decomposition. 

Arii, T., and Fujii, N., Controlled Rate Thermal Analysis Kinetic Study in Thermal Dehydration of Calcium Sulfate Dihydrate, J. Anal, and Appl. Pyrolysis, 39 129-143 (1997)... 

Under certain conditions of pressures and temperature, the formed sohd B can present a distance to stoichiometry with respect to gas or dissolve gas significantly. We assume that only one gas is produced with molar mass Mq. Take s as the amount of gas that remains fixed per mole of B. We will quote as an example the dehydration of calcium sulfate dihydrate (gypsum) into plaster, which is, in fact, a sohd solution of water in calcium sulfate whose composition varies practically from 0 to 0.66 and which takes value of 0.5 under normal conditions of temperature and moisture from where the name of semi-hydrate comes, which is sometimes given to this solid solution. 

Calcium. sulfate occurs naturally as its dihydrate, (natural gypsum) as anhydrous anhydrite and rarely as its hemihydrate in the form of the mineral bassanite. Only the deposits of natural gypsum and anhydrite are of economic interest. For applications in the construction industry only the hardenable modifications, calcium sulfate hemihydrate (a- and P-form) and anhydrite, which are manufactured by dehydrating the dihydrate, are important. The properties and formation conditions of the different calcium sulfate modifications are given in Table 5.3-11. 

RG. 1—Stepwise fixation trf citrates on the dehydrated calcium sulfate (Step I) and the nuclei or on the growing dihydrate crystals, respectively (Step 11). 

The main or sole constituent of plaster of Paris is calcium sulfate hemihydrate. If natural gypsum is used as the starting material the plaster may also contain some anhydrite, calcite, or clay minerals. The binder consists of individual particles whose size and shape corresponds to those of the starting dihydrate. They exhibit a distinct internal porosity and a relatively large specific surface area as a result of the dry dehydration process. 

Dihydrate calcium sulfate whiskers (CaS04-2H20 molecular weight, 172.18) are white fluffy solids in appearance and fibrous or acicular crystals long fibers under a microscope. Their diameters are 10-50 pm, lengths are >500 pm, and L/D ratios are 20-100. They have relatively poor hardness, heat resistance, and intensity. These whiskers will dehydrate at room temperature and transform to amorphous calcium sulfate particles at about 110°C. Thus, applications of dehydrate calcium sulfate whiskers in composites are limited. ... 

Decomposition of model substances method The third method of calibration is by carrying out an experimental run using certain well studied model substances such as copper sulfate pentahydrate, calcium carbonate, calcium oxalate mono hydrate, potassium carbonate, sodium hydroxide, zinc oxalate dihydrate, and benzoic acid. These model substances show well resolved dehydration and decomposition temperatures over a wide temperature range. 

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