Hemihydrate addition

P-Hemihydrate. The dehydration of gypsum, commonly referred to as calcination in the gypsum industry, is used to prepare hemihydrate, or anhydrite. Hemihydrate is generally called stucco in North America and plaster in many other continents. In North America, plaster is differentiated from hemihydrate or stucco by the inclusion of additives to control intended use properties, eg, rehydration time, density, coverage, strength, and viscosity. 

Hemihydrate. The abiUty of plaster of Paris to readily revert to the dihydrate form and harden when mixed with water is the basis for its many uses. Of equal significance is the abiUty to control the time of rehydration in the range of two minutes to over eight hours through additions of retarders, accelerators, and/or stabilizers. Other favorable properties include its fire resistance, excellent thermal and hydrometric dimensional stabiUty, good compressive strength, and neutral pH. 

Special grades of gypsum hemihydrate are blended with Portland cement for types with reduced thickening time and setting time. Gypsum requires significantly more water addition. The maximal application temperatures are 70° to 80° C. Sodium silicate is used for cement slurries with diatomaceous earth. It can be used up to 7% by weight. 

Keough, B.K. "Influence of Additives and Particle-Size Classification on the Continuous Crystallization of Calcium Sulfite Hemihydrate," M.S. Thesis, University of Arizona, Tucson (1983). 

If soluble anhydrite is desired, firing is maintained until a second boil occurs accompanied by a second temperature plateau at about 190°C. Virtually all the water of crystallization has been removed at 215°C. Soluble salts are impurities that increase the vapor pressure within the kettle. Aridized stucco refers to ketde-calcined hemihydrate that has been made with the intentional addition of 0.55—1.1 kilograms of NaCl or CaCl per metric ton of land plaster. The stucco characteristic of lower water demand permits higher density and higher strength casts. The hygroscopic nature of the chlorides prevents the use of aridized stucco for some applications. 

Stoichiometric hydrates are the most important solvates affecting the solubility of marketed pharmaceuticals. Hemihydrates, monohydrates, and dihydrates are the most common stoichiometric ratios of water incorporated into the crystalline lattice of drugs. Pfeiffer et al. (1970) have shown how different hydrates of cephalosporins could be isolated from solvent systems of varying water activity. Cephalexin has a monohydrate and a dihydrate form, which are stable under different relative humidity conditions. Cefazolin has a monohydrate, a sesquihydrate (1.5 moles water), and a pentahydrate form (Byrn and Pfeiffer, 1992). Jozwiakowski et al. (1996) have found that lamivudine can form a 0.2 hydrate, where only one of L ve lamivudine molecules in the crystal lattice is associated with a water molecule. Multiple solvates can be formed for the same drug Stephenson et al. (1994) have shown that dithromycin can crystallize in at least nine solvate forms, including a cyclohexane trisolvate and an acetonitrile trihydrate. In addition, Byrn et al. (1995) have noted that desolvated forms of some drugs have unique properties that differ from their nonsolvated counterparts. 

Very similar results were found in open pans, although the dihemihydrate and the hemihydrate Form II yielded an additional exotherm at 133°C, which was attributed to recrystallization of the anhydrous form. 

Knauf process in different variants after washing and flotation, drying is carried out with partial dehydration. The resulting mixture of dihydrate and hemihydrate is recrystallized with the addition of lime hydrate and stoichiometric quantities of water, then pelletized and calcined... 

Results obtained with an industrial hemihydrate. hydrated with setting additives, are shown in Table 3. Conversion rate values, calculated fiom weight loss, are in a good agreement with those given by thermal flow integration (Fig. 9). 

In the present research work the addition of citric acid did not cause any change in the solubility of the hemihydrate. But with an equal water/gypsum ratio a decrease of the viscosity was observed with increased citric acid addition. 

FIG. 2—Grain structure parameter as a function tf citric acid admixture (addition in weight % of hemihydrate). 

Additional hardened gypsum was examined with regaid to its creep behavior. Figure 4 shows the deformation time diagram of hemihydrate as a function of the citric acid addition and of the equilibrium humidity. 

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