Calcium Sulfate. Laboratory Synthesis. Industrial Dehydration of Gypsum 4. Energy Aspects.

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Calcium Sulfate. Laboratory Synthesis. Industrial Dehydration of Gypsum 4. Energy Aspects. Structure, Mixed Compounds, Solubility. Occurrence, Raw Materials. Gypsum and Anhydrite Rock. Other Byproduct Gypsums. Natural Gypsum to Calcined Prod- ucts. Phosphogypsum to Calcined Prod- ucts. Anhydrite Plaster. Hydration, Setting, Hardening.

Prefabricated Gypsum Building Components. Gypsum Plaster. Other Uses. Material Testing and Chemical Analysis. Chemical Analysis. Phase analysis. Economic Aspects. Recycling and Disposal. Only in volcanic regions are gypsum and anhydrite rock completely absent. Gypsum is useful as an industrial material because 1 it readily loses its water of hydra- tion when heated, producing partially or totally dehydrated calcined gypsum, and 2 when wa- ter is added to this calcined gypsum, it reverts to the original dihydrate — the set and hardened gypsum product.

These two phenomena, dehy- dration and rehydration, are the basis of gypsum technology:. KGaA, Weinheim. Gypsum and anhydrite are nontoxic. Both gypsum plaster and lime were used as mortar in antiquity. Gypsum was called gatch in Persian, gypsos in Greek, and gypsum in Latin.

The Iranians, Egyptians, Babylonians, Greeks, and Romans were familiar with the art of work- ing with gypsum plaster, examples being the. In Germany gypsum plaster was used as mortar in walls and buildings dur- ing the early Middle ages, e. It gained popularity tremendously, reaching its peak dur- ing the Baroque and Rococo periods. Examples are the Wessobrunn school for stucco workers [17] and the stucco decorations in Charlotten- burg palace, Berlin.

The expansion of the cement industry in the second half of the nineteenth cen- tury also considerably increased the use of gyp- sum. Over the centuries the gypsum industry has developed empirically out of the old craft of gyp- sum plastering.

The distinctions between gyp- sum plaster and lime, however, remained ob- scure up to the eighteenth century. Research into the principles of gypsum technology was begun in by L avoisier , and has continued to this day.

Four exist at room temper- ature: calcium sulfate dihydrate, calcium sul- fate hemihydrate, anhydrite III, and anhydrite II. They differ from each other in their applica- tion characteristics, their heats of hydration, and their methods of preparation see Table 8 [21]. Figure 1. Figure 2.

Lehmann et al. Anhydrite II is the naturally occurring form and also that. Table 1. Calcium sulfate. Anhydrite III. Anhydrite II. Anhydrite I. CaSO 4. AII-s, slowly soluble. Other names, often based on the application. FGD gypsum. Table 2. Refractive indices. Optical character. Axial angle 2 V Lattice symmetry Space group. I C Lattice spacing, nm. The most important physical properties of the calcium sulfate phases are shown in Table 2. The thermodynamic stability ranges for the cal- cium sulfate phases are shown in Table 1.

The other phases are obtained at higher temper-. Under normal atmospheric conditions hemi- hydrate and anhydrite III are metastable, and be-. Gypsum dehydration kinetics have been in- vestigated by several authors.

Neutron and X- ray powder diffraction studies have shown that the dehydration and hydration mechanism is strongly topotactic in the temperature range of. With high local steam pressure, a subhydrate with 0. H 2 O was found [25]. According to another pa-. Table 2 also lists the well-established crystallographic data of gypsum [27] and of anhydrite II [28], [29].

Anhydrite II is formed at temperatures bet-. In this case anhydrite II with or- thorhombic crystal structure is produced by neo- formation [30]. Industrial Dehydration of Gypsum. Industrially it is most important that dehydra- tion is achieved in the shortest time with the lowest energy consumption, i. Because of kinetic inhi- bitions calcination is carried out at much higher. Rarely are pure phases produced dur- ing manufacture; rather, mixtures of phases of the CaSO 4 —H 2 O system are produced.

Three types of calcined anhydrite II anhydrous gyp- sum plaster or overburnt plaster are manufac- tured, depending on burn temperature and time:. In use the difference among these products lies in the rates of rehydration with water, which for anhydrite II-s is fast, for anhydrite II-u slow, and for anhydrite II-E in between, a little faster than anhydrite II-u see Fig. Transitions bet- ween these different stages of reaction are pos- sible. The presence of impuri- ties lowers the normal dissociation temperature of anhydrite II, ca.

Kelly et al. Structure, Mixed Compounds,. Crystal Structure. All structures in the. CaSO 4 —H 2 O system consist of chains of alter-. These CaSO 4 chains. Calcium sulfate dihydrate has a layered structure, and the water of crystallization is embedded between the layers. When calcium sulfate dihydrate is dehydrated to hemihydrate, a tunnel structure. The relative ease of escape of these water molecules explains the facile conversion from subhydrate to anhydrite III.

Anhydrite II exhibits closest packing of ions, which makes it the densest and strongest of the calcium sulfates. However, lacking empty channels, it reacts only very slowly with water.

Isomorphic incorporation of chemical compounds into the lattice of. Isomorphic incorporation of calcium hydrogen- phosphate dihydrate occurs because CaHPO 4. Monosodium phosphate, NaH 2 PO 4 , can also be incorporated into the gypsum lattice [35]. Kitchen et al. Eipeltauer [20], [37]. Chlorides are not incorporated. Double and Triple Salts. There are triple sulfates of calcium with the divalent ions of the iron and zinc subgroups and of manganese, copper, and magnesium along with the univalent alkali metals, also includ- ing ammonium.

Table 3. Heat of hydration. Phase change.


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Anrufer, die nicht mit Telefonnummer in der Knauf Adressdatenbank angelegt sind, z. The process completely expels the water of crystallisation from the FGD gypsum and the surface of the anhydrite CaSO4 that is formed is reduced by sintering to ensure a more favourable mixing water requirement. As a moist, particulate FGD gypsum, the thermal anhydrite is directed straight into the economic cycle of the building material industry, and its uniform grain size makes it the exemplary manufacturing solution for many products. Gypsum and anhydrite rock formations originate from the Permian geological epoch and were formed to million years ago. The solubility of carbonate, sulphate and chloride degraded with time due to the crystallization of shallow seas. Gypsum and anhydrite only differ by half a water molecule before being processed, and by the addition of water, the water-free anhydrite is converted to gypsum.


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