Anhydrite solubility

Calcined Anhydrite. Soluble anhydrite, or second-settle stucco, has physical properties similar to those of gypsum plaster. It hydrates to the dihydrate rapidly in water. Its outstanding property is its extreme affinity for any moisture, which makes it a very efficient drying agent (see Desiccants). In ambient moisture-laden air, it readily hydrates to hemihydrate. Soluble anhydrite, under the trade name Drierite, is widely used as a desiccant in the laboratory and in iadustry. A small amount is also used as an insecticide carrier. Small amounts of soluble anhydrite are unintentionally produced in most commercial calciners during hemihydrate production. 

Blount and Dickson CG9) have also suggested possible reasons for some of the disparities in the data. They note that very fine grained or poorly crystalline forms of gypsum may have resulted in higher apparent solubilities in some experiments. Anhydrite solubility results may also vary due to poorly crystallized anhydrite or hemlhydrate contamination. Most reported saturation curves were the result of approaching saturation from undersaturated solutions. Few experimenters have presented results from both undersaturated and supersaturated solutions. 

Q22. Glew, D.N. D.A. Haroes, "Gypsum, disodium pentacalcium sulfate, and anhydrite solubilities in concentrated sodium choride solutions". Can. J. Chem., v48, pp3733-3738 (1970)... 

Anhydrite also has several common classifications. Anhydrite I designates the natural rock form. Anhydrite 11 identifies a relatively insoluble form of CaSO prepared by high temperature thermal decomposition of the dihydrate. It has an orthorhombic lattice. Anhydrite 111, a relatively soluble form made by lower temperature decomposition of dihydrate, is quite unstable converting to hemihydrate easily upon exposure to water or free moisture, and has the same crystal lattice as the hemihydrate phase. Soluble anhydrite is readily made from gypsum by dehydration at temperatures of 140—200°C. Insoluble anhydrite can be made by beating the dihydrate, hemihydrate, or soluble anhydrite for about 1 h at 900°C. Conversion can also be achieved at lower temperatures however, longer times are necessary. 

Anhydrite. In addition to ketde calcination (Fig. 1), soluble anhydrite is commercially manufactured in a variety of forms, from fine powders to granules 4.76 mm (4 mesh) in size, by low temperature dehydration of gypsum. 

The precipitation of anhydrite (anhydrous calcium sulfate, CaS04) may also occur. Under ambient temperatures, anhydrite is much more soluble than calcium carbonate, but because calcium sulfate, in common with other calcium salts such as calcium phosphate (also known as tricalcium phosphate [Ca3(P04)2]), has an inverse-temperature solubility, it deposits more rapidly on the hottest heat transfer surfaces. 

Sulfates in surface MU water sources usually are present at lower concentrations (typically 20-60 ppm) but this level may rise to several hundred ppm in subsurface waters. The maximum solubility of calcium sulfate is dependent on temperature but is in the range of 1,800 to 2,000 ppm in cold water. This rate is significantly less in hot BW where boiler deposits occur, the sulfate scale normally is present as anhydrite (CaS04). Sulfate scales are hard and very difficult to remove, so treatment programs employed must be carefully controlled to avoid risks of scaling. 

Gypsum is a relatively soluble mineral and can undergo dissolution vhereas anhydrite is less soluble. 

In the three simulations, the sulfate minerals form at mixing ratios related to their solubilities. Barite, the least soluble, forms early, when small amounts of seawater are added. The more soluble celestite forms only after the addition of somewhat larger quantities of seawater. Anhydrite, the most soluble of the minerals, forms from the Amethyst fluid at still higher ratios of seawater to formation fluid. 

In contrast to the fate of silicates, a catchment exhibiting a small area of so-called Biindner Schiefer, a sandy-marly schist containing soluble anhydrite or gypsum, will produce a remarkably high weathering rate for the entire basin. This effect arises in the alpine catchments of the Ticino, Rhine, and Rhone. The occurrence of Biindner Schiefer also causes sulfate concentrations in the range of 0.5-1 mmol in rivers. Natural and anthropogenic atmospheric sulfur... 

Uses. The insoluble anhydrite is used in cement formulations and as a paper filler the soluble anhydrite is used as a drying agent the hemihydrate is used for wall plaster and wall-board gypsum is used in manufacture of plaster of paris and portland cement. 

Anhydrous calcium sulfate is a crystalline substance orthorhombic the color may vary as white, gray, blue or brick-red occurs as insoluble anhydrite or porous soluble anhydrite density 2.96 g/cm hardness 3.5 Mohs insoluble anhydrite is practically insoluble in water (0.21% at 20°C) soluble anhydrite readily absorbs moisture and is soluble in water. 

EFFECT OF TYPE OF GYPSUM AND GYPSUM HEMIHYDRATE OR SOLUBLE ANHYDRITE RATIO (ABNORMAL SETTING AND LOSS OF WORKABILITY)... 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO3 and the... 

The uncontrolled variation in the gypsum-hemihydrate or soluble anhydrite ratio (G-H or A) that occurs during production, storage and transportation of cement can cause compatibility problems with certain superplasticizers, particularly a reduction in workability. The extent of the reduction on workability properties is dependent on the G-H ratio (ranging from 80 20% to 20 80%), level of C3A and alkalis present in the cement and the fineness of... 

Fig. 7.42 SO3 solubilities of gypsum and natural anhydrite in saturated lime water with and without CLS admixture (Dodson [127]).

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