Calcium hemihydrate, 3.25

Properties and reactivity of a calcium sulfate 3-hemihydrate are widely dependant on the preparation procedure. Although kinetical studies on the hydration reaction are done by a number of researchers, there is no standard procedure to elaborate a stable, reproducible calcium -hemihydrate specimen. This paper presents a very simple apparatus and an experimental procedure setup to elaborate a specimen that is stable and allows kinetical studies even after a long time of conservation. This apparatus also allows the simultaneous determination of the hydration rate for six different species of incompletely hydrated plasters and demonstrates experimentally that the amount of heat involved in the hydration... 

The physical properties of the three forms of calcium sulfate differ somewhat from each other, but their chemical properties are essentially the same. Anhydrous calcium sulfate and calcium hemihydrate are fine white odorless powders or crystalline solids, while the dihydrate may occur either as a powder or as white lumps. Both hydrates are... 

The primary use of calcium hemihydrate is, of course, in the production of plaster of Paris. Plaster of Paris has a number of important applications, such as ... 

Type M expansive cement is a combination of ordinary Portland cement, calcium aluminate cement, and additional gypsum or calcium hemihydrate, typically in the ratio 66 20 14 (Mikhailov, 1960 Budnikov and Kravchenko, 1968). It may be produced by mixing these constituents in a plant. Alternatively, an expansive additive consisting of high-alumina cement, gypsum or henuhydrate, and hydrated lime may be added to the fresh concrete mix during mixing. By this approach the extent of expansion may be controlled by the amount of additive added to the mix. 

It is common that commercial calorimeters have internal, automatic calibration. Although this makes a calorimeter user friendly, it is problematic if the user does not know whether the calibrations are accurate. One way to check whether the instrument is working properly and whether the user is performing the measurement in a correct way is to run a validation procedure, i.e. an experiment with a known outcome (proficiency test). A number of such chemical calibration systems are described by Wadso and Goldberg (2001) however, none is similar to cement hydration measurements. It is therefore of interest to establish reference cements - or other similar systems, for example, based on calcium hemihydrate - that can be used to validate the quality of calorimetric cement measurements in a laboratory. 

Two main categories of the wet process exist, depending on whether the calcium sulfate is precipitated as the dihydrate or the hemihydrate. Operation at 70—80°C and 30% P20 in the Hquid phase results in the precipitation of CaSO 2 filterable form 80—90°C and 40% P20 provide a filterable CaSO O.5H2O. Operation outside these conditions generally results in poor filtration rates. A typical analysis of wet-process acid is given in Table 4. For more detailed discussion of the wet-process acid, see Fertilizers. 

Gypsum is widely distributed naturally as calcium sulfate dihydrate [101012-1-4], CaS04-2H2 0. When partially calcined, the hemihydrate, CaS04 H2 0, is formed (see Calciumcompounds-calciumsulfates). Gypsum has been used in one form since 1756 for making dental casts, and in another form since 1844 for dental impressions (101). 

Calcium sulfate (anhydrous). (Prepared by heating the dihydrate or the hemihydrate in an oven at 235° for 2-3h it can be regenerated.) Available commercially as Drierite. It forms the hemihydrate, 2CaS04.H20, so that its capacity is fairly low (6.6% of its weight of water), and hence is best used on partially dried substances. It is very efficient (being comparable with phosphorus pentoxide and concentrated sulfuric acid). Suitable for most organic compounds. Solvents boiling below 100° can be dried by direct distillation from calcium sulfate. 

Calcium sulfate hemihydrate []0034-76-1 j M 145.2. Sol in H2O (0.2 parts/100 at 18.75°). Completely dehydrated >650°. Dry below 300° to give a solid with estimated pore size ca 38% of vol. Anhydrous CaS04 has high affinity for H2O and will absorb 6.6% of its weight of H2O to form the hemihydrate (gypsum). It sets to a hard mass with H2O, hence should be kept in a tightly sealed container. 

The dehydration and interconversion reactions of the various forms of calcium sulphate [dihydrate, hemihydrate (a and 3 pseudo morphs) and anhydrous salt (hexagonal and orthorhombic structures)] have been studied by Ball et al. [281,590,591] who compared their observations with the available rate and microscopic data. The important features of the complicated behaviour found are summarized in the scheme [281]... 

Likewise, the calcination of gypsum drives away water. Calcinating gypsum (composed of hydrated calcium sulfate) causes that part of the water combined with the calcium sulfate to evaporate, leaving a solid, friable residue usually known as plaster of Paris (composed of calcium sulfate hemihydrate) plaster of Paris is used as a cement and mortar as well as an efficient casting material (see Textbox 35). 

Plaster of Paris has long been used as a casting material, a cement, and a mortar. If mixed with water, plaster of Paris forms a very soft and pliable mixture. After a very short time, lasting only 5-8 minutes, the wet, pliable mixture sets, that is, it hardens into a stable, firm solid. The setting process entails the incorporation of water molecules (a process known as hydration) into the calcium sulfate hemihydrate and the consequent formation and crystallization of hydrated sulfate of calcium. In other words, when water is added to plaster of Paris, the two combine, again forming gypsum, which soon crystallizes into a hard solid mass ... 

HDH (1) [Hemi dihydrate] A Wet Process for making phosphoric acid. The calcium sulfate is first produced as the hemihydrate and then in another stage this is converted to the dihydrate. Developed by Fisons, UK, and operated in Yugoslavia and the United Kingdom. 

The calcium sulfate by-product separates as either the dihydrate or the hemihydrate, depending on the conditions. The process originates from the work of J. B. Lawes in 1842 who patented a method of making a fertilizer by treating bones with sulfuric acid. Many variations are practiced today. See also Dorr and Haifa. 

Calcium sulfate a-hemihydrate, 4 582 manufacture, 4 590-591 Calcium sulfate anhydrous (anhydrite), 4 582, 583t manufacture of, 4 591 physical properties of, 4 585t uses of, 4 599... 

Calcium sulfate P-hemihydrate, 4 583 manufacture of, 4 588-590 Calcium sulfate dihydrate (gypsum), 4 582 5 479t... 

Calcium aluminate monosulfate hydrate CaNH4P04-8H20 Calcium sulfate hemihydrate CaS04 CaS04-2H20... 

Benson et al. (2) showed that high Mg ion concentration also inhibits nucleation during the precipitation of calcium sulfite hemihydrate from simulated FGD liquors. 

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). 

A computer model has been generated which predicts the behaviour of a continuous well mixed gypsum crystallizer fed with a slurry of hemihydrate crystals. In the crystallizer, the hemihydrate dissolves as the gypsum grows. The solution operating calcium concentration must lie in the solubility gap. Growth and dissolution rates are therefore limited. 

Calcium sulfate crystals were precipitated in a Continuous Mixed Suspension Mixed Product Removal (CMSMPR) crystallizer by mixing of calcium phosphate and sulfuric acid feed streams. The formed calcium sulfate hydrate (anhydrite, hemihydrate and dihydrate) mainly depends on the temperature and the solution composition. The uptake of cadmium and phosphate ions in these hydrates has been studied as a function of residence time and solution composition. In anhydrite, also the incorporation of other metal ions has been investigated. The uptake was found to be a function of both thermodynamics and kinetics. 

It combines with sulfur dioxide to form calcium sulfite hemihydrate, CaSOs MiH20 which can oxidize in air in the presence of moisture to give calcium sulfate dihydrate, CaS04 2H2O. However, when SO2 is passed through a solution of calcium hydroxide, calcium bisulfite, Ca(HS03)2 is obtained. The solution is yellowish when it contains bisulfite in aqueous SO2. 

Synonyms anhydrous calcium sulfate-anhydrite muriacite karstenite anhydrous gypsum anhydrous sulfate of lime hemihydrate-plaster of Paris annalin dried gypsum dried calcium sulfate dihydrate-gypsum alabaster satin spar mineral white terra alha satinite light spar selenite precipitated calcium sulfate native calcium sulfate... 

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