Hydrated plaster

Produced by grinding from gypsum, or hydrated plaster of Paris, calcium sulphate is inert. It is whiter in colour and of a lower density than barium sulphate. 

While hydrating plaster of Paris, students feel heat—the energy being released by the hydration of the plaster. 

ABSTRACT In the frame of calcium sulfate p-hcmihydratc hydration kinetical studies, a simple experimental set up has been developed to determine the rate of hydration for different species of incompletely hydrated plaster specimens. This apparatus may also be used to determine widi accuracy the amount of gypsum contained in plaster rocks, and acquire simultaneously quantitative analysis for six specimens. The apparatus is described, and the effect of vapor pressure is studied. 

Experimental results on incompletely hydrated plaster specimens show that either the weight loss or the integration curves allow the determination of the hydration ratio of the specimen with high accuracy. Il has been experimentally shown that the amount of heat involved in the hydration reaction is proportional all along the reaction to the conversion rate. 

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... 

As gypsum dehydration is realized in quite reproducible conditions, the reactor may be used to determine with accuracy, the amount of gypsum contained in a plaster rock or an incompletely hydrated plaster. 

With a very simple apparatus and experimental procedure it is possible to elaborate a stable, reproducible standard specimen for kinetic studies. With this apparatus, the hydration ratio of incompletely hydrated plasters or plaster rocks can be determined with accuracy. Finally this apparatus demonstrates experi-... 

Karmazsin s paper describes a simple apparatus for measuring the degree of hydration of plaster and gypsum rock. The reactor consists of a vertical stainless steel cylinder capable of holding six small stainless steel sample tubes that are suspended from the removable roof of the cylinder by a stainless steel thermocouple well. The cylinder is fitted with a heater controlled by an external thermocouple, and a reservoir containing water vapor at 81% relative humidity is connected to the reactor. The reactor may be used to determine accurately the amount of dihydrate in either gypsum rock or incompletely hydrated plaster. 

Fig. 12 Schematic representation of the build up of a cast layer (wall) from a suspension of clay particles (slip) in contact with a gypsum(hydrated plaster of Paris) mold (reproduced by permission of Addison-Wesley from F.H. Norton, Elements of Ceramics, Addison-Wesley Publishing, Reading, MA, 1952) [22]...

Finishing lime is a refined hydrated lime, milled to make it suitable for plastering, particularly the finish coat. Putty derived from this hydrate possesses unusually high plasticity. 

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. 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

Hydraulic cements. These cements are formed from two constituents one of which is water. Setting comprises a hydration and precipitation process. Into this category fall Portland cement and plaster of Paris. 

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 ... 

Vermiculite and as a filler in soap, putty and plaster. An expanded mica (hydrated magnesium-aluminium-iron silicate). Used in lightweight aggregates, insulation, fertilizer and soil conditioners, as a filler in rubber and paints, and as a catalyst carrier. 

The inclusion of solvent molecules as part of the crystal lattice is another common phenomena in both organic and inorganic systems. Calcium phosphate used in modem building plaster, sets when it reacts with water and crystallizes as a stable deca-hydrate. In pharmaceutical systems it is common... 

Calcium carbonate occurs in nature as hmestone in various forms, such as marble, chalk, and coral. It is probably the most widely-used raw material in the chemical industry. It has numerous apphcations, primarily to produce cement, mortars, plasters, refractories, and glass as budding materials. It also is used to produce quicklime, hydrated lime and a number of calcium compounds. It is produced either as powdered or precipitated calcium carbonate. The latter consists of finer particles of greater purity and more uniform size. They also have many important commercial apphcations. Various grades of precipitated calcium carbonate are used in several products, such as textiles, papers, paints, plastics, adhesives, sealants, and cosmetics. 

The normal selenates of potassium, sodium, calcium and magnesium are very similar in properties to the corresponding sulphates, except that the potassium salt is much more soluble in water than potassium sulphate.3 Sodium selenate gives a decahydrate which effloresces, and the transition-point between this and the anhydrous salt is 31-8° C., above which point the solubility falls as the temperature rises. Calcium selenate gives a hemihydrate resembling plaster of Paris, and also a hydrate of composition CaSe04.l-5H2O.4... 

Plaster of Paris is supplied as a white powder which can be mixed with water for producing impressions of footprints, tire tracks, and the like. Plaster of Paris is a hydrated calcium sulfate, CaSC>4, containing 6.20% H2O. What is the formula for Plaster of Paris ... 

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