Chemical gypsum

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Chemical characteristics Pure gypsum is saturated with water and has 32.6% CaO, 46.5% SO3 and 20.9% H2O. Chemically, gypsum is acidic because of the SO3 radical. It reacts with HCl acid, and it dissolves in excess water (400-500 times). It has low oil absorption (less than 5%). 

FGD DeNO Newsletter, 1993B, Gypsum Has Its Problems (based on a paper by W. Ellison presented at the Third International Conference on FGD and Chemical Gypsum in Toronto in September), Mcllvaine Company, Northbrook, IL, November, pp. 3-4. 

Saleem, A., 1991 A, GE s Worldwide Experience with IFO Based Gypsum Producing Flue Gas Desulfurization Systems, presented at the Second International Conference on FGD and Chemical Gypsum (sponsored by ORTECH), Toronto, Canada, May 12-IS. 

Tested super sulfated cements comprise mainly recycled industrial by-products blast furnace slag (Table 1) and heat-activated chemical gypsum, Kerysten K Co (the estimated CO2 balance (excluding transport) is 50 kg/t of manufactured SSC). Tests were conducted on the basis of standard mortar (binder/sand mass ratio = 1/3) with two W/B ratios (0.4 and 0.5), which correspond to characteristics of the old and new standards for SSC. 

An additional study permits to evaluate the effect of the nature of the calcium sulfate addition on SSC setting and hardening phenomena. The SSC formulation displaying the best mechanical performance is selected. Mechanical performances of SCC obtained with heat-activated and nonheat-activated chemical gypsum are compared. The weight and dimensions of the test specimens formulated in this manner were then monitored over a standardized storage period of 28 days (immersion at 20°C). Mechanical performances were assessed at 2,7 and 28 days. 

The formulation of the selected SSC (C cement content) was used as a reference for comparison with an identical SSC but formulated with non-activated chemical gypsum (NAG). The performance levels achieved (Table 3) were lower than those observed in the case of heat-activated gypsum, whatever the W/B ratios used. 

Bijen J., Niel E., Supersulphated cement from blastfurnace slag and chemical gypsum available in the Netherlands and neighbouring countries, Cem. Cone. Res. 11 (3) (1981) 307-322. 

It is well known that chemically, gypsum is calcium sulfate dihydrate "CaS0. 2H20" which on heating to a temperature of about 130°C loses three fourths of its water and becomes the hemihydrate CaSO,. HpO (Plaster of Paris). 

Cements are commonly made by heating a mixture of limestone and clay to about 1700 C. The product is ground with gypsum. Chemically cements consist of a mixture of calcium silicates and aluminates with some sulphate present. World production 1976 730 megatonnes. 

Other fibrous and porous materials used for sound-absorbing treatments include wood, cellulose, and metal fibers foamed gypsum or Pordand cement combined with other materials and sintered metals. Wood fibers can be combined with binders and dame-retardent chemicals. Metal fibers and sintered metals can be manufactured with finely controlled physical properties. They usually are made for appHcations involving severe chemical or physical environments, although some sintered metal materials have found their way into architectural appHcations. Prior to concerns regarding its carcinogenic properties, asbestos fiber had been used extensively in spray-on acoustical treatments. 

Soil conditioners are materials that measurably improve the physical characteristics of the soil as a plant growth medium. Typical uses include erosion control, prevention of surface sealing, and improvement of water infiltration and drainage. Many natural materials such as peat and gypsum are used alone or in combination with synthetics for soil conditioning. This article is concerned with synthetic soil conditioners, many of which are introduced as polymeric systems similar to the gels and foams formed in situ by chemical grouts. 

This article discusses traditional hull ding and construction products, ie, not made from synthetic polymers (see Building materials, plastic), including wood, asphalt, gypsum, glass products, Pordand cement, and bricks. The article presents information about each basic material, the products made from it, the basic processes by which the products or materials are produced, estimates of the quantity or doUar value of the quantities produced or used in the United States, and some pertinent chemical or physical properties related to the material. More detailed chemical and physical property data can be found in articles devoted to the individual materials (see Asphalt Cement Glass Wood). 

Gypsum is also obtained as a by-product of various chemical processes. The main sources are from processes involving scmbbing gases evolved in... 

Fluorogypsum is the name ascribed to by-product gypsum from fluorspar acidulation to produce hydrofluoric acid. The chemical reaction... 

C471-91 Chemical Analysis of Gypsum and Gypsum Products ... 

Each 100 g of calcined gypsum theoretically requires only 18.6 mL of water to complete the chemical reaction from the hermhydrate to the dihydrate. Any amount of water greater than 18.6 mL/100 g of powder is excess and reduces the strength of the hardened plaster. When a mixture of the hermhydrate and water hardens, linear expansion takes place. This expansion may amount to as much as 0.5% for plaster. Dental stones also expand on setting, but the amount is significantly less than that permitted in plaster, ie, 0.2% for type III, 0.1% for type IV, and 0.3% for type V. 

Typical applications in the chemical field (Beaver, op. cit.) include detarring of manufactured gas, removal of acid mist and impurities in contact sulfuric acid plants, recovery of phosphoric acid mists, removal of dusts in gases from roasters, sintering machines, calciners, cement and lime Idlns, blast furnaces, carbon-black furnaces, regenerators on fluid-catalyst units, chemical-recoveiy furnaces in soda and sulfate pulp mills, and gypsum kettles. Figure 17-74 shows a vertical-flow steel-plate-type precipitator similar to a type used for catalyst-dust collection in certain fluid-catalyst plants. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

National Association of the Chemical hidustry (ANIQ), 262 National Biochemicals Corporation, 239 National Capital Poison Center, 312-313, 318 National Chemical Corporation, 260 National Chemicals Inspectorate, 264 National Envhonmental Health Association (NEHA), 279 National Federation of Italian Chemical Manufacturers (FEDERCHIMICA), 261 National Fire Protection Association (NFPA), 279 National Gypsum Company, 240 National Health hiformation Center (NHIC), 286 National histitute for Environmental Studies (NIES), 283 National histitute for Occupational Safety and Health (NIOSH), 286... 

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