Gypsum hardening

Hydrated lime is also used to stabilize the calcium sulfite—sulfate sludge derived from thickeners at SO2 scmbbing plants that use limestone—lime. Hydrated lime (2—3%) is added to react with the gypsum sludge and flyash or other added siHceous material. Under ambient conditions the lime and siHca serve as a binder by reacting as calcium siHcates so that the material hardens into a safe, nonleaching, stable, sanitary landfill or embankment fill. 

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. 

The hardness test sometimes is performed on the mud as well as the mud filtrate. The mud hardness indicates the amount of calcium suspended in the mud as well as the calcium in solution. This test usually is made on gypsum-treated muds to indicate the amount of excess CaSO present in suspension. To perform the hardness test on mud, a small sample of mud is first diluted to 50 times its original volume with distilled water so that any undissolved calcium or magnesium compounds can go into solution. The mixture then is filtered through hardened filter paper to obtain a clear filtrate. The total hardness of this filtrate then is obtained using the same procedure used for the filtrate from the low-temperature low-pressure API filter press apparatus. 

Silver white, relatively soft metal that is only applied in alloys. Oxygen and water attack pure Ca. The most prominent compound is the oxide (CaO) = burnt calcium, which hardens to calcium carbonate in mortar. Annual production of about 120 million tons. Burnt gypsum (CaS04 0.5 H20) hardens with water. A great step in evolution was the replacement of hard shells of brittle calcium carbonate by an internal skeleton of tough calcium phosphate (hydroxylapatite)-protein composite. Calcium is essential for all life forms. The daily requirement is 0.7-1.0 g. Humans (70 kg) contain 1 kg of calcium. Calcium silicate is the main component of cement. Marble is calcium carbonate in polycrystalline form and the favorite material of sculptors. 

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 ... 

When Plaster of Paris, CaSC>4 5H2O (s), is mixed with water, solid gypsum, CaS04- 2 H2O (s), is formed in an exothermic reaction, that is, a heat-releasing reaction. Thus, as the cast hardens, the plaster cast becomes warm ... 

The primary industrial use of calcium metal is as an alloying agent to harden aluminum. Calcium compounds such as lime and gypsum are used for many purposes throughout the chemical and construction industries. Portland cement, for example, contains approximately 70% CaO. In addition, calcium is the primary constituent of teeth and bones. 

There has been more work on monocyclic systems and a number of patents have been taken out on the amino-diacid 4 (75GEP2343147,75GEP2343195, 75GEP2417534 76GEP2456667). Uses proposed for the compound include detergent builder, hardening retardant for gypsum, reduction of calcification... 

The situation of the Cl ion in the hardened paste is important in relation to the possibility of promoting corrosion. A study on the hydration of CjA and C3S in the presence of gypsum and CaCl2 showed that C3A CaCl2-10H2O was formed after all the had been used up (T53). No phase containing Cl was detected in the mixes with C3S except at a... 

To improve the resistance of the catalyst towards poisoning and deactivation the tresh catalyst was immersed into a slurry of CaS04 x V2 H2O, 30 weight-% in H2O. The nets were blown clean with compressed air, and then left to harden. This procedure was repeated until 130 g/m of gypsum were deposited on the nets. 

Expanding cement (shrinkage-compensated cement) do not shrink upon hardening. Addition of 10 to 15% of calcium sulfate or aluminate even effects an expansion ( managed gypsum bursting ). 

Calcium. sulfate occurs naturally as its dihydrate, (natural gypsum) as anhydrous anhydrite and rarely as its hemihydrate in the form of the mineral bassanite. Only the deposits of natural gypsum and anhydrite are of economic interest. For applications in the construction industry only the hardenable modifications, calcium sulfate hemihydrate (a- and P-form) and anhydrite, which are manufactured by dehydrating the dihydrate, are important. The properties and formation conditions of the different calcium sulfate modifications are given in Table 5.3-11. 

CaSO, 2H,0 gypsum mono- < 40 only stable <40 <40 hardened gypsum , by ... 

GYP5UM is a common mineral. Calcined at around 150 C most of its chemically bound water is driven off to produce FLA5TEK. Mixing dry plaster powder with water reforms gypsum, firstly as a paste then apanding and hardening into a solid. A good preparation should be evenly mixed. 

Heating limestone, CaC03, produces quicklime, CaO, which can then be treated with water to form slaked lime, Ca(OH)2, an inexpensive base for which industry finds many uses. When slaked lime is mixed with sand and exposed to the CO2 of the air, it hardens to form mortar. Heating gypsum, CaSO - 2H2O, produces plaster of Paris, 2CaS04... 

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