Gypsum hydration

Calcium. Calcium is the fifth most abundant element in the earth s cmst. There is no foreseeable lack of this resource as it is virtually unlimited. Primary sources of calcium are lime materials and gypsum, generally classified as soil amendments (see Calcium compounds). Among the more important calcium amendments are blast furnace slag, calcitic limestone, gypsum, hydrated lime, and precipitated lime. Fertilizers that carry calcium are calcium cyanamide, calcium nitrate, phosphate rock, and superphosphates. In addition, there are several organic carriers of calcium. Calcium is widely distributed in nature as calcium carbonate, chalk, marble, gypsum, fluorspar, phosphate rock, and other rocks and minerals. 

Gypsum. Hydrated calcium sulfate (CaSO4-2H20). It occurs naturally and - is used as a source of sulfur, as a drying agent, an organic liquid precursor, and in polishes. 

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]...

Sodium chloride, gypsum, and potassinm salts are all important minerals that are recovered as evaporites remaining from the evaporation of seawater and from brines pumped from below the ground. Sodium chloride in the form of mineral halite is used as a raw material for the production of industrially important sodium, chlorine, and their compounds. It is used directly to melt ice on roads, in foods, and in other applications. Potassium salts are, of course, essential ingredients of fertilizers and have some industrial applications as well. Gypsum, hydrated calcium sulfate, is used to make plaster and wallboard and is an ingredient in the manufacture of portland cement. 

Alabaster is a form of gypsum (hydrated calcium sulfate q.v), occurring as a fine-grained, massive mineral which is used as an ornamental stone. It can become coloured, a property dependent on associated minerals - for example, hematite q.v.) will impart a red colour. Oriental alabaster is a term for a stalagmitic variety of calcite q.v) characterised by well-marked banding it should therefore not be confused with alabaster proper. 

DTA results of C3A + 25% gypsum mixtures hydrated with 0% and 1% TEA are shown in Fig. 11. The unhydrated mixture exhibits an endothermal doublet between 100 and 150°Cduetoastepwise dehydration of gypsum. Hydration seems to start from the first minute of contact of water with the sample. At 5 minutes, a larger amount of gypsum disappears from... 

Figure 5. DIG of the mixture C3A-Ca(OH)2-gypsum hydrated for different periods in the presence of SNF admixture.

Figure 8.25 Blend of calcium sulfoaluminate cement with gypsum hydrated for 14 days. (Courtesy of Julien Bizzozero.)...

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. 

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. 

Fig. 5. The early hydration reactions of tricalcium aluminate in the presence of gypsum and calcium hydroxide. Initial molar proportions l-C A ...

The composition of the Hquid phase during the early hydration of Portiand cements is controlled mainly by the solution of calcium, sulfate, sodium, and potassium ions. Very Httie alumina, siHca, or iron are present in solution. Calcium hydroxide, as calcium oxide, and gypsum, as calcium sulfate, alone have solubihties of about 1.1 and 2.1 g/L at 25°C, respectively. In the presence of alkaHes released in the first 7 min, the composition tends to be governed by the equiHbrium ... 

The CaS04 or its hydrate (gypsum) acts only as an unwanted diluent. Its presence can he avoided by using H1PO4 instead of H1SO4 for the acidulation, thus giving rise to triple superphosphate ... 

Inhibited Muds—Dispersed Systems. These are water-base drilling muds that repress the hydration and dispersion of clays. There are essentially four types of inhibited muds lime muds (high pH), gypsum muds (low pH), seawater muds (unsaturated saltwater muds, low pH), and saturated saltwater muds (low pH). 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

Hydration is the incorporation of water mole-cule(s) into a mineral, which results in a structural as well as chemical change. This can drastically weaken the stability of a mineral, and make it very susceptible to other forms of chemical weathering. For example, hydration of anhydrite results in the formation of gypsum ... 

Water-based drilling muds that repress hydration of clays (lime muds, gypsum muds, seawater muds, saturated saltwater muds)... 

Hydrates quickly, responsible for strength of cement in early stage setting time can be controlled by addition of gypsum Responsible for strength in all stages Responsible for final strength Little effect on physical properties... 

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