Gypsum, Classification of Gypsum, POP, GWP

Gypsum is a non-hydraulic binder occurring naturally as a soft crystalline rock or sand. Pure gypsum is a white translucent crystalline mineral and is so soft that it can be scratched by a finger nail. When heated to 205°C, pure gypsum loses its luster and its specific gravity is increased from 2.3 to 2.95 due to the loss of water of crystallization. Gypsum has a unique property of moulding. When heated it gives up combined water and easily turns into powder.On adding water to the powder it can easily be shaped and moulded, and in a short time it hardens again and becomes similar to what it was in its natural state. When water is added the gypsum forms interlocking crystals. As the gypsum hardens it is this crystallisation that makes it such an effective fire resisting material.

There are two commercial varieties of crude gypsum, rock gypsum and gypsum or gypsite used for the manufacture of gypsum binding material. These substances consist principally of a hydrous sulphate of lime (CaSO4 + 2H2O) with varying percentages of silica, carbonate of lime, carbonate of magnesia, and iron oxide. Building gypsum is an air-setting binder composed mainly of semihydrate gypsum and obtained by processing gypsum at temperatures 150°C -160°C.

Gypsum

Gypsum items have a number of valuable properties like relatively small bulk density,incombustibility, good sound absorbing capacity, good fire resistance, rapid drying and hardening with negligible shrinkage, superior surface finish, resistance to insects and rodents and low energy input during burning to produce gypsum plaster. The major shortcomings are its poor strength in wet state and high creep under load. Gypsum plaster, e.g., Plaster of Paris,wall plaster stucco, and hard finish plaster are extensively used in wall construction. Flooring plaster, made by calcining gypsum at a high temperature has been considerably used. In all of these powders, gypsum in a more or less dehydrated state is the essential element. Gypsum based items should be used only in dry state and in premises of not more than 60 percent relative air humidity.

Classification of Gypsum

Gypsum binders are classified as low and high burning varieties. The low burning variety is manufactured by heating dehydrated gypsum to a temperature of about 160°C. The examples of low burning variety are building and extra strong gypsums. The high burning (anhydrite) variety is obtained by burning dehydrated gypsum at 700°C–1000°C, when the chemically bound water is lost totally. Gypsum may also be classified as low strength gypsum—obtained by heating natural gypsum rock at normal pressure, the resultant gypsum ( modification) is very hygroscopic (60–65%) and porous (40%), and extra strong gypsum—obtained by heating gypsum at pressure of 2–3 atm followed by drying at 160°C–180°C (modification). The extra strong gypsum is used in metallurgical industries for manufacture of moulds.

Low Burning Variety: A 75 per cent dehydrated gypsum is referred to as Plaster of Paris. The pulverized Plaster of Paris is the basic material used to make many of the gypsum building materials. For refined grade of Plaster of Paris the oven, kettle and rotary processes are used. Hard finish plaster is made in kilns similar to that used in calcining lime The excavated raw materials are crushed, and if the kettle process is used, ground until about 60 per cent pass No. 100 sieve. In the rotary process the final pulverization is omitted until calcination is completed.The kettles employed for calcinations are 2.5 or 3 m in diameter and about 2 m high. The pulverized material is chuted into the kettle and temperature raised gradually so as to drive off the mechanically held water. At about 100°C the whole mass bubbles up violently and then sinks. At 150°C the combined water begins to boil out and between 170° and 200°C the process is stopped. The kettle process requires about 2 to 3 hours to calcine a charge yielding 5 to 6 tonnes. The calcined product is then cooled partially in a vat and is sent to the screens. Residues from the screen are ground; the fines are stored in bins.In the rotary process the raw material is crushed to pass through 25 mm mesh and is then fed into a rotating cylinder inclined to the horizontal. Calcination is accomplished with the introduction of hot furnace gases. The roasted material is conveyed to calcining vats in which further changes are bought by the heat within the material. The product is then ground screened and stored.

In case of Plaster of Paris or stucco the time of setting is delayed by adding fraction of one per cent of retardant like glue, saw dust or blood after the plaster has cooled to increase the handling time. Cattle hair or wood fibre is introduced for cohesiveness of plastics. Wall plasters made from pure raw materials are adulterated with 15–20% of hydrated lime, the addition is not required for the raw materials containing considerable amount of clay. If instead of using moderate heating the gypsum is heated sufficiently to drive off all the water, the product no longer combines readily with water to form a useful plastering material. If small quantity of accelerating salts is added to it, a useful range of materials is again formed.These are known as anhydrous gypsum plasters or hard burnt plasters.

High Burning Variety: Anyhydrite cement is obtained by burning natural dihydrate gypsum at a temperature of about 700°C and then grinding the product together with hardening catalyzers (lime, mixture of sodium sulphate with green or blue vitriol, burned dolomite, granulated basic blast-furnace slag, etc).A typical anhydrite binder may be of the following composition: lime, 2–5%; a mixture of sodium bisulphate or sulphate with green or blue vitriol in amounts of 0.5 to 1% each; dolomite burned at 800–900°C, 3–8%; granulated basic blast-furnace slag, 10–15%. Green and blue vitriols consolidate the surface of hardened anhydrite cement, so that the catalyzers do not seep out and discolour the item’s surface. The action of the catalyzers is due to the ability of anhydrite to form complex compounds with various salts in the form of an unstable multiple hydrate, which then decomposes yielding CaSO4_2H2O. Anhydrite cement can also be obtained by grinding natural anhydrite with the above additives.Anhydrite cement is a slowly setting binder; its setting starts not earlier than in 30 min and ends not later than in 24 hours. It is used for preparing brick-laying and plastering mortars,concretes, heat insulating materials, artificial marble and other ornamental items A variety of anhydrite cements is the high-burned gypsum (estrich gypsum). It is manufactured by burning natural gypsum or anhydrite at a temperature between 800 to 1000°C followed by fine grinding. This results not only in complete dehydration but also in partial decomposition of anhydrite with the formation of CaO(3–5%) according to the reaction CaSO4 = CaO + SO3. When estrich-gypsum is mixed with water, CaO acts as a catalyzer which promotes the hardening of the anhydrite cement in a manner discussed above.High-burned gypsum is used to prepare brick-laying and plastering mortars, to build mosaic floors, to manufacture artificial marble, etc. Items from high-burned gypsum have low heat and sound conductivity, higher frost and water resistance and a smaller tendency to plastic deformation than products from building gypsum.

POP (Plaster Of Parish)   

It is produced by incompletely dehydrating pure finely ground gypsum at a temperature some what lower than 185°C. Most plasters theoretically approach CASO4 + 1/2 H2O which contains about 6.2 percent of water.The setting of plaster of paris is attributed to the formation of gypsum crystals from a supersaturated aqueous solution. When substances of colloidal nature (for example glue) are mixed with the plaster the formation of crystals is hindered and the time of set retarded. In hardening, Plaster of Paris first shrinks then expands. The latter property makes the material suitable for making casts, since a sharp impression of the mould can be secured. For the same reason it forms an excellent material for filling cracks, holes in the plastered surfaces and also on the wooden surfaces before painting/polishing.Owing to the rapidity of set and difficulty in working, its use in structures is limited to ornamental works. Being unstable in water it should be used for indoor works only.

plaster of perish

Properties

1. White in colour

2. Setting time is 5 to 10 minutes

3. Specific gravity is 2.57.

GWP (Gypsum wall plaster)

Gypsum wall plasters gain one-half of their one-month strength in a day. Plaster and sand mortars of 1:1 proportions may be expected to develop 80 per cent of the neat strength at corresponding ages, while those of 1:2 proportion generally possess one-half to two-third of the neat strength.The gypsum to sand neat plaster in proportion of 1:3 should set in 2 to 32 hours and in 1.5 to 8 hours when mixed with wood fibres. The dry set density of gypsum wall plaster is 850–1040 kg/m3, and compressive strength of 1:2 gypsum wall plaster is 6 to 15 N/mm2.Gypsum wall plasters are divided into following four categories.

* Gypsum Neat Plaster is 60.5 per cent or more of calcined gypsum (plaster of paris) with material added to control workability, time of set and cohesiveness.

* Gypsum wood fiber plaster is 60.5 per cent or more of calcined gypsum and, wood fibre 1.0 per cent or more to increase cohesiveness, and the remaining material to control work ability and time of set.

* Calcined Gypsum is used for finishing coat. It may or may not carry a retardent. Calcined gypsum may be white or grey.

* Gypsum ready sanded plaster consists of cementing material, predominantly calcined gypsum, which has been mixed at the mill with the proper proportions of sand and other desirable constituents. It is prepared for use simply by adding water. There are two grades of Gypsum Ready Sanded Plaster, the scratch or first coat, and the browning or the second coat.The scratch coat contains 2 sand to 1 cementing material by weight. The browning coat contains 3 sand to 1 cementing material by weight. The cementing material carries at least 60.5 per cent by weight of calcined gypsum and other ingredients to control set and work ability