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1. Introduction 

   
Gypsum is a non-hydraulic binder occurring naturally as a soft crystalline rock or sand. Puregypsum is a white translucent crystalline mineral and is so soft that it can be scratched by afinger nail. When heated to 205°C, pure gypsum loses its luster and its specific gravity isincreased from 2.3 to 2.95 due to the loss of water of crystallization. Gypsum has a uniqueproperty 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 ithardens again and becomes similar to what it was in its natural state. When water is added thegypsum forms interlocking crystals. As the gypsum hardens it is this crystallisation that makesit such an effective fire resisting material.

 

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

 

Gypsum items have a number of valuable properties like relatively small bulk density,incombustibility, good sound absorbing capacity, good fire resistance, rapid drying andhardening with negligible shrinkage, superior surface finish, resistance to insects and rodentsand low energy input during burning to produce gypsum plaster. The major shortcomings areits 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. Flooringplaster, made by calcining gypsum at a high temperature has been considerably used. In all ofthese 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 per centrelative air humidity

2. Effect of Heat and Moisture

 

The water of crystallization in the gypsum (CaSO42H2O) is not held firmly by the mineral.Therefore, when it is heated to about 160°C it loses a part of water of crystallization and isknown as half-hydrate gypsum.

 

At still higher temperatures (About 200°C), gypsum loses all its water of crystallization and turns out into an hydrate gypsum.

 

The lost water of crystallization can be regained under favourable damp or moist conditions.

 

 

3. Setting and Hardening

 

The setting and strengthening of gypsum are due to intergrowth of very fine and poorlysoluble crystals of dehydrated gypsum as they precipitate from a solution which remainsoversaturated as long as the hydration of gypsum proceeds. Following are the two theories ofsetting of gypsum.

 

According to the crystallization theory proposed by Le-chatelier when water is added togypsum, the latter dissolves forming a saturated solution of dehydrate gypsum. Since thesolubility of semihydrate gypsum is about 3.5 times more than of dehydrated gypsum, thesoloution that is saturated with respect to the semihydrate gypsum causes dehydrated gypsumto crystallize. In this process the concentration of semihydrate gypsum is reduced causingmore of it to dissolve until again the solution is oversaturated and consequently again yieldingcrystals of dehydrate gypsum. The process continues until all the semihydrate gypsum ishydrated and crystallized.

 

According to colloidal theory when water is added to gypsum, the semihydrate gypsumgoes into solution until the latter is saturated. In an oversaturated solution, the interaction ofwater with the solid semihydrate continues on their surface due to high mutual chemicalaffinity. The resultant dehydrated gypsum fails to dissolve further and precipitates as anunstable disperse colloid mass in the form of gel, the process being accompanied by the settingof the mass. The resultant crystals grow both in number and size, while orienting randomlyand interwining, convert the jelly like mass into a crystalline growth. The resultant CaSO42H2O crystals grow into a single crystalline concretion which on drying becomes very strong.

 

Gypsum sets within 20 minutes and it is difficult to use it for some purpose. Suitable settingretarders like lime-kerat in glue and sulphite -alcohol vinasse may be used.

 

 

4. Classification

 

Gypsum binders are classified as low and high burning varieties. The low burning variety ismanufactured by heating dehydrated gypsum to a temperature of about 160°C. The examplesof 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 chemicallybound water is lost totally. Gypsum may also be classified as low strength gypsum—obtainedby heating natural gypsum rock at normal pressure, the resultant gypsum ( modification) isvery hygroscopic (60–65%) and porous (40%), and extra strong gypsum—obtained by heatinggypsum at pressure of 2–3 atm followed by drying at 160°C–180°C (    modification). The extrastrong gypsum is used in metallurgical industries for manufacture of moulds.

 

 

5. Manufacture

Law Burning Variety

   
A 75 per cent dehydrated gypsum is referred to as Plaster of Paris. The pulverized Plaster ofParis is the basic material used to make many of the gypsum building materials. For refinedgrade of Plaster of Paris the oven, kettle and rotary processes are used. Hard finish plaster ismade in kilns similar to that used in calcining lime.

 

The excavated raw materials are crushed, and if the kettle process is used, ground untilabout 60 per cent pass No. 100 sieve. In the rotary process the final pulverization is omitteduntil calcination is completed.

 

The kettles employed for calcinations are 2.5 or 3 m in diameter and about 2 m high. Thepulverized material is chuted into the kettle and temperature raised gradually so as to drive offthe mechanically held water. At about 100°C the whole mass bubbles up violently and thensinks. At 150°C the combined water begins to boil out and between 170° and 200°C the processis stopped. The kettle process requires about 2 to 3 hours to calcine a charge yielding 5 to 6tonnes. The calcined product is then cooled partially in a vat and is sent to the screens. Residuesfrom 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 fedinto a rotating cylinder inclined to the horizontal. Calcination is accomplished with theintroduction of hot furnace gases. The roasted material is conveyed to calcining vats in whichfurther changes are bought by the heat within the material. The product is then groundscreened and stored.

 

In case of Plaster of Paris or stucco the time of setting is delayed by adding fraction of oneper cent of retardant like glue, saw dust or blood after the plaster has cooled to increase thehandling time. Cattle hair or wood fibre is introduced for cohesiveness of plastics. Wall plastersmade from pure raw materials are adulterated with 15–20% of hydrated lime, the addition isnot required for the raw materials containing considerable amount of clay. If instead of usingmoderate heating the gypsum is heated sufficiently to drive off all the water, the product nolonger combines readily with water to form a useful plastering material. If small quantity ofaccelerating salts is added to it, a useful range of materials is again formed.These are known asanhydrous gypsum plasters or hard burnt plasters.

 

 

High Burning Variety

 

Anyhydrite cement is obtained by burning natural dihydrate gypsum at a temperature ofabout 700°C and then grinding the product together with hardening catalyzers (lime, mixtureof sodium sulphate with green or blue vitriol, burned dolomite, granulated basic blast-furnaceslag, etc).

 

A typical anhydrite binder may be of the following composition: lime, 2–5%; a mixture ofsodium bisulphate or sulphate with green or blue vitriol in amounts of 0.5 to 1% each; dolomiteburned at 800–900°C, 3–8%; granulated basic blast-furnace slag, 10–15%. Green and bluevitriols consolidate the surface of hardened anhydrite cement, so that the catalyzers do notseep out and discolour the item’s surface. The action of the catalyzers is due to the ability ofanhydrite to form complex compounds with various salts in the form of an unstable multiplehydrate, which then decomposes yielding CaSO42H2O.

 

Anhydrite cement can also be obtainedby grinding natural anhydrite with the above additives.Anhydrite cement is a slowly setting binder; its setting starts not earlier than in 30 min andends 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 ismanufactured by burning natural gypsum or anhydrite at a temperature between 800 to1000°C followed by fine grinding. This results not only in complete dehydration but also inpartial decomposition of anhydrite with the formation of CaO(3–5%) according to the reactionCaSO4 = CaO + SO3. When estrich-gypsum is mixed with water, CaO acts as a catalyzer whichpromotes 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 mosaicfloors, to manufacture artificial marble, etc. Items from high-burned gypsum have low heatand sound conductivity, higher frost and water resistance and a smaller tendency to plasticdeformation than products from building gypsum.

 

 

6. Plaster of Paris or Stucco

 

It is produced by incompletely dehydrating pure finely ground gypsum at a temperature somewhat lower than 185°C. Most plasters theoretically approach — 421CaSO +  HO2— whichcontains about 6.2 per cent of water.

 

The setting of plaster of paris is attributed to the formation of gypsum crystals from asupersaturated aqueous solution. When substances of colloidal nature (for example glue) aremixed with the plaster the formation of crystals is hindered and the time of set retarded. Inhardening, Plaster of Paris first shrinks then expands. The latter property makes the materialsuitable for making casts, since a sharp impression of the mould can be secured. For the samereason it forms an excellent material for filling cracks, holes in the plastered surfaces and alsoon the wooden surfaces before painting/polishing.

 

Owing to the rapidity of set and difficulty in working, its use in structures is limited toornamental works. Being unstable in water it should be used for indoor works only.

 

Properties

 

1.White in colour

2.Setting time is 5 to 10 minutes

3.Specific gravity is 2.57.

 

7. Gypsum Wall Plasters

 

Gypsum wall plasters gain one-half of their one-month strength in a day. Plaster and sandmortars of 1:1 proportions may be expected to develop 80 per cent of the neat strength atcorresponding ages, while those of 1:2 proportion generally possess one-half to two-third ofthe neat strength.The gypsum to sand neat plaster in proportion of 1:3 should set in 2 to 32 hours and in 1.5to 8 hours when mixed with wood fibres. The dry set density of gypsum wall plaster is850–1040 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) withmaterial added to control workability, time of set and cohesiveness.

 
Gypsum Wood Fibre  Plaster is 60.5 per cent or more of calcined gypsum and, wood fibre 1.0per cent or more to increase cohesiveness, and the remaining material to control workabilityand time of set.

 

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

Gypsum Ready Sanded Plaster consists of cementing material, predominantly calcinedgypsum, which has been mixed at the mill with the proper proportions of sand and otherdesirable 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 second coat.The scratch coat contains 2 sand to 1 cementing material by weight. The browning coatcontains 3 sand to 1 cementing material by weight. The cementing material carries at least 60.5per cent by weight of calcined gypsum and other ingredients to control set and workability.Some of the properties of gypsum plasters are given in Table 19.1

 

 

8. Hard Finish Plaster

When gypsum is burnt at considerably high temperature than that for calcining of cementplaster, and treated with certain solutions like alum and Glauber’s salt (Na2SO4), the plastersso produced show slow setting but ultimately become very hard. Such plasters may be polishedto form a smooth surface and make a very satisfactory finish for interior walls. Often walls ofthese plasters are marked to imitate tiling with pleasing effects.Two commercial hard finishplaster cements are available. 

Keene’s Cement is made by burning a very pure rock gypsum at a red heat (700°C), cooling,and then adding 1.0 per cent of potassium and aluminium sulphates to accelerate the set.Subsequently the material is ground so that 90 per cent or more passes No. 100 sieve. It is pureCaSO4 of pure white colour. Keene’s cement is not injured by storage and mortars of it may beretempered. Set occurs between 20 minutes to 6 hours. At 7 days the tensile strength is 3.16N/mm2. It is used as a finish plaster only where a greater resistance to moisture and surfaceabrasion is required.

Mack’s Cement is made by burning gypsum at a very high temperature and adding about0.4 per cent of burnt Glauber’s salt or potassium sulphate. It is said to form unusually hard,dense and durable surface which will take paint very well.

 

 

9. Gypsum Plaster Boards

 

It is a gypsum product of recent origin made of thin layers of card board or wood cementedtogether with wall plaster, used for lining walls and ceiling of buildings. The boards may bestrengthened by incorporating fibres as fibrous gypsum plaster boards. Sissal or coconut fibresare generally used. The weight of plaster in the later variety is 10 kg/m2 of board and that offibre is 250 g/m2 of board. They are very light weight and have high fire resisting properties.Gypsum plaster boards can be sawn to desired size and shape. They are available in widths400, 600, 800, 900, 1200 mm; in length 1200, 1500, 1800, to 3600 mm in steps of 100 mm and; inthickness 9.5 to 15 mm. They are classified as

 

The breaking load of the boards are given in Table 19.2.

 

 

10. Non-Load Bearing Gypsum Partition Blocks

These can be solid or hollow, rectangular with straight and square edges and true surfaces. Thecompressive strength of these partition blocks should not be less than 50 N/m2 on gross area.These boards are available in sizes as given in Table 19.3.

 

11. Pyrocell

 

It is finely ground powder containing an admixture, forms a gas on being mixed with waterand expands the mixture to 3 or 4 times its volume. This inflated paste hardens into a light,cellular, fire resistant mass possessing good acoustical and insulating properties.