PRE-MIXED GYPSUM PLASTERS

• Lime is the one which has been used as the only basic material of plaster available for centuries.

• But lime has recently been superseded by gypsum in all over the world.

The reasons why gypsum is preferred over lime as the basic material of plaster include

the following:

1. Drying shrinkage of lime plaster which causes cracking.

2. Slow drying out process of lime which can take several weeks causing delays for the following trades.

3. Need to apply lime plaster in several coats, to reduce the amount of shrinkage.

Gypsum plasters like lime plasters must

fulfil the following requirements:

1. Camouflaging irregularities in the backing wall.

2. Providing smooth continuous surface which is suitable for direct decoration.

3. Providing sufficiently hard surfaces to resist damage by impact upon them.

• Gypsum is crystalline combination of calcium sulphate and water. In its natural state it may be grey, pink or white. The word gypsum came from the Greek word gypsas, meaning chalk.

• Gypsum is found in various parts of the world: Canada, U.S., France, England, Italy, China, Russia and some countries in South America has large deposits.

• İt is obtained from open pits and underground mines.

• Gypsum rock used in the construction industry is crushed and heated (calcined) in rotary kilns to temperatures of 160-260 ºC until much of its moisture is driven off, depending on the class sought.

• When the powder-dry gypsum is recombined with water, it again returns to its rock like form.

• Wall plaster contains fibres for added strength. It is a hard plaster widely used to form the first and second coats on plastered walls and ceilings. Water and aggregate is added on the job to obtain the correct body and consistency.

• The aggregate may be send or a lightweight material such as vermiculite (hydrous silicates resulting usually from expansion of granules of mica) or perlite. A lightweight base coat plaster is also a product which has the gypsum and aggregate mixed in it. Water is added on the job.

• Finish plaster is made especially to produce the finish coat for plastered surfaces.

• This plaster is mixed with hydrated lime and water.

• Hydrated lime is an inert compound formed by the chemical combination of water and calcium oxide.

TYPES OF READY MIXED GYPSUM PLASTERS

• The types of ready-mixed gypsum plasters differ from one another basically on the basis of amount of water retained at the end of the process of heating or dehydration within the powder gypsum.

• When gypsum is heated to about 170 ºC it looses about 75% of its combined water and is called hemi-hydrate gypsum plaster but this plaster is better known as “plaster of Paris”.

• If the temperature is raised to about 260 ºC it looses nearly all of its combined water and is called anhydrous.

According to British standards (BS 1191), gypsum plasters are classified as follow:

Part 1 :

• Class A : Hemi hydrate gypsum plaster (plaster of Paris)

• Class B : Retarded hemi hydrate gypsum plaster.

• Class C : Anhydrous gypsum plaster

• Class D : Keene’s plaster

Part 2 :

• Pre-mixed lightweight plasters, consisting of suitable lightweight aggregates and retarded hemi-hydrate gypsum plasters of part 1 class B.

• Type a : undercoat plaster

• Type b : finish plaster

Class A: Plaster of Paris

• This is a hemi-hydrate plaster having approximately 25% of its original combined water left and has a very rapid setting time of about 10 minutes.

• It can be used neat or mixed with a little sand and is suitable for use as a filler, moulding and repair coat.

Class B : Retarded Hemi-hydrate Gypsum Plaster

• This is a hemi-hydrate plaster to which an animal protein such as keratin has been added during manufacture to retard the setting time. It is generally produced in two types:

• Type a : undercoat plasters for use when mixed with sand.

• Type b : final coat plasters used for applying a smooth finish to undercoat plaster or plasterboard.

Class C and D : Anhydrous gypsum plaster and Keene’s plaster

• Anhydrous plaster is formed when most of the moisture has been driven off and to which ½ or 1% of aluminium or zinc has been added to speed up the set otherwise it would be unsuitable.

• Because it is used as a finishing plaster and has the advantage of a slower setting time than Class B plasters.

• It can be trowelled to a high polished finished and is generally harder than Class B plasters.

• Class D or Keene’s Plaster is similar to class C except that during manufacture its temperature is raised to about 370 ºC resulting in a finishing plaster with a very hard surface.

APPLICATION OF PLASTER

• The mix proportion (plaster : sand) the number of coats and the thickness of coats of plaster will depend largely on the nature of the background to which the plaster is applied.

• Backgrounds vary in their qualities of flatness and / or roughness, and ‘suction’.

Flatness :

• Irregular Surfaces such as brickwork will require at least two coats of plaster.

• Where a surface is out of plumb three coats of plaster may be necessary.

• On an uneven surface one coat plaster tends to sag and leave surface of plaster wavy.

• By using a second undercoat the surface can be finished level.

Key:

• An irregular, rough background surface such as brick wall provides depressions into which the plaster will ‘key’ or lodge.

• The key will provide support the weight of the plaster.

• On smooth, flat, dense surfaces, a bounding agent might be applied to the wall, prior to plastering.

• There are emulsions based on polymer which provide an adhesive surface suitable for the application of plaster.

Suction :

• Suction (the ability of background to absorb water) will affect the drying rate of plaster.

• Where the suction of a background is too high, the plaster will fail to set properly, resulting in a loss of adhesion of the plaster to the background.

• Where the suction of a background is too low, the plaster will retain excessive moisture, and drying shrinkage cracks could occur in the plaster.

OTHER PLASTERS

• Other Plasters are available which are used for their specific functional properties.

• These includes:

Fibrous Plaster,

Lightweight Plasters, and

Acoustic Plasters

Fibrous Plaster

• Fibrous plaster consist of plaster of paris reinforced by materials such as jute scrim or metal lathing.

• It may be moulded into complex ornamental forms for such purposes as cornices, covings, and decorative work to walls and ceilings.

Lightweight Plaster

• These are premixed Class B gypsum plasters with aggregates of eitherexfoliated vermiculite or expanded perlite.

• Expanded perlite is a form of volcanic glass which is expanded by heat.

• When these materials are used as aggregate in plaster, the plaster is much lighter in weight than gypsum / sand plaster.

The advantages of lightweight plasters are:

• İmproved thermal insulation, hence condensation is less likely to occur;

• İmproved fire resistance.

Acoustic Plasters

• These are premixed Class B gypsum plasters with a porous aggregate such as pumice, which absorbs sound.

• Acoustic plaster may be applied to curved or irregular surfaces.

Plaster Undercoats for Lime and Cement Plasters:

Background Coat Thickness

Uneven 1st Undercoat

10 mm

Uneven 2nd Undercoat

6 mm

Uneven Finishing Coat

3 mm

Volume of Ingredients in the Plaster Mix

• Lime Plaster:

Lime: 1 part / volume

Sand: 3 part / volume

• Lime and Cement Plaster:

Lime: 1 part / volume

Cement: 1 part / volume

Sand: 6 part / volume

Examples for Drawing Techniques

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