gypsum pointing | Gypsum plaster

What is Gypsum Plaster? | Types of Gypsum Plaster | Uses, Properties, Advantages & Disadvantages

 

  1. Introduction  

Plastering is the process of covering the rough surfaces of walls, columns, ceilings, and other building components with a thin coat of plaster to make the surface smooth and durable. 

Plastering not only provides beautiful aesthetic beauty but also protects the surface from weathering agencies and vegetation growth.

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate (CaSO4·2H2O). It is generally found in the form of rock and is white in color.

Gypsum plaster is a paste formed by mixing the gypsum powder and water in a suitable proportion.

gypsum pointing | Gypsum plaster

No sand is used in gypsum plaster. Only mixing of gypsum powder and water is done to prepare gypsum plaster.

 

  2. Constituents of Gypsum Plaster  

Gypsum plaster consists of dry gypsum powder, water, and sometimes a small amount of accelerator to make anhydrous plaster.

Retarders like organic acids, soluble salts, basic phosphates, and proteins are used in gypsum plaster to reduce its setting time.

Hardeners also can be added to gypsum plaster for improving strength and non-brittleness properties.

 

  3. History of Gypsum Plaster  

Ancient people used to plaster their shelters with mud to make their structures durable and stable. With time, they started to replace mud with various materials like gypsum.

Gypsum plaster was used as an internal plaster in the Giza Pyramid. This provides strong evidence about the performance and durability of the gypsum plaster.

 

  4. Precautions for Storing Gypsum  

Gypsum has a self-life of 3-4 months from the date of manufacture. But self-life can be increased further for additional 6 months if stored properly with great care.

Gypsum has a setting time of 25-30 minutes which may be reduced due to exposure to humidity, moisture, and frozen backgrounds. Hence, it should be stored in a closed area such that its hydrating property won’t let it to be expired soon even in humid conditions.

 

  5. Thickness of Gypsum plaster  

While applying gypsum plaster on walls, ceilings, or other surfaces; its thickness really matters.

Even experts recommend applying gypsum plaster with a minimum thickness of 6 mm to avoid cracks and peels-off problems.

Thus, gypsum plaster should be applied in the thickness of 6-20 mm.

For undercoat, the thickness of gypsum plaster is:

For walls = 11 mm,

For ceilings = 8mm

For finishing coat = 2 mm.

If thick plastering is required more than 20 mm thickness, then an extra coat of cement sand plaster can be applied within the thickness of 8-12 mm and over it, gypsum plaster of not less than 6 mm is coated for smooth finishing.

Mainly, the gypsum bag has a weight of 25 kg and plaster prepared from each bag provides the coverage of  21 ft² with a thickness of 12 mm.

For aesthetic beauty; after complete drying of plaster, it can be painted.

 

  6. Types of Gypsum Plaster  

 

  i. Based on heat generation during the manufacturing process  

 

a. Anhydrite Gypsum Plaster

Anhydrite gypsum is manufactured by heating gypsum to 170 °C.

And plaster prepared using Anhydrite gypsum is known as Anhydrite Gypsum Plaster.

 

b. Hemihydrite Gypsum Plaster

When gypsum in hydrated form is heated to more than 170°c, Hemihydrite gypsum is formed.

Plaster prepared using Hemihydrite Gypsum is known as Hemihydrite Gypsum Plaster.

 

  ii. Based on the Application  

 

a. Casting Gypsum plaster

b. Undercoat Gypsum Plaster

c. Finish Gypsum plaster

d. One coat Gypsum plaster

e. Machine applied Gypsum Plaster

 

  7. Process of Applying  

 

a. Preparation of surface 

All the joints and surfaces of the masonry are well cleaned with a wire brush.

It should be confirmed that the surface is free from oil, grease, etc.

 

b. Preparation of Plaster

Gypsum plaster comes in bags in powder form. It is then mixed with water in a clean dry vessel and stir for 2 to 3 minutes.

 

c. Application of Plaster

After preparing the plaster, apply it to the surface with firm pressure.

Gypsum Plaster gets stiffen quickly so continue flattening. When the plaster becomes sufficiently firm, scour the surface with sponge float & water if needed. For a smooth surface, trowel the surface progressively.

The surface after plaster should not be exposed to moisture and painting is restricted until the plaster dries completely.

Plaster can be applied in multiple coats also. The total thickness of this plaster should vary between 6 mm to 20 mm.

 

  8. Properties of Gypsum Plaster  

a. It is practically unaffected by bacteria.

b. It is light in weight.

c. The plaster shows good adhesion to fibrous materials.

d. It is slightly soluble in water ( hence it cannot be adopted for damp conditions and external work).

e. It is set by the natural process by crystallization, hence it can be applied with ease and without wastage.

f. It sets with little volume change and with negligible shrinkage on drying.

g. Since it dries up quickly, the finishing coat can be applied soon after the undercoat has set, usually the same day.

h. A porous gypsum is an important sound-absorbing material while dense and hard plasters are efficient reflectors of sound up to 90% energy incident at their surfaces.

i. It has high tensile and flexural strength.

j. It prevents rusting of metal by increasing its durability.

k. It has low thermal conductivity.

 

  9. Uses of Gypsum Plaster  

a. It is used as a heat-insulating material to protect wood or metal columns and beams from high temperatures.

b. It is used for ornamental plasterwork.

c. The gypsum plasterboards which are cheap, light in weight, fire-proof, and easy to work are used for the ceiling, for internal lining of walls, and partition walls.

d. Gypsum blocks are very useful in the construction of non-load bearing partitions in dry conditions because of good thermal insulation and small dead load and therefore facilitate speedy construction. These blocks are also used for the protection of columns, beams, and elevated shafts against fire.

e. Slotted gypsum plaster tiles 13 mm thick measuring 300 mm × 300 mm size can be prepared using calcined gypsum and sawdust mix in a proportion of 65:35 parts by weight. These acoustic tiles are used in large offices and auditoriums to reduce the echo. The effectiveness of tiles depends on their surface porosity and the extent to which pores are interconnected.

 

  10. Advantages of Gypsum Plaster  

a. It is less costly and time-saving.

b. Negligible wastage during application of plaster.

c. It is easily workable and also provides an excellent finish.

d. No need to do the curing, hence directly saves both water and time.

e. It has good binding properties and only a single coat is sufficient for plastering RCC ceilings, internal bare-brick walls, AAC blocks, and concrete columns.

f. It is ecofriendly and unaffected by bacteria.

g. It avoids the growth of fungus.

h. It is easy to obtain various surface textures and surface hardness.

i. It provides a smooth finish to the surface.

j. It is easy to paint on a gypsum plastered surface with any type of paint.

k. Various shapes for decorative purposes can be obtained with the help of gypsum plaster.

l. The time gap required between two successive coats is lesser and the setting time of gypsum plaster can be controlled.

m. It is pre-mixed material and easily available.

n. Due to its white color, it gives a beautiful appearance.

 

  11. Disadvantages of Gypsum Plaster  

a. It is expensive than cement plaster for the same thickness.

b.  It is unfavorable for exposing walls since they are prone to dampness. Also, it is undesirable in the area like a bathroom, kitchen, toilet, etc.

 

  12. Gypsum Board  

Gypsum board is made up of gypsum having a paper surface face and long and square edges.

It is also known as drywall, plasterboard, or gypsum wallboard.

It is used as false ceiling material having different beneficial properties.

Some of the beneficial properties are as follows:

a. It has thermal resistant property thus maintains the room temperature.

b. Gypsum board absorbs excess sound and provides better sound insulation.

c. It can be molded in different designs and patterns.

d. LED lights can be attached to the false ceiling of the gypsum board which provides efficient lighting.

e. It is fire resistant and to some extent moisture-free.

f. It is durable and economical.

 

  13. How much does the gypsum board cost for making a false ceiling? 

Gypsum board can be used for false ceilings with the process of framing, boarding, and finishing. Up to these processes, we require materials, labor, transportation, and installation.

It generally costs 1 $ to 5 $ per square foot in the US. And in India, it costs around Rs 35 to Rs 500 per square foot.

The cost totally depends upon multiple factors like:

a. Quality and type of gypsum boards used.

b. Quantity of designs made.

c. Type of structure ( Eg. False ceiling of truss structures is costlier than that of concrete structures. )

 

Read Also: Cement Plaster
Read Also: Lime Plaster

 

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