What Is Plaster And Plastering?
The process of applying plastic mortar on the surfaces of rough ceilings, walls, etc in order to obtain an even, smooth, and clean surface is known as plastering and the plastic mortar is called plaster. When plastering is done on external exposed surfaces, it is known as rendering. In this article, we will discuss different types of plaster used in construction.
Purpose Of Plastering
Plastering is done mainly for the following purposes:
i) To protect the surface from harmful atmospheric effects such as rain, wind, sun, snow, etc.
ii) To create decorative effect as well as to improve the aesthetics.
iii) To hide defective and poor workmanship.
Types Of Plaster
Conventional types of plaster are as follows:
- Cement plaster
- Lime plaster
- Mud plaster
1. Cement Plaster
Proportions of cement and sand may vary according to the requirements of the plaster. But, the usual proportion are 1:3 and 1:4 which gives the best results.
Cement plaster is used on the interior walls and exterior walls, ceilings, and other building elements. For interior walls, a single coat of plaster is usually enough to achieve the desired surface.
For exterior walls, another coating is required depending on the thickness of the plaster and the nature of the surface. The thickness of cement usually ranges between 12 and 20 millimeters, depending upon the type of wall or ceiling surface.
2. Lime Plaster
Lime plaster consists of lime, sand. and water. The proportion of lime and sand is equal. Sometimes, little amount of cement may also be added to improve the strength of the plaster. In lime plaster mostly fatty lime is used. Hydraulic lime results in blisters on the plastered surface.
Gulal, a type of fragrant gum may be added at a rate of about 1.6 kg/m3 of mortar. It gives good binding properties to the mortar. Chopped hemp is also be used at the rate of about 1kg/m3 of mortar.
It provides improved adhesive and tensile properties to the plaster. Types of plaster are now being replaced with cement plaster.
3. Mud Plaster
This is the cheapest type of plaster. It is made up of suitable proportions of clay and sand. The clay content should be free from grass, roots, organic matter, stone pebbles, etc.
Chopped straw, hay, or hemp is also used in the mixture at the rate of 30kg/m3 of earth content. Mud plaster is mostly used in villages and temporary constructions.
Special Types Of Plaster
Besides the above plastering materials, there are some special types of plasters. They are as follows:
1. Waterproof Plaster
As the name suggests, this type of plaster protects the masonry wall from ingress of moisture and thereby eliminating or reducing dampness of the wall.
The mortar for waterproof plaster is prepared by mixing cement, sand (1:2), and pulverized alum at the rate of about 12 kg/m3 of sand content.
Water to be used in the mix is not normal water but the water is prepared by dissolving 75gm of soft soap per liter of water.
2. Stucco Plaster
Stucco plaster is a type of decorative plaster that provides excellent finishing. This type of plaster is usually done in three coats with a thickness of about 25 mm.
Each coat needs to be dry completely before applying the next coat. The first coat in stucco plaster is called scratch coat.
The second coat is called finer coat or brown coat and the final coat is known as a white coat or finishing coat. Stucco plaster can be used for internal as well as external walls.
3. Gypsum Plaster
Gypsum plaster is a factory-made product produced by partial or complete dehydration of mineral gypsum. It is available in powder form and in white color.
Dry gypsum powder is mixed with a suitable amount of water to form a ready paste that can be easily applied to the ceiling and wall surfaces.
The thickness of gypsum plaster is usually 6 mm- 20 mm. It dries very fast, so painting on the plastered surface can be started 72 hours after application.
It gives a very good smooth finish and level surface with excellent proper angled corners. This type of plaster can only be used in the interiors of the building, not suitable for external plastering. It can also be used to fabricate ceiling cornices and mouldings.
4. Keen’s Cement
It is produced by calcinating alum with gypsum. It is the hardest and densest form of gypsum plaster. It is pure white in color and gives a very fine glass-like polish. This type of plaster is mostly used for ornamental works.
5. Martin’s Cement
Martin’s cement, another type of special plaster material is produced by calcinating peal as with gypsum. It also sets quickly and results in a hard and white surface.
6. Parian Cement
It is obtained by calcinating borax with gypsum. It is an alternative to keen’s cement and used for ornamental purposes.
This product is produced by slaking gypsum in petroleum. It is a good fire resisting plaster material.
It is obtained by dissolving keen’s cement and color pigments in glue. It gives marble like finishing, used for plasters, panels etc.
9. Acoustic Plaster
It is a gypsum product that is mostly used as a final coat. During final coat finishing small pores are left on the surface to absorb sound.
10. Barium Plaster
This type of plaster is made from barium sulphate. It is used as final coat in X-ray rooms.
11. Asbestos-marble plaster
It is prepared by mixing finely crushed marble, asbestos, and cement. It gives a beautiful marble-like finish on the surface.
12. Snow-crete and Color-crete cement
These are patented cement. They are mostly used on the exterior walls to create a very good appearance from the outside.