What is plaster?
Plaster is the protective layer of the mortar along with the purpose of the protection of the structure from external environmental exposure as well as to increase the protection of the aesthetic beauty of it. There are different types of plaster used for the wall.
Plaster is made by well covering of the plastic mortar which later om harden to rigid. Plastering mortar can be shaped into different designs as required by a skilled mason. But, the thickness of the plaster must be taken in care.
These days even moulded decorative plasters template is more popularly available to attach it on wall and ceilings.
Plaster is generally done over the exposed part of the structure. Mortar for plaster is generally prepared by mixing the fine sieved and well-washed sand, cementing material (lime or PPC or clay or gypsum ), water, and sometimes some additional admixture.
The normally adopted ratio for plaster is 1:4, but different ratios like 1:6,1:3 are also adapted according to need and types of plaster. It must be noted that to dry or too wet mortar cannot be used for plaster.
Plaster alone doesn’t play any significant role in the structural strength of the building components, however, it protects the structural component like a shear wall, beam, column, slabs, from direct exposure of sunlight, moisture and different gases and chemicals present in the atmosphere.
Plaster, however, applied with reinforcement fibers or mesh can provide some structural strength as well as gives proper bonding to the entire structure. The process is also known as the retrofitting of walls with galvanized wire mesh.
Plastering is generally preferred in rough surfaces of the structure and avoided into the steel and woods as it doesn’t form any proper bonding.
Why to use plaster?
Plaster alone doesn’t contribute any structural strength but helps to form a monolithic component with the wall and beam, a column with the use of galvanized reinforcement mesh.
So, plaster with reinforcement is quite useful to renovate the building, to avoid cracks and damages of the wall due to seismic activities.
Normally, plaster’s main purpose to be used is to provide protection from external environmental exposure along with good finishing of work to increase aesthetic value. Thus, the durability of structural supporting members as they are protected.
The plaster is also used for preventing water seepage and transfer of moisture to the internal compartment. But the amount of cement proportion in such plaster is relatively high along with the addition of some waterproofing admixture. Such a plaster has a very smooth finishing.
Plaster increases the thermal insulation so it protects from the heat transfer to some extent. And moreover, they also act as fire-resistant material preventing the propagation of fire.
Also, plaster is very widely used to smoothen the rough and undulating surfaces. Use of plaster in water tanks, channels are done in order to smoothen the surface, prevent leakage and growth of plants.
Usable thickness of plaster
The thickness of the plaster is generally dependent upon the number of coats, types of plaster, and the surface of the wall or ceiling to be plaster.
Normally about 12 mm – 15 mm of total plaster is preferable in the inner smoothen face of the wall and about 20mm – 25 mm of plaster is preferred in the outer or exterior wall. The plaster shouldn’t be too thick, else there might be cracks, falling, and blistering.
According to the reference of IS code 1661:1972, plaster with different numbers of coat and their relative thickness are mentioned below.
- Single coat plaster – 10mm-15mm
- Double coat plaster
- Backing coat – 10mm-12mm
- Finishing coat – 3mm-8mm
- Three coat plaster
- Basecoat – 10mm-15mm
- Second coat – 3mm-8mm
- Finishing coat – 3mm-5mm
Normally, there is an adaptation of using the gypsum plaster, lime wall putty plaster as only finishing plaster. For which their thickness is,
For gypsum coat finishing – 2mm
For lime coat finishing – 4 mm- 7mm
Materials and equipment used for plaster
The general raw materials used for plaster are sand, water, and cementing material, and sometimes additional admixtures. The commonly used equipment for manual work base plastering is,
- Battern (wood loath)
- Squares or rulers
- Iron pan
Types of plaster
There basically 5 types of plaster depending upon mortar used. They are,
- Clay plaster
- Gypsum plaster
- Lime plaster
- Cement plaster
- Heat resistant plaster
These types of plaster are one of the ancient methods of building technology. The old and traditional houses are made up of the bricks, clay, woods, and naturally available raw materials.
These houses can also be seen in remote areas of Nepal, India, Bangladesh, South Africa, and even in native America. Clay is one of the most abundantly available raw materials all over the world. So the remote parts where new technologies haven’t been in reach and access the clay plaster are still in use.
Humans have adopted different locally available admixture like cow dunk, straw, fibers, etc. for the proper adhesion and prevention of cracks. These plaster are generally applied with hands.
Cement plaster is a commonly adopted technique in the modern era. The cement mortar is prepared by adding pozzolanic cement, sand, water, and sometimes admixture in different proportions.
These types of plaster are a commonly used material to bond the masonry units so cement plaster is more likely to adhere over the walls and such bonding. Also as people are familiar with the cement mortar, a normal mason can easily handle it too.
The cement plaster neither sets too fast like gypsum plaster nor sets too slow like lime plaster.
Moreover, these plaster are used more quantity of sand as constituent material so they help in forming granular interconnection with bricks, binding material, mesh(if use), and concrete. They give a rough texture.
Due to this reason, they are used in most cases as primary coats of plaster. And above them, the secondary coat of plaster of lime wall putty or gypsum is applied
Gypsum plaster(Plaster of Paris)
Gypsum plaster is also called plaster of Paris. The mortar used in such a plaster is prepared by the use of calcium sulfate hemihydrate, which is formed by heating the gypsum to 150°C.
CaSO4·2H2O + heat (150°C) → CaSO4·0.5H2O + 1.5H2O
The powder of calcium sulfate hemihydrate powder is called plaster of Paris powder, which is mixed with a sprinkle of water to form a slurry. The slurry again rehydrates to form the gypsum crystal.
The paste form of plaster of Paris is applied to the rough walls or ceiling by the help of trowels and metallic float. They are well leveled overall on the surface. And after drying they are polished with sandpaper to give smooth finishing.
The initial settings start in 10 min, thus only the required quantity of slurry that can be applied within 10 min is prepared. And the final set of it is about 45min, however, it takes 72 hrs for the complete formation of gypsum crystals. They are white in color naturally, buy the pigmentation can be added later, or paint can be easily applied over it.
The lime plaster is made of mortar by mixing sand water and lime (usually hydraulic lime). Lime plaster is well adopted traditional techniques that seem to be adopted since 7200BC /5000BC according to a record of archeological study.
The lime is sufficiently wetted in the water and then the mix is formed which is applied over the walls and ceilings. And then left for air dry with the access to free airflow. The CO2 from free air turns the lime into the Calcium carbonate forming the hardened layer.
The lime plaster takes a very long time to set and may extend for several weeks. The lime plaster needs to be cured more than other plaster. But after hardening the lime plaster may even gain more strength than other plaster.
These types of plaster are less brittle and formless cracks. Some admixtures like pozzolanic material, gypsum, and admixture also added to improve the quality of lime plaster and shorten it’s settings time.
Heat resistant plaster
These types of plaster are used to form chimney and fire barriers. These plaster mortar are specially prepared from inert substances like fly ash along with a proper mix of sand and cement.
Types of finishing of plaster
- Smooth coat finish – the higher proportion of cement is used in such plaster to keep plaster smooth, waterproof, and shiny surface
- Rough coat finish – The surface is made rough intentionally in a random pattern by scrappers and brush. This is generally done to apply tiles.
- Sand faced finish – this is normal plaster with a high quantity of sand giving the granular texture of sand grains.
- Pebbledash finish – clean pebbles are embedded in plaster during plastic coverage state
- Scrapped finish – vertically scratch plaster surface in a good manner. The depth of scrap is 3mm. It is done in order to prevent event the crack-like in the Contraction joint
- Depeter finish – pieces of Flint gravel are embedded over plaster at plastic state
- Textural finish – different kind of texture in the plastic state of plaster cover is made as a kind of artwork.
- Pointing – pointing is part of plaster finish work, in which duffer shapes are made especially at joints. However, pointing itself is also sometimes considered a separate topic.
Procedure to prepare and apply different types of plaster
The plaster mortar is prepared in a distinctive ratio as required. Moreover, the number of coats and the type of plaster coat use is decided according to which work is performed.
The general procedure for apply plaster(usually in the normal building) in three coats includes preparing the surface of work, applying the rendering coat, and applying the final coat.
Initially, the mortar joints and extruded bricks are racked out to a depth of 20mm and the surface is cleaned and sufficiently water.
However, the watering for celling is prohibited as a high probability of falling off the stuck plaster mortar due to high water content. If the surface to be plaster is very rough and uneven then a preliminary coat is applied so that the holes and unevenness are roughly brought to level. Then the 1st coat of plaster is applied on the surface.
The mortar can be stick with the help of trowels and can be leveled in the plastic state by the wooden patterns. The thickness of such an initial coat should lie within 10-15 mm. Now, another coat of plaster is applied after some initial settings of the plaster. Such coats are limited to be very thin and cover the well level of the surface.
To maintain uniform thickness, the small lump of screeds with well level by measuring device is initially attached to a different places of the wall. And according to the reference of such dots of lump plaster is brought to uniformly level.
Such lumps are initially applied on the rough surface approximately at the spacing of 2m in a square pattern over the wall.
Then the surface can be successfully plaster to finish plaster with either cement mortar or lime mortar or plaster of Paris by use of float.
However, the plaster procedures for massive structure, traditional temples, architectural building might be different than this normally explained method. Also, in modern days, there is also the availability of the mechanical machine which applies The plaster over the wall within a few minutes.
Special types of plaster- BAJRA (Extra content)
“Bajra” literally means very strong or unbreakable in Sanskrit. ‘Bajra’ is the special traditional mortar commonly used in ancient structures and monuments of Kathmandu Valley of Nepal. Bajra mortar is prepared by the proper mix of the kiln burnt bricks dust also called “Surkhi”, clay and lime.
The bajra is very weld mix by kneading with hands or mechanical equipment to form a dough-like mass of the floor. This has a very unique quality and significant property than lime mortar. However final coat of lime mortar is applied for smooth finishing.
The famous stupas of Nepal like Swayambhunath stupa, Boudhanath Stupa with their unique dome shape structure are made with such bajra. Bajra takes an extremely long time to set as well as a long time for preparation. They are thus used in the conservation of old ancient monuments of Nepal.
It also has been found that some organic additives like lentils, chaku, egg white, milk, and cow dunk are added to improve its quality. But this process makes it more uneconomical in the present generation. The bajra mortar can also be used as binding material as well as plaster.
In Nepal, the 9 stored tower of height 61.88 m tall, named “Dharahara” was built in 1832 A. D.with the use of no reinforcement. The monument was solely made up of the bajra along with the above-mentioned admixture and bricks. And was well plaster with bajra as plaster material giving long durability and aesthetic view.
I hope this article on “Types of plaster” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha