When lime is used as a binding material in making mortar for plastering purpose, it is called lime plaster.
A mixture of lime and sand taken in equal proportions in a mortar mill, mechanically prepared with water to make a paste of required consistency.
The sand proportions in lime plaster control various properties such as shrinkage, strength, porosity, adhesiveness & fine sand is more suitable for making the plaster.
How to apply LIME plasterS:
- It is more desirable to apply more than one coat of the lime plaster, so before applying the second coat the mortar is dashed against the surface of the wall in a layer of 6 to 10 mm thickness with a trowel to give a good grip.
- The second coat is then applied to the first coat (not dry).
- In the second coat the mortar is well pressed and rubbed along the straight edge of the wood and leveled through a wooden float to finish evenly.
- After 5 days, the third coat called the finishing coat, is applied over the second coat, the thickness of the third should not exceed 3mm.
- The third coat is finished by applying cream of white or fat called neeru with the help of a metal float.
Lime plaster mix:
Sand should not more fine and it should pass more than 5 % through a 100 mesh sieve or more than 20% through a 50 mesh sieve.
There should be the gradation of sand for making the plaster more effective and durable, the sand should be free from any impurities such as pebbles, rubbish, grits, mud, etc.
Thick lime or poor lime is generally recommended for plastering work due to the slow process of hydraulic lime.
For external plasterwork, sometimes cement is added into the lime mortar so as to improve the strength and properties of lime mortar.
Advantages of lime plaster:
- Lime plaster is set to a solid mass that is durable, relatively flexible.
- It is permeable and allows for moisture diffusion and evaporation.
- It is less affected by water and will not dry or dissolve like clay or gypsum plaster.
- Plasters made of hydrated lime is less brittle and less prone to breakage, which does not require expansion joints.
- The high pH of lime acts as a fungicide; preventing mould from growing in the lime plasters.
Disadvantages of lime plaster:
- Hydraulic lime plaster is not as hard as cement plaster.
- When the plaster dries, the pH drops to about 8.6.
- Non-hydraulic lime plaster requires moisture to set and prevents it from drying for several days.
Application of lime plaster:
- This plaster is used foror the plain side of a brick wall, 13 mm thick plaster is provided and for the rough side of a brick wall, 16 mm thick plaster is suggested.
- Also, for the stone masonry wall, 19 mm thick plaster is provided due to surface unevenness.
- Lime plaster can be applied in one, two or even three coats on the surface of the finished wall between the screeds.
- For a coat plaster, mortar is applied in a uniform surface that is slightly higher than the required one.
- This surface is then crushed within the thin strips of bamboo, so that the joints can be filled and conveyed to the required surface.
- Applying a solution of lime putty to the surface eliminates plastering work, making it smooth with a metal float.