Cement Plaster: All You Need To Know!
Plastering your home is an important task, as it helps in providing a perfect wall finish. It conceals rough and uneven surfaces to obtain even, smooth, clean and durable surface. Plastering provides a satisfactory base for decorating the surface by painting, distempering, white washing or colour washing. It also gives protection against rain and other atmospheric agents.
Plastering is done by using a plastic material or mortar, which is a mixture of various building materials such as cement, lime, gypsum, fine aggregates (sand) and water. Nowadays, cement plaster is mostly used in plastering the interior as well as exterior walls of the house. Here we have given brief information on cement plaster, which a homeowner must know, before plastering the home.
Courtesy - UK Construction College
Cement plaster is a homogenous mixture of Portland cement, fine aggregates, and water. Generally, sand is used as fine aggregates in cement plaster hence it is also called as cement-sand plaster. Cement and sand are mixed in different proportions to obtain the cement plaster. For preparation of cement plaster, OPC 43 grade and OPC 53 grade are mostly used. PPC cement can also be used in place of OPC cement.
Plaster is applied in different coats, according to the requirement. Generally, for interior walls, a single coat of plaster is sufficient to achieve a finished surface. Sometimes, when thickness of plaster is more than 15 mm or when it is desired to have a finer finish, plaster is applied in two coats. If the surface to be plastered is very uneven, plaster is applied in three coats wherein the first coat is known as the dash coat or scratch coat. Then the base coat is applied, and lastly the finish coat is applied to give finishing to the surface.
Thickness of Cement Plaster
The thickness of cement plaster depends upon its composition and the surface to be plastered. Minimum thickness of plaster should be 10 mm to provide a satisfactory smooth surface. For concrete surface a minimum of 10 mm plaster in single coat is applied. For brick masonry surface which is uneven as compared to concrete surface, cement plaster thickness must be 12 mm or 18 mm. If the surface is more uneven the thickness of cement plaster can be increased to 20 mm. The recommended thickness of an individual coat of plaster is as follows:
Coat of Cement Plaster
|Single coat plaster||10 to 15 mm|
|Under coat for Two coat plaster||10 to 12 mm|
|Finishing coat for Two coat plaster||3 to 8 mm|
|Base coat for Three coat plaster||10 to 15 mm|
|Second coat for Three coat plaster||3 to 8 mm|
|Finishing coat for Three coat plaster||3 to 5 mm|
Finishing coats shall be of such minimum thickness as to provide a sufficient body of material to harden satisfactorily under the site conditions in any particular case. The total thickness of two-coat plaster generally, should not exceed 20 mm. The thickness of three-coat plaster should not normally exceed 25 mm. In general case, 20 mm thick plaster is provided on the outer wall, 15 mm thick plaster in inner wall and 10 mm plaster in ceiling.
Grading of Sand for Cement Plaster
Sand for cement plaster must be clean, sharp, suitably graded, and free from impure matter. According to Indian Standards, most suitable particle size grading of sand for cement plaster work for internal and external walls and ceilings is given below:
IS Sieve Designation
The sand for final and base coat should be 100% passing 2.36 mm sieve and not more than 5% should pass 150 micron sieve. For finishing coat, the maximum size should be restricted to that passing 1.18 mm sieve.
The required grading adjustment is often done by screening and/or by blending together either natural or crushed sands which may be by themselves of unsuitable grading.
Also Read: Ways to Check the Quality of Sand at Site!
Cement Plaster Mix
There are different proportions of cement and sand in plaster to obtain a homogeneous mixture of plaster. The proportions of cement and sand in plaster will depend on several factors such as:
- The environmental conditions existing in the surrounding
- The type and grade of cement
- The type and grading of sand
- Location of plastering, i.e internal wall or external wall
According to Indian Standards, the following cement plaster mixes are recommended:
Coat of Cement Plaster
Cement sand ratio (Cement Plaster mix)
|Single coat plaster||1:3, 1:4, 1:6|
|Under coat for Two coat plaster||1:3, 1:4, 1:6|
|Finishing coat for Two coat plaster||1:3 to 1:6|
|Base coat for Three coat plaster||1:3, 1:4, 1:6|
|Second coat for Three coat plaster||1:3 to 1:6|
|Finishing coat for Three coat plaster||1:3 to 1:6|
The mix for the finishing coat depends on the texture and colour of the surface desired. It is preferable to mix the ingredients in a mixer. Dry mortar is mixed initially and thereafter water is added to the dry mix to get the required consistency.
For making cement plaster mix, water of a quality suitable for drinking purpose must be used. It should be free from chlorides and organic impurities. Water used for mixing must be clean, reasonably clear and free from objectionable quantities of silt, oils, alkalis, acids, salts so that the mix is not weakened.
The quantity of water should be equal to 28% by weight of cement plus 4% by weight of total aggregates. It is assumed that the materials are saturated surface dry, non absorbent and their use results in a workable mix with almost maximum strength.
The quantity of water for cement plasterwork with 1:3 proportions is about 70 percent of weight of cement. The quantity of water may vary depending on the following factors:
- The condition and nature of the fine aggregate;
- The temperature and humidity at the time of plastering;
- Richness of the mix, whether rich or leaner than 1:3;
- The varying quantities of lime in composite mortars;
- The admixtures added for improving the workability.
For normal residential plaster work, the quantity of water is taken as 20% of the total weight of cement and aggregates. However for accuracy, the quantity of water should be worked out depending upon the mix and thickness of plaster.
Curing of Cement Plaster
Curing of cement plaster is necessary as it helps in reducing cracking on the surface and also helps in gaining the strength. Curing of cement plaster shall be started after 24 hours after finishing it. The cement plaster shall be kept wet for a period of 7 days. There should be a gap of 7 days between first and second coat of plaster. Each coat should be kept damp continuously for at least 5 days. Then each plaster coat should be allowed to dry through before the next coat is applied. However, just before application of the next coat the base coat should be evenly dampened.
During the cold weather, longer curing periods are necessary. In hot/warm weather, in windy conditions and / or low humidity, curing of cement plaster should be commenced as soon as the mortar shows evidence of setting.
Applying cement plaster on your walls makes it look really finished and it adds value to your house. Cement plaster actually gets mixed with water and later get applied to the wall to give a smooth finish and base to the wall so that any paint can be applied to the walls easily.