When cement is used as a binding material in making mortar, for plastering purpose, it is called cement plaster.
Cement plaster is a mixture of portland cement, sand (fine aggregate) and water in appropriate proportions, usually applied to masonry interiors and exteriors to obtain a smooth surface.
Objectives of Plastering:
- To provide a uniform, smooth, regular, clean and durable surface with improved appearance.
- It preserves and protecting the surface.
- To cover the use of porous materials of masonry work.
- To hide faulty workmanship.
Cement plastering work:
- The cement plaster is normally applied in a single coat, the thickness of the coat might be 12 mm, 15 mm or 20 mm depending upon the kind of building to be plastered.
- It consists of the usual proportions 1:4 (i.e. 1 part of cement and 4 part of sand) and higher proportions can also be used depending upon nature.
- When the thickness of plaster is more than 15 mm or when finer finish is required, then cement plaster is required to be applied in two coats.
- For one coat plaster, cement plaster is applied on the prepared surface between screeds with mason’s trowel.
- After application of this plaster; the floor is levelled with the help of wooden float and straight edges, then the floor is lastly polished with a trowel.
- For two coats plaster, the primary coat also called rough coat, it is initially dashed in opposition to the surface of the wall in a layer of 8 mm to 10 mm in thickness,
- Note that mortar is dashed over the surface with a trowel, so to carry the next coat of plaster, the surface of first coat allow to set but it should not dry.
- Then this coat is roughened by means of a second scratching tool so to have a key for a second coat or finishing coat.
- Then finishing coat is applied over the tough and damp surface of the primary coat within 48 hours, then the coat is correctly toweled and finished smooth by the wooden float.
- Finally, plaster is cured for at least seven days.
- For external and internal plastering of building, the proportion of cement mortar is varied.
- External plastering work needs stable, firm and rigid scaffolding.
Advantages of cement plaster:
- If cement properly mixed and applied, a plaster coating creates a stronger and more durable wall finish.
- It is used on both internal and external surfaces.
- This plaster is not a rust inhibitor.
- It has more or less thermal conductivity
- It has more or less identical acoustics properties.
Disadvantages of cement plaster:
- This plaster is mainly weak in tension and flexural strength, due to this fact liable to cracking.
- Cement vapour is not 100% permeable to water vapour.
- This plaster requires a considerable amount of water throughout its curing period.
- The percentage of wastage throughout the application is excessive.
- The application of this plaster is a time-consuming process so increases the project cost.
Application of cement plaster:
- For preparation of surface for plastering.
- The groundwork for plaster.
- Application of plaster coats.