Plastering is essential to building construction, offering both functional and aesthetic benefits. Cement plaster is commonly used to strengthen walls, give them a smooth finish, and protect them from external elements.
Cement plaster is key to its main benefits, ensuring a home’s walls are durable, smooth, and ready for painting or other finishes. Let’s explore the main aspects of cement plaster and how it benefits construction projects.
Let’s get straight to the point.
Cement plaster combines cement, sand, and water to create smooth, durable wall finishes. Depending on surface conditions, it’s commonly applied in one or two coats and is ideal for interior and exterior walls.
Benefits include structural reinforcement, weatherproofing, fireproofing, soundproofing, and thermal insulation. However, proper water content and thickness are needed for effective application.
Plaster must be cured by keeping it moist for at least seven days to prevent cracking.
Various finishes, such as smooth, roughcast, or textured, can be achieved. Modern plastering equipment helps reduce labour and material costs for large projects.
What Is Cement Plaster?
Cement plaster is a mixture of Portland cement, sand, and water. It’s commonly referred to as cement-sand plaster.
The main component, cement, is often of two types: PPC (Portland Pozzolana Cement) or OPC (Ordinary Portland Cement). PPC is the preferred choice for its advantages in durability and performance.
Cement plaster is typically applied to interior and exterior walls to create a smooth, even surface, making it easier to paint or apply other finishes.
Plaster is usually applied in a single coat to interior walls, though multiple coats may be necessary for uneven surfaces or surfaces that require a finer finish.
It’s common to use two coats of paint on exterior walls, with an overall thickness ranging from 12 to 18 mm.
Benefits Of Using Cement Plaster
Cement plaster offers several benefits in construction, including:
- Structural Reinforcement: Plaster helps to reinforce weak areas in walls, protecting them from deterioration over time.
- Weather Resistance: Cement plaster shields the walls from rain, wind, and other external elements that could cause damage.
- Fireproofing and Soundproofing: Plaster provides a degree of fireproofing and soundproofing, improving the safety and comfort of buildings.
- Thermal Insulation: The thermal properties of cement plaster help maintain a balanced temperature within the structure.
- Smooth Finish: It conceals flaws in the construction, providing a smooth, aesthetically pleasing finish ready for painting.
Water Requirements For Cement Plastering
Water is an important component in cement plastering, and the amount used depends on the moisture content in the cement and sand.
The typical water content needed for residential plastering ranges from 15% to 18% of the dry materials’ total weight.
For a 1:3 ratio mix of cement to sand, approximately 70% of the cement’s weight in water is required.
However, this can vary depending on humidity, temperature, and fine aggregate conditions.
Tools For Cement Plastering
Several basic tools are necessary for plastering work, including:
- Trowel
- Paintbrush
- Bucket
- Utility knife
- Handsaw
- Hammer
These tools help ensure an even and efficient plaster application, resulting in a smooth and durable finish.
Thickness Of Cement Plaster
The thickness of the plaster depends on the surface being worked on:
- Concrete walls: Minimum thickness of 10 mm for a smooth finish.
- Uneven brick masonry: Typically requires 12 to 18 mm.
- Extremely uneven surfaces: May need up to 20 mm of plaster.
Two coats of plaster, usually 12 to 18 mm thick, are applied to the exterior walls. A third coat may be necessary for highly uneven surfaces, starting with a scratch or dash coat.
The standard thickness for ceilings is 10 mm.
Steps For Plastering Work
1. Surface Preparation
Before applying plaster, the surface must be thoroughly cleaned, and any loose or defective materials should be removed. For older surfaces or uneven areas, filling holes and raking out the mortar joints is essential to improve the bond.
The surface should be washed and moist for at least six hours before plastering.
2. Applying The Base Coat
- Preliminary Work: Apply dots of plaster (15 mm x 15 mm) spaced 2 metres apart to ensure even thickness. Check verticality with a plumb bob.
- Base Coat Application: The base coat is applied with a trowel, typically with a 1:6 cement-to-sand ratio, and levelled using flat timber floats. The thickness for brick masonry is around 10 mm, while concrete surfaces require 9 to 15 mm.
- Scratch Coat: The base should be roughened with a scratching tool before applying the second coat to ensure proper bonding.
3. Applying The Finishing Coat
The second coat of plaster, or finishing coat, is applied after the base coat has been moistened. This coat is usually 2 to 3 mm thick and is applied in one operation, working from top to bottom to avoid joining marks.
The cement-to-sand ratio for this coat is typically between 1:4 and 1:6.
4. Curing The Plaster
Proper curing is essential for ensuring the strength and longevity of the plaster. Each coat should be kept moist for at least seven days to prevent cracks and improve its hardness.
This can be done by sprinkling water on the surface or using gunny sacks to keep it damp. In colder weather, curing times may need to be extended.
Types Of Cement Plaster Finishes
Cement plaster offers several finish options, depending on the desired appearance and function:
- Smooth Cast: Achieved using a wooden float and a fine sand-cement mixture. It provides a smooth surface, ideal for exterior walls, but can be prone to cracking if overworked with a steel float.
- Roughcast: This finish includes sand, cement, and coarse aggregate. It’s applied by throwing the mixture onto the surface and then levelling it with a wooden float.
- Sand-Faced: A waterproof finish, often used on masonry, where the second coat is smoothed using a sponge to reduce the appearance of cracks.
- Scraped Finish: After the final coat has partially dried, it is scraped with a toothed tool to give the surface a textured look.
- Textured Finish: Special tools create decorative patterns or textures on the plaster surface.
Curing Process For Cement Plaster
Curing is vital in ensuring the plaster’s durability and crack resistance. After application, cement plaster requires at least 24 hours to set.
During this period, it must be kept moist. The surface should remain damp seven days after plastering to prevent cracks or surface damage.
In colder weather, additional time is needed for the plaster to cure properly.
Modern Plastering Equipment
Traditionally, plastering has been a labour-intensive process. However, modern plastering tools and machines have made it easier and more efficient.
Plastering machines can reduce labour costs, minimise material waste, and ensure more accurate application, particularly for large projects.
Benefits Of Plastering Machines
- Labour Cost Reduction: Machines reduce the need for manual labour, though skilled workers are still required for operation.
- Reduced Material Waste: With more precise application, there is less cement and mortar waste.
- Faster Application: Machines can cover large surfaces more quickly than manual methods.
- Cost Efficiency: Due to the speed and precision of machine application, large-scale projects benefit from reduced costs per square metre.
Conclusion
Cement plaster is a critical element in construction. It offers durability, weather resistance, and a smooth, aesthetically pleasing finish.
Whether applied manually or with modern equipment, it strengthens walls, protects against the elements, and prepares surfaces for painting or other finishes.
With proper preparation, application, and curing, cement plaster ensures a long-lasting, durable finish that enhances buildings’ structural integrity and appearance.
By using the correct techniques and materials, builders can maximise the benefits of cement plaster, ensuring that homes and other structures are visually appealing and structurally sound.
FAQs About Plastering
How Long Does Cement Plaster Take To Dry And Cure?
Cement plaster typically sets within a few hours but requires several days to cure fully. The drying time may vary depending on environmental conditions such as temperature and humidity.
Can Cement Plaster Be Reinforced Or Modified For Specific Applications?
Yes, cement plaster can be reinforced with fibres or additives to improve strength, flexibility, or resistance to cracking, depending on the project’s requirements.
Is Cement Plaster Compatible With Other Building Materials?
Yes, cement plaster is compatible with many substrates, including concrete, masonry, brick, and metal, making it suitable for various construction and renovation projects.
Can Cement Plaster Be Painted Or Finished In Different Ways?
Yes, once fully cured, cement plaster can be painted with suitable exterior or interior paints. It can also be textured or finished using decorative techniques to achieve the desired aesthetics.
How Do I Ensure The Quality Of Cement Plastering Work?
Hire experienced plasterers who follow proper preparation and application techniques to ensure quality. Regular maintenance and timely repairs can also help preserve the integrity and appearance of cement plaster surfaces.
