For optimal performance and durability, the ideal sand for lime plaster is a washed, sharp sand with angular grains. This type of aggregate ensures superior bonding qualities and contributes significantly to the strength and longevity of the plaster. While sand and grit are the most common aggregates, the quality and characteristics of the sand are paramount.
Essential Qualities of Sand for Lime Plaster
Selecting the right sand involves considering several key attributes that directly impact the plaster's workability, strength, and resistance to cracking.
1. Purity (Washed Sand)
- Cleanliness is Crucial: Sand must be clean and free from impurities such as silt, clay, organic matter, and soluble salts. These contaminants can weaken the plaster, inhibit proper curing, lead to efflorescence (white salt deposits on the surface), or cause aesthetic issues.
- Washed Sand: Opting for "washed" sand ensures that fine particles and undesirable elements have been removed, leaving a cleaner, more reliable aggregate.
2. Particle Shape (Sharp, Angular Grains)
- Mechanical Key: Angular grains create a strong mechanical bond within the lime matrix, providing excellent 'key' or interlocking, which is vital for the plaster's structural integrity. This is often described as "sharp" sand.
- Reduced Shrinkage: The irregular shapes of angular grains allow for a higher volume of aggregate with less water, leading to reduced drying shrinkage and a lower likelihood of cracking.
- Workability: While rounded sands might seem easier to work with initially, they require more water for a given consistency, which ultimately weakens the final plaster.
3. Grading (Particle Size Distribution)
- Well-Graded Sand: The sand should have a good range of particle sizes, from coarse to fine. A well-graded sand fills voids efficiently, creating a dense, strong, and durable plaster with reduced permeability.
- Minimizing Voids: A mix of different sized particles minimizes the air voids within the mortar, leading to a denser mix that requires less binder and is less prone to cracking.
- Table: Typical Sand Grading for Lime Plaster (Sieve Analysis)
| Sieve Size (BS/EN Standard) | Typical % Passing | Purpose |
|---|---|---|
| 4.75 mm (No. 4) | 95–100% | Coarse aggregate for bulk and strength |
| 2.36 mm (No. 8) | 80–100% | |
| 1.18 mm (No. 16) | 50–90% | Main body of aggregate |
| 0.600 mm (No. 30) | 25–65% | |
| 0.300 mm (No. 50) | 10–40% | Finer particles for workability and density |
| 0.150 mm (No. 100) | 0–15% | |
| < 0.075 mm (Silt/Clay) | < 3% | Minimal to avoid weakening |
Note: Specific grading requirements can vary based on the type of lime, application, and desired finish.
Types of Sand Commonly Used
- Sharp Sand / Washed Concrete Sand: These are often excellent choices, typically consisting of angular, well-graded particles suitable for base and scratch coats.
- Pit Sand: Sourced from quarries, pit sands can be highly variable. It's crucial to ensure they are washed and contain sufficient angular grains and a good grading.
- River Sand: While often clean, river sands can sometimes be too rounded. If it's sharp and well-graded, it can be suitable.
- Crushed Rock Fines: Crushed rock can produce angular particles, but it's essential to verify the grading and ensure there are no excessive fines or dust.
Practical Considerations for Sand Selection
When choosing sand for your lime plaster project, consider these practical aspects:
- Application Layer:
- Base Coats (Scratch & Float): Generally benefit from coarser, well-graded sand to provide bulk, strength, and a good key for subsequent layers.
- Finish Coats: Often require a slightly finer, but still well-graded, sand for a smoother aesthetic finish and ease of application, without sacrificing durability.
- Local Availability: While adhering to quality standards, utilizing locally sourced sand can be cost-effective and environmentally friendly. Always test local sands for suitability.
- Testing Sand Quality: Simple field tests can help assess sand quality:
- Jar Test: Mix sand with water in a jar and let it settle. Layers of silt and clay will be visible above the sand, indicating impurity levels.
- Rub Test: Rub a small amount of damp sand between your palms. If it leaves a fine, dusty residue, it may contain too much silt or clay.
Why the Right Sand Matters
The aggregate constitutes a significant proportion (often 2-3 parts sand to 1 part lime) of the lime plaster mix. Its quality profoundly impacts:
- Durability and Strength: Directly influences the long-term performance and resistance to environmental factors.
- Workability: Affects how easily the plaster can be mixed, applied, and finished.
- Shrinkage and Cracking: Poorly graded or impure sand can lead to excessive drying shrinkage and the formation of cracks.
- Aesthetics: The color and texture of the sand can significantly influence the final appearance of the plaster, from a smooth render to a textured finish.
