A good plaster mix is primarily defined by the ratio of binding material (like cement or lime) to sand, which varies depending on its intended use.
Achieving the correct mix ratio is crucial for the strength, durability, and workability of plaster. The ideal ratio ensures the plaster adheres properly, resists cracking, and provides a suitable finish for its specific application, whether indoors or outdoors, or for repair work.
Understanding Plaster Mix Ratios
The ratio typically represents the proportion of binder to sand, for example, 1 part binder to 3 parts sand (written as 1:3). Sometimes, lime may also be included, affecting the ratio further (e.g., cement:lime:sand). The provided reference suggests specific ratios are broadly used for different purposes:
- Rich Mortar (1:3): Suitable for repair work and filling cracks. This higher proportion of binder provides greater strength for patching and structural repairs.
- External Plastering (1:4): Recommended for exterior walls. This mix offers a good balance of strength and weather resistance.
- Internal Plaster Work (1:5 and 1:6): Used for interior walls. These mixes are less 'rich' as internal plaster doesn't require the same level of strength or weather resistance as external plaster, making them more cost-effective and easier to work with for large interior surfaces.
Recommended Mix Ratios
Based on common practices and the provided reference, here is a breakdown of suggested plaster mix ratios for various applications:
| Application | Recommended Ratio (Binder:Sand) | Notes |
|---|---|---|
| Repair Work | 1:3 | Also used for filling cracks. Rich mix. |
| Filling Cracks | 1:3 | Strong mix for detailed or structural needs. |
| External Plastering | 1:4 | Durable mix for exterior walls. |
| Internal Plastering | 1:5 | General interior wall finish. |
| Internal Plastering | 1:6 | Suitable for less demanding interior surfaces. |
Note: The "binder" is typically cement, or a mix of cement and lime, depending on traditional practices and desired properties (e.g., breathability with lime). The consistency or "right consistency" is also a common query among those new to plastering, as mentioned, highlighting that getting the water content right is key for workability after the correct dry ratio is measured.
Choosing the appropriate ratio ensures the plaster performs correctly for its intended environment and function. Using a ratio that is too rich (too much binder) can lead to cracking, while a mix that is too lean (too much sand) can result in poor adhesion and a weak finish.
