Lime plaster has been a primary building material for centuries, serving as a key component in construction from ancient times to the present day. Its flexibility, breathability and sustainability make it a favourite among modern builders and environmentalists.
Egyptians, Greeks, and Romans used lime-based plasters for both structural and decorative purposes. This blog explores everything you need to know about lime plaster in construction, including its history, types, advantages, disadvantages, and application methods.
What is Lime Plaster?
A building material called lime plaster is made by mixing lime, sand and water. It uses lime as the binding agent instead of cement. This mixture is applied to ceilings and other surfaces, where it hardens through a chemical reaction called carbonation, in which lime reacts with carbon dioxide in the air to form calcium carbonate, resulting in a strong, sustainable, and flexible protective layer.
Limestone is processed into hydrated lime, which, when combined with water, undergoes a chemical reaction that allows the material to harden over time. This material is used for lime plaster walls, both interior and exterior, due to its natural qualities that promote healthy building environments by regulating moisture and providing a long-lasting finish.
There are two basic types of lime used in plastering:
1. Non-hydraulic lime
It is made of pure limestone. It sets slowly by absorbing carbon dioxide from the air and is a flexible and highly breathable form of lime.
2. Hydraulic lime
It contains impurities like clay, causing it to set faster than non-hydraulic lime, even in damp conditions. It is usually used for exterior applications.
Components of Lime Plaster
1. Lime: It can be quicklime (calcium oxide) or hydrated lime (calcium hydroxide). When water is added to quicklime, it goes through a chemical reaction called slaking, creating hydrated lime. Hydrated lime is the form used in lime plaster.
2. Sand: It is mixed with lime to give the plaster strength and prevent shrinkage during the drying process. The quality of the sand affects the finish and durability of the plaster. For good bonding and durability, well-graded sharp sand is used.
3. Water: It is added to the lime and sand to create a workable plaster mix, and the amount of water depends on the specific application. However, it is important to control the moisture content to avoid cracking during the drying process.
4. Additives: Additives like pozzolans, which react with lime to form stronger bonds and pigments, can be added to improve the properties or appearance of the plaster.
How to Apply Lime Plaster?
Lime plaster application needs careful planning and attention to detail. The general steps are explained below.
1. Surface Preparation: The surface must be clean and free of any debris, dust, or loose particles. For old walls, the surface should be moistened to prevent plaster from drying too quickly.
2. Mixing: Limestone plaster is mixed on-site to ensure proper consistency. The lime, sand, and water are combined in a specific ratio and mixed properly until the desired texture is achieved. Additives, if needed, can be added at this stage.
3. First Coat (Scratch Coat): The first layer is usually around 5-7 mm thick and serves as a base for subsequent layers. The surface is scratched with a tool to create a rough texture that helps the next coat to adhere properly.
4. Second Coat (Brown Coat): Once the scratch coat has dried and carbonated slightly, the second layer is applied. It is thicker than the first and helps even out the surface. It is normally around 8-10 mm thick and should be left to dry thoroughly before applying the final layer.
5. Final Coat (Finish Coat): The final coat is applied to achieve the desired texture and appearance. This layer is thinner, and so for aesthetic preferences, we can smoothen or use textures.
6. Curing: After application, a long curing period is needed. The plaster should be kept moist to prevent cracking during the initial drying phase. Full carbonation can take several weeks or even months, depending on the environmental conditions.
Types of Lime Plaster
There are a few types of lime plaster, each with its special properties and uses.
1. Hydrated Lime Plaster
2. Cement Lime Plaster
3. Gypsum Lime Plaster
4. Lime Concrete Plaster
Let us briefly discuss these types of lime plaster.
1. Hydrated Lime Plaster
Hydrated lime plaster is made from slaked lime, which results when quicklime is mixed with water. This type of plaster is highly flexible and permeable, making it ideal for historic restorations and traditional buildings. The plaster will grow stronger over time because of its slow curing process.
2. Cement Lime Plaster
Both lime and cement act as binding agents in this mixture. Cement gives the plaster more strength and rigidity, while lime imparts flexibility and breathability. This type of plaster is suitable for areas that need additional durability, but cement lime plaster lacks the same permeability found in pure lime plaster.
3. Gypsum Lime Plaster
This plaster combines gypsum with lime, giving it faster-setting properties. It is commonly used for internal lime plaster wall finishes, especially where faster drying times are needed. Gypsum lime plaster is not suitable for damp environments because it is less permeable than traditional limestone plaster.
4. Lime Concrete Plaster
It is made by mixing lime with concrete, making the plaster strong and durable. It is often used in high-traffic areas where impact resistance is needed. Lime concrete plaster retains some breathability but is more rigid compared to pure lime plaster.
Lime Plaster Mix
The basic lime plaster mix includes lime, sand and water, and its proportion depends on the type and application of lime. The ratio is one part lime to three parts sand. The sand provides structure and texture, while the lime acts as a binding agent. Maintaining the correct mixing and setting times for lime and plaster before application is essential.
For exterior lime plaster, coarse sand is used to add durability and resistance to weather. For internal lime plaster, finer sand or aggregates are used for a smoother finish.
Advantages and Disadvantages of Lime Plaster in Construction
| Advantages | Disadvantages |
| Good breathability. It allows moisture and vapour to pass through, reducing the risk of trapped moisture which leads to mould growth or structural decay. | Longer drying time. Lime plasters take longer to dry and fully carbonate and can be a disadvantage for projects that are time-sensitive. |
| Highly flexible. It is less prone to cracking and is important in buildings subject to movement due to thermal expansion or settling. It can self-heal small cracks over time as it continues to absorb carbon dioxide and carbonate. | The initial cost is higher than cement-based plaster, particularly when using high-quality materials and skilled labour. However, long-term durability and low maintenance costs can offset the initial investments. |
| As it is obtained from natural limestone, it is environmentally friendly. Lime production emits less carbon compared to cement. It absorbs carbon dioxide during the curing process, reducing its environmental impact. | Lime plaster needs special knowledge and skills to apply correctly. It must be applied in thin coats, and the mixture must be carefully managed to prevent cracking or uneven drying. |
| More durable. If applied correctly, lime plaster can last for centuries. | Lime plaster often necessitates hiring experienced craftsmen, which can add to the cost of the project. |
| Lime plaster offers a smooth, matte finish that is aesthetically pleasing. It can be pigmented or textured to suit different design preferences. | Lime plaster is highly breathable, but this feature can make it unsuitable for environments where excessive moisture is present. For this, hydraulic lime plaster, which sets in wet conditions, can be used. |
Lime plaster continues to be a popular choice for construction projects because of its application in interior and exterior plaster of walls and for the restoration and conservation of historic buildings. Whether using hydrated lime plaster, cement lime plaster or gypsum lime plaster, lime offers flexibility, durability, breathability and an aesthetically pleasing finish.
Lime plastering cost can vary based on the type of lime used, location and difficulty of the project. Lime plastering cost is normally more than cement-based plastering due to high material costs and skilled labour requirements. Limestone plaster walls are easy to maintain and repair and can heal minor cracks on their own through the carbonation process. Lime-based materials for repair ensure compatibility with the original plaster. By choosing the right lime plaster mix, you can ensure beautiful and long-lasting results.