Lime concrete is a natural and eco-friendly construction material. It has been used for centuries, and before the invention of modern cement, people used lime, sand, and gravel to construct durable and breathable buildings. The concern for the environment and the conservation of old buildings is increasing. So, again, lime is gaining popularity.
In this blog, we will guide you through everything you need to know about lime concrete, including what it is, how it works, its benefits, and its limitations.
What is Lime Concrete?
Lime concrete is a traditional type of concrete where lime is used as the primary binding material instead of modern-day cement. It’s made by mixing slaked lime (calcium hydroxide), sand, coarse aggregates like gravel or crushed stone, and water. Unlike cement-based concrete, which sets through hydration, lime concrete hardens through a process called carbonation, where lime reacts with carbon dioxide in the air to gradually form calcium carbonate.
The lime used in this concrete is produced by heating limestone to create quicklime (calcium oxide), which is then combined with water to form slaked lime. This method has been used for centuries, especially in historical and heritage structures, thanks to lime’s natural ability to “breathe.”
One of the most notable benefits of lime concrete is its breathability—it allows moisture to escape from within the structure. This helps prevent dampness, mold growth, and decay, making it an excellent choice for walls, domes, vaults, and floors, particularly in hot or humid climates. It’s also more environmentally friendly than cement-based alternatives, as lime production emits less carbon dioxide and partially reabsorbs CO₂ during the setting process.
Key Components of Lime Concrete
Lime concrete consists of four significant ingredients,
1. Lime: It is the primary binder. It keeps the rest of the materials together after they have set.
2. Sand: It is a fine substance. It fills the spaces and voids between large aggregate pieces, helping to increase toughness.
3. Gravel Fill or Crushed Stone: These large, coarse materials make the concrete harder.
4. Water: Water combines with the lime and aids in mixing and placing the concrete.
Every material should be clean and free from dirt or organic matter, as these can weaken the mix and affect its quality.
Proportioning of Lime Concrete
There should be a suitable mix ratio for high-quality concrete. The most widely used proportions in lime concrete are
- Lime to Sand Ratio: 1:2
- Lime to Coarse Aggregate (Gravel) Ratio: 1:3
The ratios may be different depending on the demands of the construction work.
Keep in mind that the type of lime used—fat lime, hydraulic lime, or poor lime significantly influences the mix proportion, as well as the setting time, strength, and workability of the concrete. Therefore, the mix ratio may need to be adjusted to achieve the optimal performance for a specific application.
Types of Lime Used in Lime Concrete
Three basic types of lime are used in construction. They are poor lime, hydraulic lime, and fat lime. Each one possesses unique features and is used for a specific purpose.
1. Fat Lime
Fat lime, or the white lime or rich lime, is the purest. Fat lime contains a very small amount of impurities, typically less than five percent. When water is added to it, it reacts rapidly. It also expands to about two or three times its original size.
Fat lime takes a long time to set and becomes smooth and sticky. Due to this, it is used primarily for whitewashing and plastering. Fat lime is extremely white and gives a fine finish on walls.
2. Hydraulic Lime
Hydraulic lime has a higher amount of impurities, typically 20-30 percent. Hydraulic lime is unique because it can be set even if submerged in water or moist conditions. It takes longer to react than fat lime, but sets very quickly and hardens.
Hydraulic lime is typically used in lime concrete, on foundations, and in areas with abundant water. It is used in construction where strong and long-lasting structures are needed.
3. Poor Lime
Poor or lean lime has a high amount of impurities, more than 30 percent. Poor lime does not gain much when mixed with water and hardens very slowly. It is not strong at all and does not form a hard bond. For this reason, poor lime is never used in important building projects.
Poor lime is used only in low-budget or temporary buildings. Poor lime will be brownish-white and won’t give a smooth finish like fat lime.
Pros and Cons of Lime Concrete
Pros:
- Environmentally Friendly: Lime absorbs carbon dioxide when setting. It reduces its effect on the environment.
- Breathable: It allows water vapour to pass through. Thus, dampness in walls and flooring will be reduced.
- Flexible: Unlike hard, brittle cement concrete, it can handle minor movements without cracking.
- Long-lasting: Buildings constructed using lime can last hundreds of years.
- Antiseptic Nature: Lime possesses weak antibacterial properties, which dissuade mould formation.
Cons:
- Slow Setting Time: Lime concrete has a slower setting and hardening time, usually days or weeks.
- Lower Strength: Less strong. Thus, it is not suitable for heavy loads or tall buildings.
- Regular Maintenance: It may need occasional recoating or restoration of the surface in extreme weather.
- Skill Required: Everyone working now is not skilled at working with lime. Thus, experience is needed.
Applications of Lime Concrete
Lime concrete is used for numerous applications, primarily where the new cement would be hard to place.
- Historical and Heritage Buildings: Lime concrete is used in restoration as it is the same as the original material and facilitates the wall’s breathing.
- Ecological Houses: Lime concrete can be used in eco-friendly building construction, such as earth houses or solar passive houses.
- Underfloor Spaces: Lime concrete is used as a substrate for limecrete floors, ensuring they remain dry and mould-free.
- Domes, Vaults, and Arches: Lime concrete is used in circular or irregular forms due to its flexibility before setting.
- Moisture-Prone Areas: Lime concrete is applied to basements or rising damp areas with natural insulation.
Properties of Lime Concrete
Lime concrete offers several beneficial properties that make it a preferred choice for certain construction applications.
- Workability: Lime concrete is easy to spread and shape before setting, which makes it suitable for intricate designs and detailed work, especially in restoration projects.
- Permeability: The material’s permeability allows water to evaporate through the surface, preventing the buildup of trapped moisture. This helps in avoiding dampness, especially in old or heritage buildings where moisture control is critical.
- Thermal Mass: Lime concrete has excellent thermal mass, meaning it can absorb and store heat, gradually releasing it over time. This helps regulate indoor temperatures, improving energy efficiency.
- Aesthetic Appeal: It provides a natural, earthy appearance, which makes it ideal for traditional or eco-friendly buildings, giving a rustic charm.
- Shrinkage in Concrete Resistance: Lime concrete is more flexible than cement-based concrete and is less prone to cracking during curing, ensuring long-term durability.
How to Make Lime Concrete?
Proceed as follows to prepare and use lime concrete,
1. Slake the Lime
When using lime concrete, mix lime with water to form slaked lime before adding other materials. Allow it to settle until it cools and forms a paste.
2. Prepare the Mix
Mix sand, lime, and gravel in the given proportion. Use a clean vessel and utensil.
3. Add Water
Add water to the dry mix gradually, mixing well until the mixture forms a thick, uniform paste.
4. Pour the Concrete
Pour the mix at the point where it is needed. Level it and spread it using hand tools.
5. Compaction and Curing
Gently compact the mix to remove air gaps. Keep the surface moist for at least 7-14 days for efficient carbonation.
Conclusion
Lime concrete is a traditional construction material that is experiencing a revival in modern sustainable building practices. Its exceptional air permeability, flexibility, and minimal environmental footprint make it the ideal choice for anyone looking to construct naturally, restore existing buildings, or utilise sustainable materials.
Proper technique and attention make lime concrete a durable and unprocessed foundation.
Apoorva H P Achar