In the world of building materials, you may come across two types of lime: hydraulic and non-hydraulic lime. Understanding the differences between these two materials is crucial for choosing the right one for your construction or restoration project.
Hydraulic lime is made from impure limestone, which gives it the ability to set through hydrolysis, a reaction caused by water. This type of lime offers a faster initial set and greater compressive strength when compared to non-hydraulic lime. It is even capable of setting under water and is commonly used for exterior work, especially in exposed or damp conditions.
On the other hand, non-hydraulic lime, also known as air or putty lime, sets by carbonation, meaning it requires air to harden. It is often sold as a putty rather than a powder like hydraulic lime. While it may take longer to set and does not offer the same level of strength, non-hydraulic lime is a more breathable and flexible material, making it suitable for certain restoration projects and eco-builds.
Types of Lime
When working with lime in construction, you’ll come across two main types: hydraulic lime and non-hydraulic lime. Understanding their differences is important so you can choose the right material for your project.
Hydraulic lime sets by hydrolysis, which is a chemical reaction caused by water. This type of lime can set underwater and is known for its resistance to weathering, making it suitable for projects exposed to water and harsh environments. Hydraulic lime is typically sold as a powder and can be found in various degrees of strength:
- NHL 2: A low-strength hydraulic lime suitable for interior work, sheltered outdoor work, and conservation work
- NHL 3.5: Medium-strength hydraulic lime, commonly used for standard construction work, masonry, and plastering. It is suitable for moderately exposed areas.
- NHL 5: A high-density, strong, fast-setting hydraulic lime suitable for paving, roofing, bridges, and seafronts. This type of lime is the hardest but also has the least flexibility and breathability. It is suitable for very exposed areas.
Non-hydraulic lime, on the other hand, sets by carbonation, meaning it re-absorbs carbon dioxide from the air. Due to this, it requires exposure to air to carbonate and set. Non-hydraulic lime is softer and sets more slowly than hydraulic lime, as the carbonation process is slower. It is typically sold as a putty or pre-mixed mortar and is made from pure limestone, also known as calcium carbonate.
This type of lime remains softer for longer periods, making it more flexible and ideal for conservation and restoration work where flexibility and breathability are important factors. Remember that non-hydraulic lime should not be used in areas with severe water exposure or where rapid setting is needed.
When working with lime, choosing the appropriate material according to your project’s specific requirements is crucial. By understanding the properties and differences between hydraulic and non-hydraulic lime, you can make an informed decision contributing to your construction project’s success and longevity.
Composition and Properties
Hydraulic Lime
Hydraulic lime is produced by burning limestone that contains clay impurities. This type of lime can set under water due to a process called hydrolysis, which is a reaction caused by water.
It provides a faster initial set and greater compressive strength than non-hydraulic lime and can set in more extreme conditions, including underwater. There are various types of hydraulic lime which differ in their properties.
Some key features of hydraulic lime include:
- Hydraulic lime usually comes in 3 strengths: NHL 2, NHL 3.5 and NHL 5. The number denotes the compressive strength in N/mm². Therefore, a higher number means a stronger lime.
- The stronger the hydraulic lime, the less breathability and flexibility the finished mortar or plaster will have.
- It sets through hydrolysis and has a higher compressive strength than non-hydraulic lime.
Non-Hydraulic Lime
Non-hydraulic lime, also known as air lime, lime putty, hydrated lime or fat lime, is made from pure limestone. It sets through a process called carbonation, which involves the absorption of carbon dioxide from the air. As a result, non-hydraulic lime takes longer to set and possesses lower compressive strength than hydraulic lime. However, it is more flexible and breathable, which makes it suitable for a wide range of building applications.
Some key features of non-hydraulic lime include:
- It does not contain clay and is often referred to as ‘pure’ lime, air lime, or putty lime.
- It sets through carbonation and has a lower compressive strength than hydraulic lime.
- It is breathable and flexible, making it more suitable for use in plaster and other building materials.
In summary, both hydraulic and non-hydraulic limes offer unique properties for construction purposes. Hydraulic lime provides a faster set and higher compressive strength, making it suitable for more demanding applications. In contrast, non-hydraulic lime is known for its breathability, flexibility and suitability for a wide range of building applications.
Applications and Uses
In this section, you will learn about the various applications and uses of hydraulic and non-hydraulic lime. Each type of lime offers unique properties that make it suitable for specific construction needs.
Hydraulic Lime
Hydraulic lime is often used in areas where strength and water resistance are required. With its ability to set underwater and its faster setting speed, it is an ideal choice for masonry work, including:
- Pointing: When repointing bricks or stones, hydraulic lime mortar offers a secure bond, ensuring the longevity of the joint.
- Lime Renders: With its water resistance, hydraulic lime can be used in external renders to protect walls from moisture.
- Masonry repair: For repairs in older buildings, hydraulic lime is compatible with existing materials, and its breathability can prevent moisture damage in walls.
Hydraulic lime comes in different grades depending on its application, speed of set and strength. When choosing a hydraulic lime product for your project, consider factors like the location, exposure to moisture, and the desired finish.
Non-Hydraulic Lime
Non-hydraulic lime is more flexible and breathable than hydraulic lime, making it suitable for various applications in older buildings and restoration projects. Some of its uses include:
- Lime Mortar: Non-hydraulic lime putty mixed with aggregates creates a flexible lime mortar that allows moisture to evaporate and can accommodate the natural movement in older buildings.
- Lime Plasters: For internal walls and ceilings, non-hydraulic lime plasters provide a smooth finish and good breathability, which is essential for preventing damp.
- Hot Lime Mixes: By mixing quicklime with aggregates, hot lime mortars can be created, allowing for a faster set than traditional non-hydraulic mortars. This isn’t usually done, as it can be dangerous.
When using non-hydraulic lime, remember that it may require longer set times, and additives such as pozzolans may be needed if increased setting speeds are desired. Additionally, due to its weaker set, non-hydraulic lime may not be suitable for all structural elements or where significant moisture levels are present.
Advantages and Disadvantages
Hydraulic Lime
Hydraulic lime is a type of lime that sets through a chemical reaction with water. This process is known as hydraulicity. Hydraulic lime has several advantages and disadvantages when used in building projects:
Advantages:
- Strength: Hydraulic lime offers higher compressive strength, making it suitable for exterior work, such as brickwork and damp conditions.
- Durability: Eminently hydraulic lime is more durable than non-hydraulic limes, making it ideal for long-term applications.
- Setting Time: Hydraulic lime sets faster than non-hydraulic lime, which could mean reduced labour costs and less time spent waiting for it to harden.
Disadvantages:
- Moisture: Although hydraulic lime is more resistant to moisture, it can be less permeable, which means that it might not allow moisture to escape as freely, potentially trapping moisture inside the structure.
- Cost: Hydraulic lime can be more expensive than non-hydraulic lime, given its higher performance qualities.
- Less Flexibility: Hydraulic lime can be less flexible due to its chemical bonding, increasing the risk of cracks when used in weaker foundations.
Non-Hydraulic Lime
Non-hydraulic lime, also known as air lime or fat lime putty, relies on natural carbonation (exposure to air) for hardening. There are both advantages and disadvantages to using non-hydraulic lime for your building projects:
Advantages:
- Permeability: Non-hydraulic lime is highly permeable and breathable, allowing excess moisture to escape more freely than hydraulic lime. This is particularly beneficial when restoring older buildings or working with cob and other natural materials.
- Flexibility: Non-hydraulic lime mortar provides a softer, more flexible bond, accommodating small movements in weaker structures and reducing the likelihood of cracks.
- Compatibility: Non-hydraulic lime is compatible with almost all building materials used today. Additionally, due to its breathability and flexibility, it is perfect for use when constructing eco-homes which use other natural materials.
Disadvantages:
- Setting Time: Non-hydraulic lime takes longer to set than hydraulic lime, possibly increasing labour costs and the length of the overall project.
- Weakness: Non-hydraulic lime has lower compressive strength compared to hydraulic lime, which might not be adequate for certain heavy-duty applications.
- Susceptibility to Rain: Non-hydraulic lime is more likely to be damaged by rain or wet conditions before it has fully set, potentially causing delays in your building project.
Conclusion
In conclusion, when choosing the right lime for your construction or restoration project, it is important to understand the differences between hydraulic and non-hydraulic lime. Hydraulic lime offers a faster initial set and greater compressive strength, making it suitable for exterior work and harsh environments.
On the other hand, non-hydraulic lime is more breathable and flexible, making it a better option for restoration projects where flexibility and breathability are important factors. Both types of lime have unique advantages and disadvantages, and choosing the appropriate material according to your project’s specific requirements is crucial for its success and longevity.
