Natural Hydraulic Lime (NHL) is a high-quality lime binder primarily used in construction, particularly for historic building restoration and new eco-friendly builds, valued for its ability to set both with and without the presence of air.
Understanding Natural Hydraulic Lime
According to Lime Stuff, NATURAL HYDRAULIC LIME (NHL) is produced by burning and slaking limestone which is quarried with a proportion of silica and trace elements. This unique composition is what gives NHL its distinct characteristics and sets it apart from non-hydraulic limes.
The Hydraulic Set Explained
The term "hydraulic" refers to the ability of the lime to set and harden in the presence of water, even without exposure to air. This crucial property stems directly from its chemical makeup and processing:
- Reactive Elements: As explained by Lime Stuff, "It is the silica and trace elements that when burnt, become reactive with water and will cause an initial hydraulic set of the mortar when mixed with water."
- Setting Mechanism: Unlike non-hydraulic limes that rely on carbonation (a reaction with carbon dioxide from the air) to harden, NHL begins to harden hydraulically due to the chemical reaction of its silica and trace elements with water. This initial set provides early strength and allows it to be used effectively in damp or submerged environments.
Production Process
The production of Natural Hydraulic Lime involves several key steps:
- Quarrying: Limestone naturally containing a specific proportion of clay minerals, rich in silica and alumina, is extracted. The ratio of these impurities determines the hydraulic strength and classification (NHL 2, NHL 3.5, NHL 5) of the final product.
- Burning (Calcination): The quarried limestone is heated in kilns to temperatures typically ranging from 900°C to 1100°C. This process converts the calcium carbonate into calcium oxide (quicklime) and transforms the silica and other trace elements into reactive compounds.
- Slaking: The resulting quicklime is then carefully slaked with a precise amount of water. This process hydrates the quicklime while allowing the hydraulic components to remain reactive within the mixture, forming a stable powder.
Key Characteristics and Uses
NHL is highly regarded in construction for its numerous beneficial properties, making it suitable for a wide range of applications:
- Breathability: Allows moisture vapor to pass through the building fabric, preventing dampness and promoting healthy internal environments.
- Flexibility: Less rigid than cement, it can accommodate minor structural movements, significantly reducing the risk of cracking in masonry.
- Durability: Provides long-lasting performance, particularly in demanding conditions or for historical restoration where compatibility with original materials is vital.
- Environmental Friendliness: Generally has a lower carbon footprint during production compared to Portland cement, making it a more sustainable choice.
- Compatibility: Ideal for use with traditional building materials such as soft brick, natural stone, and timber frames, ensuring the integrity and longevity of heritage structures.
| Characteristic | Description | Benefit in Construction |
|---|---|---|
| Hydraulic Set | Hardens in the presence of water, independent of atmospheric carbonation. | Enables use in damp or wet conditions, offers faster initial strength and resilience. |
| Breathability | Allows moisture and vapor to pass through. | Prevents moisture entrapment, reducing issues like damp and frost damage. |
| Flexibility | Possesses a degree of elasticity. | Accommodates slight building movements, minimizing cracks and stress on masonry. |
| Durability | Forms a long-lasting, resilient bond. | Ensures longevity of structures, especially crucial for historic buildings and harsh climates. |
Example Applications of Natural Hydraulic Lime:
- Mortars: Used extensively for laying bricks, natural stone, and blocks, especially in conservation projects.
- Renders and Plasters: Applied as breathable internal and external wall finishes, contributing to thermal performance and moisture management.
- Grouts: Employed for consolidating masonry, filling voids, and repointing historic or sensitive structures.
For more detailed information on different types of lime and their properties, resources such as Lime Stuff can provide valuable insights.
