Natural Hydraulic Lime
Made from limestone with natural impurities like clay, aluminates, and silica (often referred to as silicates), Natural Hydraulic Lime (NHL) gets its name from its chemical reaction when it comes into contact with water. It reacts to form compounds that harden over time. NHL sets in both air and water, unlike Non-hydraulic lime, which requires air exposure to carbonate. This makes NHL an excellent choice for structures exposed to moisture or water. Its breathable properties also make it ideal for restoring heritage buildings and sustainable construction, helping regulate indoor moisture levels and promoting healthy environments.
NHL Grades and Clay Content
The hydraulic properties of NHL are largely determined by the amount of clay and silicates naturally present in the limestone used to produce it. These impurities react with lime during the calcination process, creating compounds that allow the lime to set in the presence of water, even without exposure to air.
NHL is available in different grades, numbered to indicate the strength and hydraulic properties of the lime:
· NHL 2 (Low Hydraulicity): The most flexible and breathable of the hydraulic limes, suitable for applications in dry or less exposed conditions, such as internal plastering or mortar in low-stress environments. NHL 2 has a lower percentage of clay, typically around 5-15% clay content. More finely ground, with a smooth texture.
· NHL 3.5 (Medium Hydraulicity): A mid-range grade that strikes a balance between workability and strength with a 15-20% clay content. It is commonly used for both internal and external plastering, as well as for mortar in walls that will experience moderate exposure to moisture. NHL 3.5 has a faster setting rate than NHL 2 and is suitable for more exposed positions but still maintains a breathable and flexible quality.
· NHL 5 (High Hydraulicity): Containing the highest amount of clay and natural impurities, around 20-30%, it’s the strongest and most durable grade of natural hydraulic lime. Often used to withstand harsh conditions, in areas with high exposure to moisture, such as foundations, external masonry, structures in damp climates and even marine environments.
The Importance of Clay content
The clay content in NHL determines its strength and setting time.
More clay means:
Faster setting times, since the higher percentage of impurities in the limestone generates more reactive compounds.
Increased strength and durability, making the lime more suitable for exposure to moisture and harsh conditions (e.g., foundation work or coastal environments).
Reactive workability, as the more reactive the lime, the faster it can become hard and difficult to manipulate.