Hydraulic lime

Hydraulic lime is a variety of slaked lime used to make lime mortar. Hydraulicity is the ability of lime to set under water. Hydraulic lime is produced by heating calcining limestone that contains clay and other impurities. Calcium reacts in the kiln with the clay minerals to produce silicates that enable the lime to set without exposure to air. Any unreacted calcium is slaked to calcium hydroxide. Hydraulic lime is used for providing a faster initial set than ordinary lime in more extreme conditions (including under water).

Use in construction

Hydraulic lime is a useful building material for the following reasons:

• It has a low modulus of elasticity.
• There is no need for expansion (movement) joints.
• It allows buildings to "breathe", and does not trap moisture in the walls.
• It has a lower firing temperature than Portland cement, and is thus less polluting.
• Stone and brickwork bonded with lime is easier to re-use.
• Lime acts sacrificially in that it is weaker and breaks down more readily than the masonry, thus saving weaker stone such as sandstone and limestone from the harmful effects of temperature expansion and mortar freeze.
• It is less dense than cement, thus less cold bridging.
• Lime re-absorbs the carbon dioxide (CO₂) emitted by its calcination (firing), thus partially offsetting the large amount emitted during its manufacture. The more hydraulic a lime, the less CO₂ is reabsorbed during set, for example, 50% of CO₂ is reabsorbed by NHL 3.5 during the set, compared to 100% of CO₂ being reabsorbed by pure calcium hydroxide (fat lime putty).

Classification

Natural hydraulic lime (NHL) is classified for different uses:

Feebly hydraulic lime

Feebly hydraulic lime (NHL 2) is used for internal work and external work in sheltered areas.

Moderately hydraulic lime

Moderately hydraulic lime (NHL 3.5) can be used for external work in most areas.

Eminently hydraulic lime

Eminently hydraulic lime (NHL 5) is used for external work in exposed areas, such as chimneys and for floor slabs/underpinning.

External links

• The Technology and Use of Hydraulic Lime, by John Ashurst
• The National Lime Association (US & Canada)
• How to use hydraulic lime and PDF application fact sheets
• The Scottish Lime Centre

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