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).

Hydraulic Lime 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 movement joints,
• It allows buildings to "breathe",
• It has a lower firing temperature than Portland cement, and produces less CO₂ than ordinary lime, and so is less polluting,
• Brickwork bonded with lime is easier to re-use,
• It is less dense than cement, thus less cold bridging,
• Lime re-absorbs CO₂, thus partially offsetting the large amount emitted during its manufacture.

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.

References

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