Air lime is lime that hardens only in contact with air: it absorbs carbon dioxide and slowly turns back into limestone. That process is called carbonation. Without access to air, air lime does not set — hence the name.
In the European standard (EN 459-1) air lime is designated CL (calcium lime) followed by its content, for example CL 90. Its counterpart is hydraulic lime (NHL), which sets under water and without access to air.
The lime cycle
- Burning — limestone is burned at high temperature to quicklime
- Slaking — with water it becomes slaked lime: hydrated lime as a powder, or lime putty when slaked with excess water
- Using — mixed with sand and water into mortar or plaster
- Carbonation — the lime takes up CO2 and becomes limestone again
That last step works from the outside inwards and is slow: days to weeks for a surface coat, months to years for a thick mortar joint.
Properties
| Property | What it means in practice |
|---|---|
| Vapour-open | Moisture can dry outwards — essential in old, damp masonry |
| Soft and flexible | Accommodates small movements without cracking; the joint takes the damage instead of the brick |
| Self-healing | Fine hairline cracks can partly close as dissolved lime recrystallises |
| Low strength | Not suitable as a structural or heavily loaded mortar |
| Slow | Demands patience and aftercare |
Where it is used
- Restoration of historic masonry, where cement mortar would crush the soft old brick
- Lime plaster and lime finishes indoors, with a characteristically matt, lively surface
- Limewash on mineral substrates
- Damp rooms and cellars, for its high alkalinity, which discourages mould, and its vapour openness
Practical tips
- Do not work in full sun or wind — the coat then dries faster than it carbonates and ends up dusty
- Aftercare — keep the work damp for a few days, for example with a fine mist
- Never in frost — work above a few degrees and protect the work from frost for several weeks
- Dampen the substrate — a dry, absorbent background pulls the mixing water out and the lime will not bond
- Protect yourself — lime is strongly alkaline and attacks skin and eyes. Gloves and goggles are essential
- Never seal it in — a cement-based or plastic-modified coating over lime work destroys the vapour openness and simply moves the moisture problem
Related terms
- Lime
- Lime mortar
- Lime putty
- Limestone
- Carbonation
- Sand-lime brick
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.