Slaked lime (calcium hydroxide, Ca(OH)₂) is the lime produced when quicklime is slaked with water, and which is worked in construction into lime mortar, plaster and limewash. It is a centuries-old binder, used since Roman times and still indispensable today for work on old and listed buildings.
From limestone to slaked lime
The lime cycle runs in stages:
- Burning — limestone (calcium carbonate, CaCO₃) is heated, releasing CO₂ and leaving quicklime (burnt lime, CaO)
- Slaking — water is added to the quicklime; with strong heat release, slaked lime (Ca(OH)₂) forms
- Setting — worked slaked lime slowly takes up CO₂ from the air and turns back into limestone (carbonation)
That last step is what makes lime special: the cycle closes, and the mortar reverts to its original form.
Difference from quicklime
| Property | Quicklime (CaO) | Slaked lime (Ca(OH)₂) |
|---|---|---|
| State | Lumps or powder, aggressive | Powder or lime putty |
| Reaction with water | Violent, lots of heat | Already slaked, stable |
| Ready to use | No | Yes |
Quicklime is caustic and must be slaked first; slaked lime is the workable form.
Application
Slaked lime is used for:
- Lime mortar — for bricklaying, especially on historic work
- Plaster — vapour-open, breathable interior and exterior render
- Limewash — a matt, breathable finish
- Pointing — for old walls that cannot tolerate hard cement
Lime sets slowly and stays slightly flexible, so it absorbs cracking and lets moisture escape. That is why it suits heritage buildings so well.
Related terms
- Lime
- Lime mortar
- Mortar
- Bricklaying
- Plaster
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