Quicklime is burnt lime — calcium oxide (CaO) — that has not yet been mixed with water. It is the intermediate product in the lime cycle: limestone is burnt to quicklime, and quicklime is then slaked with water to produce slaked lime (calcium hydroxide), the binder for lime mortar and limewash. It is also called burnt lime or lump lime, after the lumps in which it leaves the kiln.
The lime cycle
- Burning — limestone (calcium carbonate) is heated in a lime kiln; carbon dioxide is driven off and quicklime remains
- Slaking — water is added and the lime breaks down into slaked lime. The reaction is strongly exothermic: the mix becomes boiling hot and spits
- Carbonation — in the mortar or wash, the slaked lime reabsorbs carbon dioxide from the air and slowly hardens back into calcium carbonate
The cycle closes: what came out of the stone becomes stone again in the wall.
Applications
- Raw material for lime mortar and limewash, once slaked. In conservation work, lime putty slaked from lump lime is valued for its plasticity and fineness
- Soil stabilisation in civil engineering: quicklime draws water out of wet clay, making the ground immediately more workable and better bearing
- Industrial use in flue gas treatment, steelmaking and water treatment, among others
Safety — this is not a benign material
Quicklime genuinely demands precautions:
- Reacts violently with water, including the moisture on skin, in eyes and in the airways. The reaction releases a great deal of heat
- Strongly caustic. Eye contact can cause permanent damage; on damp skin it produces burns that are often only felt some time later
- Always wear close-fitting goggles or a face shield, impermeable gloves, covering clothing and a dust mask
- Add lime to water, never water to lime, and stand back during slaking
- Store dry and sealed. Quicklime draws moisture from the air; an opened bag in a damp space can heat up
If in any doubt about handling, read the supplier’s safety data sheet before use.
Related terms
- Slaked lime
- Lime kiln
- Lime slaking
- Lime putty
- Lime mortar
- Air lime
- Limestone
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