What is the difference between quicklime and hydrated lime?
Quicklime is calcium oxide (CaO), made by heating limestone until it releases carbon dioxide. Hydrated lime is calcium hydroxide (Ca(OH)₂), made by adding a controlled amount of water to quicklime. Quicklime is more concentrated and highly reactive; hydrated lime is a dry, fine powder that is easier and safer to handle.
How are they made?
Limestone is calcined in a kiln at roughly 900–1,100 °C:
Quicklime is then slaked with water in a hydrator. The reaction releases a lot of heat:
Roughly 1 tonne of quicklime produces about 1.3 tonnes of hydrated lime, because water is chemically bound into the product.
Side-by-side comparison
| Property | Quicklime (CaO) | Hydrated lime (Ca(OH)₂) |
|---|---|---|
| Form | Lumps, pebbles or ground powder | Fine dry powder |
| Reactivity with water | Strong, exothermic | Already reacted |
| Lime concentration | Higher per tonne | Lower per tonne (contains bound water) |
| Handling | Must be kept dry; heat risk with water | Easier to store, dose and mix |
| Typical packaging | 50 kg bags, FIBC, bulk | 25 kg bags, FIBC |
| Shelf life | Picks up moisture and CO₂ if exposed | Slowly carbonates in air if exposed |
Which specifications matter?
Where is each one used?
| Industry | Quicklime | Hydrated lime |
|---|---|---|
| Steel | Flux to remove silica, phosphorus and sulphur | — |
| Water treatment | Softening, pH correction (large plants) | pH control, softening, coagulation aid |
| Flue gas treatment | Wet scrubbing slurries | Dry injection to capture SO₂ and HCl |
| Construction | Soil stabilisation, aerated concrete | Mortars, renders, lime plasters, asphalt anti-stripping |
| Chemicals | PCC, calcium carbide, sugar refining | Calcium salts, neutralisation |
| Mining | Gold and copper processing pH control | Tailings neutralisation |
| Agriculture | — | Soil pH correction |
How should lime be handled safely?
Both products are alkaline and can irritate skin, eyes and lungs. Quicklime reacts violently with water and can cause burns. Use eye protection, gloves and dust masks, keep quicklime away from water and moisture, and store both in dry, covered areas. Always follow the supplier's SDS.