Lime is made from limestone through a two-step process involving calcination and slaking.
Step 1: Calcination (Burning)
The first step involves heating limestone (calcium carbonate, CaCO3) at a high temperature, typically around 900°C (1650°F). This process, called calcination, drives off carbon dioxide (CO2) and produces quicklime (calcium oxide, CaO). The chemical equation for this reaction is:
CaCO3 (limestone) + Heat → CaO (quicklime) + CO2 (carbon dioxide)
This burning or roasting process is usually carried out in kilns. The resulting quicklime is a highly reactive substance.
Step 2: Slaking (Hydration)
The second step is slaking, which involves adding water (H2O) to the quicklime (CaO). This process creates slaked lime (calcium hydroxide, Ca(OH)2), also known as hydrated lime. The chemical equation is:
CaO (quicklime) + H2O (water) → Ca(OH)2 (slaked lime) + Heat
This reaction is exothermic, meaning it generates heat. The resulting slaked lime is a paste or powder, depending on the amount of water used. The type of lime produced (hydraulic or non-hydraulic) depends on the presence of impurities, particularly clay minerals, in the original limestone. If minimal clay is present, the resulting lime is non-hydraulic, meaning it only sets and hardens through carbonation (reaction with carbon dioxide in the air).
In summary, making lime from limestone requires first burning the limestone to create quicklime, and then adding water to the quicklime to create slaked lime.
