Limestone/chalk

• Chemical formula: CaCO₃ (calcium carbonate)
• Molecular Mass: 100.09
• Decomposes when calcined at 900°C to form calcium oxide

Calcining (or burning)

Burning limestone/chalk occurs at roughly 1000°C. The reaction for the thermal decomposition of calcium carbonate is as follows:

CaCO₃ + heat ↔CaO + CO₂
      100g                   56g      44g      

This chemical reaction produces quicklime. Different kilns are used for different types of limestone/chalk and for different types of product (see 'How Lime is made' for more detail).

Quicklime

• Chemical formula: CaO
• Molecular Mass: 56.08
• Melts at 2614°C, boils at 2850°C
• Hydration (or slaking)

Quicklime's principal component is calcium oxide. Its quality often depends on a number of certain factors including physical properties, reactivity to water and chemical composition. As the most readily available and inexpensive alkali, quicklime plays an essential part in a wide range of industrial processes.

Hydrated Lime

• Chemical formula: Ca(OH)₂
• Molecular Mass: 74.09
• Decomposes at 580°C to form calcium oxide

Adding water to quicklime produces an exothermic reaction (gives out heat) and hydrated lime.

The reaction for the hydration of quicklime is as follows:

CaO + H₂0 = Ca(OH)₂ + heat

Hydrating plants are relatively complex and can be fed with surplus grades of quicklime. Hydrated lime, despite its name is essentially dry and generally contains less than 1% of unreacted water. An estimated 10% to 15% of quicklime produced in developed countries is converted into hydrated lime (a percentage which is probably larger for countries which do not have a large steel industry). Slaking lime involves the production of a dispersion of calcium hydroxide in water, creating a product known as milk of lime or lime putty.

Recarbonation

The recarbonation process is essentially the opposite of the calcining/burning process. Both quicklime and hydrated lime, when exposed to the air for long periods, begin to draw in carbon dioxide from the atmosphere. This therefore replaces the oxide component of the chemical and turns the lime roughly back to its original state – being limestone or chalk.

The formula is as follows:

Ca(OH)₂ + CO₂ = CaCO₃ + H₂O

Recarbonation can occur in a number of lime based products and in many cases acts as a benefit. For example, when the process occurs in lime mortar it effectively increases the strength of the final bond and acts as a self healing solution if any cracking occurs.