Essentially this process is the opposite to that of the calcining process (see ‘How Lime is made’ and/or ‘Life Cycle’). Both quicklime and hydrated lime when exposed to the air 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 (Back to calcium carbonate, this is sometimes known as precipitated calcium carbonate or PCC)
Re-carbonation occurs routinely, but to a very limited extent (<0.5% by weight of H2O and <0.003% of CO2) in the cooling zone of all lime kilns but generally doesn’t cause any adverse effects. It does, however, occur to a much greater extent when quicklime is exposed to the atmosphere for excessive periods, known as ‘air slaking.’
The same process occurs for hydrated lime when it is added to a mixture for specific construction products, such as mortar. In fact, re-carbonation in mortar provides the mechanism for self healing as well as long term strength development. This self healing concept is a particularly important aspect regarding the sustainability of building products. Should hairline cracks develop in the mortar, the combination of hydrated lime, moisture and carbon dioxide from the air help to seal the crack by the formation of limestone (calcium carbonate). The crystals formed by this process then help to plug the cracks.
Recarbonation is a particularly important issue as the energy produced from non-renewable sources consumed in building services currently accounts for approximately half of the UK’s emissions of carbon dioxide. The use of lime in mortar not only enhances the performance of the product, but it also helps in the ever increasingly important fight against climate change.