There are 2 stages used to create lime mortar - carbonation and slaking:
Carbonation: Lime mortar is created by burning limestone or chalk with coal in process called carbonation. This forms calcium oxide - more commonly known as quicklime. During this process calcium dioxide is 'burnt-off', releasing itself as a gas whilst the end product (quicklime) clumps together to form lump lime.
Slaking: The quicklime is added to water, forming slaked lime (calcium hydroxide). It is then mixed with fine aggregates, such as sand or local aggrate to form mortars, render or plasters.
When lime mortar sets it slowly combines with carbon dioxide in the atmosphere to transform back into its original state - calcium carbonate. Traditionally, this could take months to set and meant a long delay before decorations could be applied.
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