Calcium hydroxide

The calcium oxide (CaO) obtained in the calcination of the limestone reacts immediately with water, transforming into calcium hydroxide (Ca (OH) 2). This phenomenon is known as hydration or quenching of quicklime (CaO).

Calcium hydroxide occupies a volume, approximately, 20% greater than that corresponding to the original calcium oxide, which produces an expansive effect, in such a way that a lump of quicklime is transformed into powdered hydrated lime or a paste more or less consistent according to the amount of water used for quenching.

Modern industrial manufacturing processes of slaked lime achieve total slaking of quicklime.
As additional information, calcium hydroxide absorbs carbon dioxide from the air (re-carbonation) to form calcium carbonate (which is a common material in nature), thus closing the so-called Lime Cycle.