When calcium oxide (chemical formula: CaO) reacts with water (chemical formula: H2O), the following reaction takes place:
`CaO + H_2O -> Ca(OH)_2`
The product of this reaction is calcium hydroxide, also known as slaked lime.
Thus, when calcium oxide reacts with water, slaked lime is produced.
Calcium oxide is also known as lime and is most commonly used for many purposes. It can be used for pH correction of water or for its disinfection (with excess lime). More importantly, lime has been used for a very long time to produce calcium carbonate by reaction with carbon dioxide. Calcium carbonate has been used as mortar for sealing the gaps between bricks, stones, etc.
The reaction for the production of calcium carbonate is:
CaO + CO2 -> CaCO3
In reality the reaction is a two-step process, with formation of slaked lime in the first step and reaction of slaked lime with carbon dioxide to form calcium carbonate in the second.
Hope this helps.