cement releases large amounts of CO2 when it is being produced, i.e. when the cement powder is being produced from limestone. this is due to a chemical reaction: CaCO3 (limestone) -> CaO (cement) + CO2

later, when you mix the cement with water and sand to make concrete, it re-absorbs (approx. 43% of) that CO2. you've got it backwards :D

curing reaction: CaO + CO2 -> CaCO3 (facilitated by water presence)

edit: ok i looked it up and concrete only absorbs about 43% of the CO2 that is emitted during cement production. Source