Concrete resorbs CO2 from the air after you pour it. This carbonation of concrete can cause corrosion to rebar or other re-inforcments. A large portion of that CO2 is eventually reabsorbed back by the concrete. Depending on the mix this process can adsorb 1/3 to 2/3rds of the CO2 produced during production. However, this is a slow process and we keep making new concrete.

https://en.wikipedia.org/wiki/Concrete_degradation#Carbonati...

Also there is the all energy used for processing concrete which a lot times is not from CO2 neutral source.