Concrete is a bit of a double whammy, because, typically, you are emitting CO2 during both the process of creating cement (to create high heat to drive oxygen off limestone), and then again when it cures.
Both contributions of the CO_2 emissions are from the production procedure: The CO_2 of the heat generation and CO_2 (not oxygen) being released from e.g. limestone CaCO_3 -> CaO + CO_2.
Concrete is a bit of a double whammy, because, typically, you are emitting CO2 during both the process of creating cement (to create high heat to drive oxygen off limestone), and then again when it cures.
Great source for information about concrete.
Both contributions of the CO_2 emissions are from the production procedure: The CO_2 of the heat generation and CO_2 (not oxygen) being released from e.g. limestone CaCO_3 -> CaO + CO_2.