The process of making clinker uses heat to separate CaCO3 into CaO and CO2 [0]. This release of CO2 is normally about half the CO2 footprint of making concrete (the rest is mostly combustion of coal to generate heat).

CO2 can penetrate into the concrete and bond with CaO, over time reabsorbing all the CO2 released during the calcination process. In practice, CO2 doesn't penetrate deeply into concrete, so depending on the concrete type and environment something like 25% of the potential absorption of concrete is realized (this is based on a conversation I had with a colleague at work, the number is not exact).

[0] https://en.wikipedia.org/wiki/Calcium_carbonate