[mchannon]

It doesn't bind CO2 from air during setting- that's the disconnect.

By converting the carbonate to oxide, the calcium in cement is free to bond with silicates (and to a lesser extent other compounds).

That CO2 released during manufacture is now looking for a new home, which could either be a plant, or a weathering deposit of limestone which could use the CO2 in the air to convert its own carbonate to stable bicarbonate.