If I knew anything about chemistry, I would say that limestone is CaCO3. And that cement is produced by causing limestone to dissociate under heat into CaO and CO2. So much would agree with the statement above.OTOH if I knew anything about chemistry I would say that when cement sets, the reverse reaction occurs and CaO and CO2 from the atmosphere combine to form CaCO3. But that cant be right, because the article clearly indicates that using cement is a net addition of CO2 to the atmosphere.
Takes a bunch of natural gas to cook limestone into cement. I think I remember hearing about a cement plant fired with old tires, now that would be nasty.