[JoeFromSidney]

Its true that to make cement you bake the carbon dioxide out of calcium carbonate. That adds carbon dioxide to the Air. However, that's only half the story. When the cement sets, it draws carbon dioxide out of the air, forming calcium carbonate again. Net contribution of carbon dioxide to the air is zero.

[Nateman]

Would not only the top layer draw any carbon? Just like a pan is protected from further oxidation by a thin oxide layer that forms?