Concrete seems pretty inoffensive. It just looks like mud, and appears to do nothing except sit there and harden. The fact is, though, concrete is the world's third-largest source of man-made carbon dioxide. Its production process accounts for at least 5% of the CO2 our species pumps into the atmosphere annually. Apparently, however, it doesn't have to be that way. Two companies are now using different technologies that not only make concrete carbon-neutral, they actually make it carbon-negative.