Cement, the glue that allows concrete to harden, is responsible for 5% of global greenhouse gas emissions. Because it requires limestone to be heated at very high temperatures, cement production emits about 800 kilos of carbon for every 1,000 kilos of cement it yields. Ferrock aims to make cement obsolete by recycling steel dust. Not only is Ferrock harder than cement, but it absorbs more carbon from the air in its hardening phase than was emitted during its production process. In other words, it is carbon-negative. The process was first discovered in 2002 by Ph.D. student David Stone. After many years of refinement, he is finally bringing it to market through his newly minted company Iron Shell.