Cement, the essential ingredient of concrete, is responsible for 7% of global man-made greenhouse emissions. A Canadian startup has invented a new system for making concrete that traps CO2 emissions forever and at the same time reduces the need for cement. CarbonCure's system takes captured CO2 and injects it into concrete as it's being mixed. Once the concrete hardens, that carbon is sequestered forever. Even if the building is torn down, the carbon stays put. That's because it reacts with the concrete and becomes a mineral.