4.4 billion tons of concrete is manufactured worldwide, 142 which generates 7-8% of total human CO2 emissions. Concrete is made from water, cement and rocks or sand (aggregate). Cement is prepared from limestone (calcium carbonate, CaCO3), silica (SiO2), iron ore, furnace slag, and clay or slate. Heating the mixture at high temperature (1800 °C) drives off carbon dioxide to generate calcium oxide and calcium silicates. 60% of CO2 emissions in concrete manufacture come from the decarbonation of limestone and the remaining 40% comes from the energy needed to make the cement. The calcium salts in powdered cement react with water at the time of use to form a paste containing calcium hydroxides and silicates which adhere to the aggregate and bind it into a single mass. Over time, the calcium hydroxides in concrete absorb carbon dioxide from the atmosphere, forming more stable calcium carbonates which strengthen the concrete in the weeks after installation and over the service life of the structure. Between 10-30% of CO2 emitted during cement manufacture is reabsorbed during its service life. Carbon dioxide can also be added during concrete pouring to incorporate more CO2 and to increase concrete strength. When concrete structures are demolished, the concrete can be broken into aggregates which can be recycled into new concrete, reducing concrete's energy consumption. Concrete wastes also absorb carbon dioxide when left exposed to air, but only 1% of concrete wastes are left exposed long enough to absorb significant amounts of CO2. Reduction of the energy needed to manufacture cement (particularly the conversion of limestone in cement to calcium oxide) by improving the efficiency of heating or of CO2 removal (either using pressure or gas transfer) would reduce energy costs and carbon dioxide emissions. Reduction of CO2 emissions can be obtained by using renewable energy sources for cement production, capturing CO2 liberated in cement manufacture, by carbonating concrete during installation, and by allowing concrete wastes to remain exposed to air during demolition. In addition, the recycling of concrete to form aggregate may reduce the amount of cement needed for new construction.