The primary component of traditional concrete is Portland cement. Portland cement is manufactured from crushed limestone (calcium carbonate). There are two main stages in the manufacturing of portland cement.
The first stage, calcination, requires heating limestone to temperatures greater than 800 degrees Celsius (~1,500 F), resulting in calcium oxide and carbon dioxide gas.
The second stage, clinker formation, combines calcium oxide with silicates at furnace temperatures greater than 1,300 degrees Celsius (~2,400 F).
Manufacturing 1 KG of Portland cement releases approximately 1 KG of carbon dioxide into the environment. Half of this comes from the calcination of the limestone, and the other half from the combustion of fossil fuels necessary to achieve the required kiln temperatures. Additional atmospheric byproducts of portland cement production include dioxin, NOx, SO₂, and particulates.