Concrete is one of the world’s favorite building materials; it’s strong, simple to mix, and generally widely available.  Its dirty little secret has always been centered around one of its main ingredients: cement.  To make cement, crushed rock and other ingredients are fed into a kiln that heats the components at temperatures reaching 2,700 degrees Fahrenheit.  Those extreme temperatures cause large amounts of carbon dioxide to be released into the air and, combined with the carbon dioxide that’s produced just to burn the fossil fuels to heat the kiln, it makes cement one of the largest producers of greenhouse gases in the world, 5% in total.