Most people don’t realize that concrete accounts for 7 percent of all greenhouse emissions.  It does so in two ways.  First, the chemical transformation of limestone into cement, the main ingredient in most concrete, emits carbon dioxide.  Second, this chemical process requires a very hot kiln, almost always using fuel that emits carbon dioxide. This is bad news given how widely concrete is used in our buildings and infrastructure.  The good news is there are a growing number of ways to significantly reduce its carbon emissions and even use new mixtures of concrete to store carbon removed from the atmosphere.