http://www.nytimes.com/2008/11/19/business/19conc ... .html?_r=1
That’s because every ton of new portland cement, the most common adhesive ingredient in concrete, releases roughly one ton of greenhouse gases from the kilns that bake it.
The most potent way to reduce that toll is to replace some portland cement with recycled material. Coal ash from power plants and blast-furnace slag are the easiest substitutes to find — and they tend to make concrete more valuable than portland cement alone.