What's that fly ash doing in my concrete?

When Paul Hummer pours the slab for the project, the concrete that he'll be using will be more sustainable than the traditional mix. That's because part of the portland cement will be replaced with fly ash. Portland cement is made from quarried limestone with some gypsum and other minerals. Fly ash is recovered from gases created by coal-fired electric power generation traditionally disposed of in land fills.
In the larger picture of sustainability the use of fly ash becomes very significant because conctete is the most used man made material on the planet. The production of portland cement for use in concrete accounts for 5 to 10 percent of the world's total carbon dioxide emmsions making it a significant contributor to global warming.

Check out the following
link for more information on the use of fly ash.