An interesting turn of events. We had planned to use fly ash - a waste product of coal-burning power plants - as a replacement for 25% of the cement in our concrete foundations and slab. Our concrete supplier, however, recommends using slag (ground granulated blast-furnace slag) to replace half the cement in the concrete mix. Slag is a waste product from steel production, which, magically, works a lot like Portland cement, so using it in the foundations will obviate the need for more virgin cement and find a useful home for an industrial waste product.
Keep in mind, too, that most of the steel rebar in the foundations is recycled. Unfortunately, we won't be using recycled aggregate, which seems to be what the hard-core sustainable types are using.
From an aesthetic standpoint, the slag cement is much lighter in color than regular Portland cement, and so our floor - an exposed concrete slab - will be brighter. This will make the whole interior even brighter than it was already going to be.