Concrete

Todd found this article interesting.

In summary:
-Production of concrete alone produces 5% of the world's greenhouse gases
-the main cause is that limestone is “cooked” to create portland cement, requiring a huge amount of energy
-the use of fly ash or slag replaces portland cement, and thus eliminates the production of these greenhouse gases (I always thought the use of fly ash was simply to re-purpose a waste material, not to cut down on emissions)
-there are concrete mixtures in the works that could be carbon neutral, or even act as carbon sinks because of their chemical structure