text: There are a few explainations.\n\n1.) Fly Ash is much cheaper than cement.  With the prices of natural gas and the fact that production of cement requires a rotary kiln that is heated by natural gas the cost of cement has gone through the roof.  From my understanding a ton of cement has increased in price by like 20% over the last year or so.\n\n2.) Fly Ash is a natural pozzulan and water reducer.  Fly ash lowers the water requirements of the concrete mix making for a much stronger concrete.  Fly ash slows early strength gains but increases late strength gains as well as increasing workability.  Think of it like this.  You put rocks in a jar and there's voids, then you fill in those voids with sand, water, and cement but there's still small voids so you use fly ash to fill those in.  You can still have voids and use other things to fill those in but that's for another question.\n\n3.) Fly Ash is a waste material (from coal fired power plants i believe don't quote me).  The federal government requires its use in some of its projects or at the very least recommends it.  It is considered a green material and i read an article in Concrete Construction magazine (Jan 2006 issue) about HVFA (high volume fly ash) mixes.  They will replace like 51% of the cement with Fly Ash in the mix and get tremendous late strengths out of the concrete.\n\nThese are three of the main reason's for the use of fly ash.  As sustainable building (green construction) continues to gain backing you'll probably find more use of fly ash in concrete.  Also with it being cheaper people will want it used.\n\nStrictly speaking Fly Ash DOES NOT increase the density of the concrete.  Fly Ash has a specific gravity of about 2.00 (meaning for a given volume it weighs 2 times the same volume of water), Cement has a specific gravity of about 3.15.  If you have 3 cubic feet of cement then it will weigh 589.68 pounds (3 x 62.4 x 3.15).  If you substitue 1.5 cubic feet of that for Fly Ash then you will have [(1.5 x 62.4 x 2.00)+(1.5 x 62.4 x 3.15)] or 187.2 + 294.84 = 482.04 lbs. (a cubic foot of water weighs 62.4 pounds i believe).  Fly ash actually lowers the unit weight of the concrete, because for a given volume it weighs 63.5% of what portland cement does.