OK, let's put our lab coats on for a minute and get down to the chemistry of how fly ash works and what makes it special. We keep saying that fly ash is a good substitute for Portland cement, but what exactly is Portland cement? Funny you should ask. It's only the most common type of cement in use around the world, and the main ingredient in most types of concrete. Lime and silica make up the bulk of Portland cement, and it also includes aluminum and iron. Most types of Portland cement contain a mix of limestone, chalk, shells, and a variety of clay, shale and sand. Producing Portland cement is a very energy- and resource-intensive process, which is why it makes sense to look for a good substitute [source: Portland Cement Association].
Geopolymer concrete is the technical name for fly ash or any other type of concrete made from synthetic aluminosilicate materials (materials made with aluminum, silicon and oxygen). When fly ash is added to a cement mixture, it isn't just filler, it actually reacts with the other compounds that are in Portland cement and becomes part of the matrix. It's rich in alumina and silicate, which causes it to react with alkaline solutions to produce a binding aluminosilicate gel. The resulting product is an excellent substitute for the traditional ordinary Portland cement [source: van Riessen].
Today, concrete mixes in which up to 25 percent of the cement is replaced with fly ash are common, and some concrete makers are replacing as much as 50 percent. But higher concentrations of fly ash require increased testing, because the results can vary.
From a builder's standpoint, the most important thing about fly ash concrete is its performance, and it doesn't disappoint. Adding fly ash to the mix has the ability to improve concrete, because changing the composition of concrete can add strength and durability. The resulting material is less porous than typical Portland cement, and it's more resistant to corrosion and premature deterioration [source: Ideker]. In addition to being more durable than ordinary Portland cement, fly ash concrete is more acid- and fire-resistant, and it demonstrated higher compressive and tensile (stretching) strength.