Date: September 10, 2013
Concrete is made with sand, rocks, cement, and water. We use many other products to improve various properties of concrete. One of those products is fly ash. Fly ash is the powdery residue left over from burning coal to produce electricity. For years, fly ash was simply dumped into land fill sites or into massive ponds as a method of disposal.
The ancient Greeks, then Romans, discovered that ash from volcanos could be mixed with lime and water to produce mortar and concrete. Many of the great roman concrete structures and pavements are still standing today. Fly ash from coal power plants today is very similar to volcanic ash.
So what are the benefits to mixing this waste byproduct into concrete? Fly ash improves concrete in many ways. Fly ash particles are nearly perfect, tiny spheres; they reduce the amount of water needed to achieve workability. Cement and sand have a jagged, angular shape, so the tiny spheres act like ball bearings to lubricate the mix. This lubrication greatly improves the ability to pump concrete over long distances. Fly ash is lighter than sand and stone, so it reduces segregation or the tendency for aggregates to sink to the bottom of the forms.
Fly ash helps protect concrete and reinforcing steel. It prevents deterioration due to a chemical reaction called alkali-silica reactivity or ASR. It makes the concrete denser and less permeable, so water and salts cannot reach the rebar and begin corrosion. It helps concrete continue to gain strength for many months after placement. It is industrial waste, so using it to improve concrete properties is recycling and it saves valuable land fill space!
In conclusion, using fly ash in concrete is a three-run homer: it improves the concrete quality; it reduces disposal in landfills or ponds; and it reduces the demand for Portland cement lowering the release of carbon dioxide.
Hank Keiper, P.E.
The SEFA Group