poz·zo·lan n. A siliceous, or siliceous and aluminous, material which in itself possesses little or no cementitious value but will, in ﬁnely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
The word “pozzolan” is taken from the name of the Italian city — Pozzuoli — that is regarded as the birthplace of ash concrete technologies. Famed Roman structures such as the Pantheon and Colosseum, as well as many Roman roads and aqueducts, are still standing over 2,000 years after their construction — in part because of the durability derived from their ash-based concrete mixes.
When mixed with lime (calcium hydroxide), pozzolans combine to form cementitious compounds. Concrete containing ﬂy ash becomes stronger over time, more durable, and more resistant to chemical attack. This is partly due to its chemical make-up and partly due to the fact that using fly ash also reduces the amount of water needed in concrete. By reducing the water-cement ratio, concrete producers are able to make less permeable, stronger, more durable concrete.
Fly ash also benefits the “finish” and “workability” of concrete because ﬂy ash particles are very fine and spherical shaped. Because ﬂy ash particles are hard and round, they have a “ball bearing” effect that allows concrete to be produced using less water. Using less water contributes to improved concrete durability.
Beneficial use of fly ash in concrete creates signiﬁcant beneﬁts for our environment. Using fly ash in concrete conserves natural resources and avoids landﬁll disposal of ash products. By making concrete more durable, life cycle costs of roads and structures are reduced. Furthermore, ﬂy ash use replaces Portland Cement in concrete. Every ton of Portland Cement replaced by fly ash is 1 less ton of Greenhouse gas in the atmosphere.
In the United States, over 12 million tons of fly ash is beneficially used as a cement replacement in concrete every year. This is perhaps the greatest beneficial re-use, recycling story in the U.S. over the last 50 years.