Pozzolanicity of fly ash

Fly Ash

The inhabitants of the small Italian town of Pozzuoli could never have imagined that the place they called home would one day be associated with an important material property of fly ash.

They discovered that the volcanic tuffs in their surroundings could be used to make hardening building materials, hence finely ground tuff also being known as natural pozzolan.

Fly ash is similar to volcanic tuffs in terms of its chemical composition and is therefore called artificial pozzolan.

Als Puzzolanität bezeichnet man die Fähigkeit von Puzzolanen, unter Anwesenheit von Feuchtigkeit und Kalk oder Zement festigkeitsbildende Produkte im Beton – sogenannte Calciumsilikathydrate (CSH-Phasen) – zu bilden.
Pozzolanicity refers to the ability of pozzolans to form strengthening products in concrete – so-called calcium silicate hydrates (CSH phases) – in the presence of moisture and lime or cement.

Like the volcanic tuffs, fly ash contains reactive silica (amorphous SiO2) which, in the presence of moisture, can react with lime (Ca(OH)2), which is split off during the hardening of the cement. The dissolved alkalis present in the cement accelerate these processes. With its high degree of fineness and the consequently large material surface, fly ash offers good conditions for the pozzolanic reaction.

The CSH phases formed during the pozzolanic reaction of fly ash in concrete are very similar to the CSH phases formed in cement. In concrete, the CSH phases formed as a product of the pozzolanic reaction not only result in increased strength, but also compact the pore structure as regards permeability to gases and liquids. The use of fly ash increases the durability and resistance of the concrete.

The pozzolanic reaction of fly ash progresses more slowly than the hardening of the cement. Due to this ongoing reaction, which usually continues for years, the concrete becomes increasingly more compact, stable, and durable. This makes it more resistant, for example against the effects of acid or sulfate. Fly ash thus lends the concrete properties that are required especially for bridges, tunnels, cooling towers, sewers, and foundations. 

Intensive research and development work since the 1970s has opened up a wide range of applications for fly ash. With its advantageous chemical and physical properties, fly ash is a component part in many building materials.

Fly Ash