Fly ash is obtained through the electro-static or mechanical separation of dust particles from the combustion gases of powder coal. In draft standard EN 450, the standard for the application of fly ash in concrete, fly ash is defined as a fine powder obtained by burning powder coal, with or without the combustion of secondary fuels (clean biomass), with Pozzolana characteristics and mainly made up of SiO2 and Al2O3.
A major characteristic of fly ash is pozzolanity. That is to say that the material is able to react with calcium hydroxide, which is released as a product on the hydration of Portland cement, whereby calcium silicate hydrate compounds are formed, which are insoluble in water. The pozzolanic properties can be traced back to the presence of vitreous phases in the fly ash. In the alkali environment of the concrete, the glass structure is broken up and the components are available to react with the calcium hydroxide to form calcium silicate hydrates.
Fly ash is a reliable raw material that is often applied in concrete for this type of constructions. With fly ash, producers are in particular able to assign the necessary specific properties to mass concrete.
Slag is a by-product that is created during the production of pig-iron in the blast furnace process. It is mainly made up of silicon oxide, calcium oxide, magnesium oxide and aluminium oxide.
Granulated slag is created by the very fast cooling of the slag from the blast furnace with a large quantity of water. We call this the granulation process. In the granulation process, the liquid slag is changed into so-called slag sand, a granular product with a grain size of 0 – 2 mm. As a result of this process, the slag also takes on an amorphous structure. Finally, the granulated slag is ground into a fine powder in a facility intended for the purpose.
Granulated slag is a raw material for the production of blast furnace cement (CEM III). In combination with Portland cement, ground granulated slag can also be used under certain conditions directly in concrete in combination with Portland cement.