Fly ash is recovered in the form of a fine dust by the dust collectors of furnaces burning pulverized coal. Fly ash may be siliceous (consisting mostly of eactive silicon dioxide (SiO2) and aluminium oxide (AI2O3) with pozzolanic properties) or calcareous (consisting of reactive calcium oxide (CaO), reactive silicon dioxide (SiO2) and aluminium oxide (AI2O3 ) and having both pozzolanic and hydraulic properties) and is consequently serves as a suitable alternative raw material. Since the reaction of pozzolanic material is slower than that of clinker, cements containing fly ash typically show a lower early strength compared to ordinary Portland cement (OPC) at similar fineness. They also exhibit a lower water demand, an improved workability, a higher long-term strength and – depending on the application – a better durability such as an increased resistance against sulfate attack. In addition, fly ash can often contain higher amounts of unburned carbon affecting their suitability for cement and concrete.