After mineral exploration projects in Cameroon and Colombia (United Nations, 1979-1984), the United Arab Emirates (1984-1985) and Zambia (cooperation project between UNZA and DUT, 1986-1994), returned to the Netherlands (DelftChemTech), where main interest was on Environmental Geochemistry and Geochemical Engineering in general and coal combustion fly ashes in particular. Technical and environmental quality improvement and development of novel applications for these residues are the main focus of research activities. Conversion of ash into useful minerals such as zeolites and synthesis of geopolymers from fly ash, are key examples of this approach. But also the synthesis of secondary industrial minerals, such as zeolites, hydrotalcite, ettringite and dawsonite from waste industrial waste solutions form good examples.
SEM of fly ash (left) and Zeolite of fly ash (right)
Coal fly ash is the most important by-product of electricity production in coal-fired power stations. It consists of the non-combustible matter in coal and is separated from the flue gasses as fine powder. In the Netherlands around a million tonnes of fly ash is produced each year. Traditionally, fly ash is being applied in cement and concrete industry, but the material has favourable properties for more valuable applications, such as the conversion into zeolites and geopolymers. Geopolymers are amorphous inorganic polymers with a chemical composition similar to that of zeolites. The building stones are Si-Al-chains that form by alkaline activation of silica and alumina containing materials at low temperature. Fly ash generally has high contents of SiO2 and Al2O3 and is an important precursor for the synthesis of geopolymers. Geopolymers are fast setting binders developing high strength up to 120 MPa. They have the potential to replace ordinary Portland cement in construction materials, as a low CO2 cement for a sustainable future.