Coal fly ash is widely used in as raw material in concrete industry because it can replace part of the cement and fly ash contributes to improve certain properties of concrete. As a way to reduce the contribution of coal fired power plants to the climate change, more and more coal is replaced by biomass. To keep fly ash as a valuable resource for concrete it is essential to create thorough understanding of the relation between the compositions of co-fired biomass, the combustion process, the properties of fly ash and concrete made with it. Fly ashes derived from co-combustion teste on both pilot-scale and real-scale were characterized physically, chemically and mineralogically. Further, the properties of cement paste, mortar and concrete made with these fly ashes were determined. The chemical composition of the fly ash is influenced due to co-combustion of biomass. Especially the content of the elements Ca, Mg, K and P increases when biomass from vegetable or animal origin is co-fired. The influence of biomass from industrial origin varied from type to type. It was possible to identify the aimed relation in the case of co-biomass from vegetable and animal origin. However, this was not always the case with biomass from industrial origin. Finally, the application of fly ashes from co-combustion is assessed from the point of view of a circular economy (technical and biological cycle). @en