Supplementary cementitious materials keep increasing their importance worldwide.Fly ash, blastfurnace slag,limestone, natural pozzolanas can substitute clinker in cement composition, improving sustainability and economics of cement manufacturing.However, their use can lead to a loss of performances (namely strengths) that limits the maximum amount of clinker substitution.The present sturly would like to be the start of a thorough practical understanding of the effect of high substitution by fly ash.Effects on strength have been studied for several fly ash and clinker combinations,correlating performance to chemical composition.In addition, treatment of fly ash cements with chemical additives (grinding aids/quality improvers to be added during cement manufacturing) has been studied.This allows the recovery of the decreased performances,opening the door for higher substitutions of cliaker without losing strength.In conclusion, the results can provide insight on specific fly ash selection by a cement manufacturer, in combination with chemical enhancers of the hydration.