The use of supplementary cementitious materials (SCMs) in blends with ordinary Portland cement (OPC) influences positively the workability, durability and strength of concrete, reduces the CO2 emission and helps to solve the environmental problems related to their disposal.Commonly used SCMs include fly ash, ground granulated blast furnace slag and silica fume.In particular, fly ashes have been used since a longtime together with OPC, utilizing their properties of low heat of hydration, development of fluidity and suppression of alkali-aggregate reaction.It is generally agreed that increasing temperature, particle fineness and pH have a positive effect on the reactivity of fly ash in blends with OPC.It is however not clear how the fly ash chemistry influences reactivity as it is challenging to obtain conclusive results due to the heterogeneity of the fly ash particles in composition and size.In order to get a deeper insight in the chemical factors controlling fly ash reactivity, the dissolution of synthesized glasses with compositions representative for siliceous fly ashes with controlled particle size in alkaline environment was studied.