© 2021, Fundatia Serban Solacolu. All rights reserved.Fly ash (FA) and granulated blast furnace slag (GBFS) produced as waste materials from coal power plants and iron/steel sector are commonly used in the concrete industry for partial substitution of cement. As the production of cement releases a substantial amount of carbon dioxide, its replacement with FA or GBFS reduces carbon dioxide emissions. This study explores the effects of FA and GBFS fineness determined by particle size distribution (PSD) on the hydration rate and compactness of cementitious materials. Firstly, FA and GBFS were classified to have three different fineness ranges with the help of PSDs without any grinding process: FA and GBFS without PSD, FA and GBFS with 0-25 µm PSD and FA and GBFS with 0-63 µm PSD. Then, flexural strength, compressive strength, and water absorption properties of cementitious composites containing FA and GBFS with four different replacement levels up to 20% wt. of cement and with three different fineness were investigated at curing ages of 7, 28, and 90 days. Results revealed that cementitious composites that contain FA and GBFS with lower PSD yield better mechanical and physical properties.