Studies on understanding the reactivity of alternative materials are typically performed on paste mixtures. This study attempts to explore the contribution of supplementary cementing materials (SCMs) to the properties of blended cementitious mortars. The strength and microstructural development of mortars are investigated through parameters such as compressive strength, porosity and chemically bound water. In addition, an attempt is made to decouple the physical and chemical contribution of SCMs with the help of inert fillers. The results indicate that there is a desired level of fineness at which an optimal performance from mineral additives is obtained. Further, the decoupling methodology was able to bring out the rate of chemical contribution from the different mineral additives; the delayed pozzolanic activity of Class ‘F’ fly ash led to significant chemical contribution at later ages. Unlike the compressive strength, where the physical filler effect did not uniformly result in a contribution to strength, the results for bound water always show a positive contribution from the filler effect. © 2018 Elsevier Ltd