Supplementary cementitious materials reduce the clinker factor in concrete. These mineral additives can vary in particle sizes (fine to ultrafine) and can influence mechanical properties and durability of concrete to a great extent. Durability of concrete depends on its intrinsic transport properties which further depend on the Interfacial Transition Zone (ITZ) present around aggregates. Available experimental investigations on the effect of fly ash on transport properties of concrete and mortar appear to be ambiguous, ranging from deleterious to enhancing effects to varying degrees. The present study examines the influence of fly ash on transport and mechanical properties of mortar and concrete to understand its influence on ITZ. Mortar and concrete having same aggregate volume fraction and water/cement ratio were tested for compressive strength, elastic modulus, oxygen permeability and sorptivity: variables include fly ash of different fineness with different percentage replacement. Same volume fraction of aggregates in mortar and concrete leads to a higher ITZ volume in mortar due to finer particle size distribution of aggregates. The strength and elastic modulus results shows that fly ash improves the mechanical properties of mortar more compared to concrete. The decrease in the fluid transport was also found more for mortar compared to concrete. This suggests that fly ash significantly improves the interfacial transition zone around aggregates. The improvement was found to increase with increasing fly ash fineness. © 2019 Associated Cement Companies Ltd.. All rights reserved.