Concrete performance is af fected by aggregates, the bulk cement paste and particularly the interfacial transition zone (ITZ). In this regard, the ITZ is generally weaker than either of two main components of concrete, namely the aggregate and the bulk hydrated cement paste. On the other hand, the strength properties are highly influenced by the ITZs complex microstructure (which has a dynamic nature) and its gradual variations as a result of environmental conditions. Many attempts have been made to overcome the heterogo nous deficiencies of concrete through the utilization of different pozzolanic materials such as nanos ilica (nS) and silica fume (SF). In the present work, nS at 0%, 1.5%, 3%, 5% and 7.5% and SF at 0%, 5% and 7.5% by weight of cement were utilized to investigate their effect on the strength properties of concrete and corresponding cement paste at early and older ages. A microstructure study was also performed by SEM, XRD and EDS to realize the reasons for the obtained results. The results demonstrated that adding 3% or 5% nS to specimens free of SF would increase both cement paste and concrete compression strength. The microstructure analysis revealed that modification of the ITZ was responsible for this strength enhanceme nt.
