© 2022 Elsevier LtdSynergistic effects of micro-silica (MS) and nano-silica (NS) on moderate and high strength concrete properties were investigated. Efficacy of micro and nano refinement remarkably increased as the W/Cm ratio decreased, and up to 37% improvement in compressive strength was obtained. Moreover, the linear-elastic region of the stress vs. strain diagrams significantly extended. As a result of the synergistic effect, a very dense interfacial transition zone was formed. The time-dependent evolution of the elastic modulus was much more effective than the compressive strength development. However, the combined use of MS and NS significantly decreased ductility.