There is a strong demand of higher volume replacement of ordinary Portland cement (OPC) in concrete and cementitious composites by supplementary cementitious materials such as blast furnace slag (BFS) and fly ash (FA) to reduce their carbon footprint. This paper presents the effect of addition of nano calcium carbonate (NC) and nano silica (NS) on improving the interfacial transition zone (ITZ) of high volume slag (HVS) and high volume slag-fly ash (HVS-FA) blended concretes. Nanoindentation, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) techniques were used to characterise the nano- and microstructure of the ITZ area between aggregates and matrix. Results show that the addition of NC and NS improved the modulus and hardness of hydration products in the ITZ area of matrix of HVS and HVS-FA concretes. The thickness of ITZ is also reduced in those concretes due to addition of NS and NC. SEM and EDS analysis also confirm the above observation where dense microstructure with less voids and micro cracks and higher peaks of Ca, Si and Al were observed. The observed improvement of 28 days compressive strengths of the HVS and HVS-FA concretes due to addition of NC and NS also correlated well with the improvement of microstructures of ITZ area.