The impact of silica-nano-particles (SNP) onto the physicochemical and microscopic characteristics of composite-cements containing 40 - 60 mass % fly-ash (FA), and/or granulated-slag (GS) were studied. The physico-mechanical and microscopiccharacteristics of the composite cement-pastes were enhanced by the replacement of 4 mass% SNP, especially the earlystrength enhancement. The behaviour of the hydration-kinetics was studied from one day up to 360 days. The consistency (W/C) ratio, setting times (STs), compressive strength (CS), total porosity, (TP), bulk density (BD), chemically combined water (Wn), free lime (FL) and gel/space ratio (X) were determined. The behaviour of SNP was proven by XRD, DTA, and SEM techniques. The 4 mass % SNP improves the characteristics and microscopic hydration behaviour of the inspected blended and composite cements matrices in the existence of FA and GS. SNP have a positive effect on the behaviour of composite cement pastes, it diminished the setting times and improved the compressive strength and gel/space ratio. The SEM micrographs showed the formation of a denser and finer structure of a CSH hydrated gel with a marked reduction in the total porosity of the hardened-cement paste to form a nano-crystalline close textured structure, which is answerable for the strength properties. It was recommended that the composite cement containing 36 mass % OPC (ordinary Portland cement) + 40 mass % FA + 40 mass % GS + 4 mass % SNP, an E4 mix, which is the suitable optimum mix composition, showed a dense compact structure mainly-composed of a nano-crystalline close-textured-matrix with a remarkable decrease in the total porosity of the hardened blended and composite cement matrices having a higher compressive strength.