This study aims to assess the effects of micro and nano-SiO2 under various dosages of carboxylated-polyether-copolymer-type superplasticizer on the rheological properties of grouts in the fresh state. In the hardened state, compressive tests and scanning electron microscopy were used to characterize type I Portland cement mortars. During Stage-I, the rheology of superplasticized grouts (w/b = 0.35 and 0.40) was obtained by the Marsh cone test. Mineral admixtures in superplasticized grouts decrease the flow times and the saturation dosages when compared to the superplasticized-control specimens. In Stage-II, experimental designs at w/b = 0.35 produced flow area, unit weight and air content in mineral admixture-systems better than in superplasticized-control samples. During Stage-III, the maximum strength in the nano-SiO2-system was obtained at 1.0 wt%, whereas in micro-SiO2-systems, this effect occurred at 15 wt%. SEM examinations in the interfacial transition zone suggest that compressive strengths of nano-SiO2-systems were controlled by both densification and filler effects, whereas micro-SiO2-systems were controlled only by the latter. Artículo Tipo A1