The present paper evaluates the pozzolanic activity of two nanostructured silicas with high specific surface area, synthetized by sol-gel technique, in comparison with silica fume. The materials were characterized by X -ray diffraction, scanning electron microscopy and nitrogen sorption. Electrical conductivity tests were performed in calcium hydroxide saturated solutions. Results revealed that both nanostructured sol-gel silicas have great pozzolanic reactivity, much higher than silica fume, especially in the early stage of the reaction. Hence, the pozzolanic reactivity of silica is affected by its specific surface area. Moreover, silica amount, test temperature and average pore size also seems to alter the silica reactivity. The reaction rate of silicas with small average pore size is reduced along the pozzolanic reaction.