Cement mortars with different contents of nano silica (NS) were fabricated and tested. Their compressive and flexural strengths showed significant increases. Theoretical calculation and thermogravimetry (TG) analysis, scanning electron microscope (SEM) and X-Ray powder diffraction (XRD), and electrical resistivity test were used to analyze the reinforcing mechanisms of NS. Theoretically, consumed calcium hydroxide (CH) increases with NS content, which indicates that NS has huge potential to react with CH. According to the results of TG, the amount of consumed CH increases and agrees with theoretical calculation when the content of NS is less than 1.5%. However, a plateau is achieved for the mass of consumed CH in results of TG when the content of NS exceeds 1.5%. SEM shows that NS can make matrix dense and also reduce the size of CH in matrix beside interfacial transition zone (ITZ). The results of XRD prove that NS can change the tendency of crystal of CH in cement matrix. However, the change degree of tendency of crystal of CH in cement matrix is lower than that in ITZ. The change trends of electrical resistivity with increasing NS content and curing age are similar with those of flexural and compressive strengths. This indicates that electrical resistivity can reflect strength and structural compactness of cement matrix.
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