Abstract A dense surface structure of cement-based material is important for its resistance to the impacts of environment. Effectiveness and mechanisms of colloidal nanoSiO2 (CNS) and its precursor, tetraethoxysilane (TEOS), on surface-treatment of the one-month-old cement mortar by brushing technique were studied in this work. It revealed that CNS decreases the water absorption ratio of mortar when cured at 50°C and/or in sealed condition, but its effect is negligible when samples are cured at 20°C. A greater reduction of water absorption ratio is found in TEOS-treated mortar at 20°C or 50°C and under sealed or unsealed condition. Pozzolanic reaction between CNS/TEOS and Ca(OH)2 was observed by XRD, IR and EDS techniques. In addition, filler effects as revealed by SEM/EDS techniques were ascribed to for the densification of the exposed hardened mortar. Pore size distribution analysis conducted on mercury intrusion porosimeter showed that TEOS is more effective than CNS in filling pores finer than 50nm, while CNS is found effective in filling capillary pores coarser than 50nm.