In this study, three modeling experiments were designed to investigate whether nano-particles incorporated in the cement paste act as nucleation sites for C-S-H gel growth during cement hydration.The nano-particles with (nano-SiO2) and without (nano-TiO2) pozzolanic reactivity were used as nano-particles.In the first experiment,both the cement and nano particles were dispersed in water to prepare dilute cement paste, in which the cement and nano-particles can contact each other.In the second one, the cement particles were laid inside a filter paper funnel and immersed in water with nano-particles dispersed, in order to separate the cement particles with nano-particles by using the filter paper.In the third one, large clinker particle was embedded in resin,surface-polished and then exposed upside down in water with nano-particles dispersed.After hydration for a certain curing age, the hydration products in the paste or the nano-paticle dispersion were observed by using TEM and the hydrated surface of the embedded clinkers were detected by using SEM.The results revealed that no C-S-H gel from cement hydration grew from nano-particles, implying that the nano-particles in the cement hydrating system cannot act as nucleation sites for C-S-H gel growth during cement hydration.