The effect of nano SiO2 on the early mechanical properties of interfacial transition zone in concrete was studied at multi scale levels. At macrolevel， the static modulus of elasticity， compressive and flexural strength were measured； at microlevel， the values and distribution of indentation modulus in interfacial transition zone were analyzed by nano indentation. Results show that the addition of nano SiO2 improves the early strength and modulus of both cement paste and concrete， in particular， a much significant increase of strength for concrete was observed. The amount of micro pores and defects decreases remarkably and C S H gel with higher density forms in ITZ of concrete modified by nano SiO2， thus the indentation modulus of ITZ is much closer to that of bulk paste.
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