Abstract: This paper describes interfacial microstructures of hydrated products of cement and three types of superfine powders, which are fly ash, silica fume and nano-SiO₂. Hydration mechanics of these three superfine powders and cement and their improvement of interfacial microstructures are researched also; therefore a concept of secondary interface is developed. The authors believe that these three superfine powders can improve interfacial microstructures and their effect degrees increased by fly ash, silica fume and nano-SiO₂. From the analysis of interfacial microstructures of hydrated products, the hydrated product of nano-SiO₂ is the densest and homogeneous when these three powders are used alone, and on the other hand, the hydration degree of mixture of these three powders shows much higher.