Many studies have focused on the fe

Many studies have focused on the feasibility of using nanoparticles including nanoalumina (NA), incement composites and reported some positive effects on the properties of concrete. However, thereare still uncertainties about the effects of NA addition on cement hydration. In this study, c-NA particles with a size of about 30 nm was incorporated into Portland cement pastes and the physicochemical and microstructural properties of the NA-modified cement pastes were analyzed. Conduction calorimetry results showed that NA accelerated the silicate and aluminate phases reactions in OPC. Compared to the plain cement paste, the NA-modified cement pastes exhibited improved compressive strength at all ages due to the reduced macropores and the accelerating effect on cement hydration induced by NA, but the strength enhancement at 28 days was less obvious. Moreover, back scattering electron imaging and energy-dispersive spectroscopy (BSE-EDS) analysis indicated that the Al-enriched outer C-S-H formed in the NA modified paste; and it was mainly due to the NA particles acting as seeding for the growth of the outer C-S-H at the early age, rather than the Al released by the aluminate phases. This also provided a possible explanation for the substantial increase in CH and chemical bounded water contents in the NA-modified pastes at 12 h, but almost no change at 28 days of hydration.