Coastal cities like Hong Kong rely heavily on their coastal and marine infrastructure for social-economic development. A major challenge for marine infrastructure is steel corrosion, which is the main cause for infrastructure deterioration. Also, fresh water is a scarce resource for some remote islands, and to meet the water requirement for concrete preparation in these regions, seawater is increasingly being considered as an alternative which would inevitable aggravate the deterioration problem due to steel corrosion. Up to now, a number of studies have focused on the effects of seawater on the workability, mechanical strength and durability (in particular corrosion of steel reinforcement) of cement pastes and reinforced concrete. However, the understanding on the influence of seawater on the composition and microstructure of cement hydration products is limited. This paper presents a preliminary study on the effects of individual salt components of seawater on the hydration process of C3S. The microstructure changes of the hydration products were also studied. The mechanism of the acceleration effect is analyzed. Na+ and Mg2+ can affect the dissolution of C3S and the participation of the hydration products. For the later age, Mg2+ can substitute part of Ca2+ and change the composition of the final reaction products.