This study investigates the early-age hydration reaction between cement hydrates and sea water through setting time, hydration heat measurement, compressive strength, thermal analysis, and microstructure study. The kinetics of hydration up to 3 days were measured using isothermal calorimetry. Microstructure of cement pastes were studied by SEM-EDX. The results indicate that the addition of sea water increases the hydration of CA and CS resulting in fast setting time. Seawater improved the cement hydration and increased the early-age strength development. The % increase in strength for seawater samples are 22.35%, 29.84% and 26.51 %, at 1 day, 3 day and 7 days comparatively. The ions present in seawater helps in acceleration of cement hydrates at early age. Friedel's salt, a Cl-AFm phase present in seawater sample, becomes unstable at later ages, followed by decrease in physically adsorbed chlorides. The difference between the seawater and freshwater cement matrix is the presence of sodium, chloride, and sulphates percentage.