In the present investigation, to evaluate the performance of a 25% cement replacement by fly ash (FA), blended composite and control (without FA) was hydrated with sea water (SW) with the water to binder ratio 0.4. Intervals of the hydrated samples are 1 hour, 6 hours, 7 hours, 1 day, 7 days, 28 days, 90 days and 180 days. Setting time and compressive strength of control and blended composites were carried out. The hydrated samples were subjected to FTIR and SEM with EDS analysis. This study reveals that the control (without fly ash) pastes can accelerate the early stage of hydration due to higher amounts of ettringite formation and retarded hydration process in later periods due to Ca(OH)2 product reacting with sodium, magnesium and chloride ions to form recrystallised gypsum and Mg(OH)2. These products are deteriorating in control paste. In blended composite, these products are free due to pozzolanic reaction of fly ash consuming Ca(OH)2. Also, the Al2O3 rich in fly ash reacts with chloride to form Friedel’s salt (C3A.CaCl2.10H2O). This reduces the amount of free chlorides and hence, the blended composite has higher strength than control in later periods and increase the resistance to sulphate attack.