Water plays an important role in the concrete industry. The research aims to study the effect of using seawater in mixing and curing concrete and indicating its effect on fresh properties of concrete such as slump and mechanical properties like compressive, tensile and flexural strength. The main variables of this paper are type of water (sea and fresh water), percentage of fly ash and metakaolin. The percentage of fly ash is 20%, 30% and 40% as a replacement of cement content. In addition, metakaolin with 3%, 5% and 7% as a replacement of cement content was used. Mineral admixtures are materials that have a softer degree compared to cement, and this leads to have denser concrete and less permeable concrete, which enhances the lower overall porosity of the system due to the filling of capillary pores. That leads to reduce the penetration of chloride and sulfate ions into the concrete. The results show that using mineral admixture gave the ability to use seawater in plain concrete, as a compressive strength of 49 MPa was achieved when seawater was used in mixing and curing concrete with 7% metakaolin and 30% fly ash. The results also showed an improvement in tensile and flexural strength.