###### Abstract  
The mixing proportions of concrete were examined with regard to the durability performance and early strength in coastal areas. Research was conducted to improve the C24 mix (characteristic strength of 24 MPa). C35 concrete (characteristic strength of 35 MPa) was selected as a comparison group, as it exhibits the minimum proposed strength criterion for concrete in the marine environment. To secure the early strength of the C24 concrete, 50% of the total ordinary Portland cement (OPC) binder was replaced with early Portland cement (EPC); and to provide durability, 20% was substituted with ground granulated blast-furnace slag (GGBS). In addition, a polycarboxylate (PC)-based superplasticizer was used to reduce the unit water content. The compressive strength, chloride ion diffusion coefficient, chloride penetration depth, and pore structure were evaluated. After one day, the compressive strength improved by 40% when using EPC and GGBS, and an average increase of 20% was observed over 91 days. EPC and GGBS also reduced the overall porosity, which may increase the watertightness of concrete. The salt resistance performance was improved because the rapid early development of strength increased the watertightness of the surface and immobilization of chloride ions, decreasing the chloride diffusion coefficient by 50%. d 20  
internal chloride as it is present in the mixing water, aggregate, cement, and admixture. Therefore, the initially introduced amount of internal chloride must be limited as much as possible to reduce the risk of corrosion of the steel reinforcements. To this end, each country regulates concrete materials and the internal chloride that is added during the manufacturing of concrete [12; 13; 14]. In general, internal chloride is periodically tested during the construction of concrete structures. This implies that external chloride is the main cause of steel corrosion. In marine environments, there are two paths for external chloride attack, i.e., direct attack from seawater and attack by sea-salt particles. Outside marine environments, chloride attack owing to deicing agents in winter is the most important path. Therefore, to prevent chloride attack, the chloride penetration path must be accurately identified, and penetration must be inhibited using appropriate methods, such as increasing the watertightness of the concrete [15; 16; 17]. The durability design standards of different countries present various methods for ensuring the durability of concrete structures during their target service lives [18; 19; 20; 21]. The durability design of concrete structures, however, is treated as part of the structural design. The service lives of concrete structures are not clearly regulated, and only general guidelines on the durability of concrete are specified. The most typical method for improving the chloride resistance of concrete is to increase the densification of the concrete structure by using concrete with high strength. Indeed, in many studies, C35 concrete (which has a characteristic strength of 35 MPa) or higher has been proposed as the minimum concrete mixture for structures in coastal areas [12; 13; 20; 21]. In addition, the watertightness of the concrete structure can be increased using mineral admixtures, such as ground granulated blast-furnace slag (GGBS), fly ash, and silica fume [22; 23; 24; 25]. Among them, using GGBS is recommended in coastal areas because it improves long-term watertightness [26; 27; 28; 29] and has an excellent ability to immobilize chloride ions. The use of these mineral admixtures, however, is recommended only to a certain level, because they do not contribute to the development of early strength [30; 31; 32]. Therefore, specifications on concrete strength must be improved to avoid early form removal and to shorten the construction period. In the field, various methods are required to improve economic efficiency and to reduce the construction time, which require mixing technology to secure early strength while maintaining concrete performance.