## 1 Introduction  
The long-term behavior of reinforced concrete structures exposed to a tidal zone has been a serious concern in marine engineering [1]. Billions of dollars are annually required worldwide for maintenance purposes. A major cause of distress can be ascribed to the reinforcement corrosion induced by chloride attack. The soluble substances of cement create an alkaline nature, with a pH value close to 13 or above in the pore solution to passivate the steel surface. The accumulation of chloride ions on steel surfaces driven by persistent diffusion, together with capillary absorption in many situations, may destroy the passivity, resulting in steel corrosion. Chloride transport takes place through the microstructure of a cementitious system. Some of the chloride ions can be physically bound by the main hydration products C-S-H [2]. The alumina-rich AFm phases [3] consist of positively charged main layers [Ca\({}_{2}\)(Al,Fe)(OH)\({}_{6}\)]\({}^{+}\) and negatively charged interlayers [X.nH\({}_{2}\)O]\({}^{-}\). They can chemically react with the chloride ions, forming the so-called Friedel's salt. Chloride ions can be bound considerably by calcium oxychloride hydrate, which is formed in particular situations such as a high chloride concentration (3.5 mol/L) and low temperature (5 \({}^{\circ}\)C). Other phases, such as hydrolatclite, a representative of the layered double hydroxides group, also play an important part in the adsorption of chlorides, especially for the concrete-containing ground granulated blast furnace slag [4]. The replacement of ordinary Portland cement OPC by ground granulated blast furnace slag, among others, at a large scale has become a common means for improved economic and sustainable considerations in marine concrete structures. The often-adopted cementitious slag has a relative density of around 2.9, and its particle fineness is usually higher than that of OPC. The use of slag has been shown to remarkably influence the initial particle packing and change the pore solution chemistry in cementitious systems. The hydration process and associated microstructure formation are expected to be quite different from those based on OPC. A practical question arises with respect to the current and future durability criteria when the slag is used as much as possible. A considerable number of scientific and engineering studies have been carried out over the past decades. The beneficial effects of slag utilization, particularly for chloride-laden environments, have always been a hot topic of research [5; 6; 7; 8; 9; 10; 11; 12; 13]. The role of slag in cementitious systems is affected by a variety of factors, such as chemical composition (CaO, MgO, Al\({}_{2}\)O\({}_{3}\), and SiO\({}_{2}\)), glass content, particle size distribution, etc. [14]. The enrichment of the SiO\({}_{2}\) is conducive to the pozzolanic reaction, a reason for the production of secondary C-S-H. The MgO of slag promotes the formation of hydrotalcite [15], contributing to chloride binding behavior. The Langmuir and Freundlich isotherms have been recognized in many cases as applicable for describing the relationship between the bound and free chlorides in cementitious systems [16]. A widespread consensus regarding the optimal inclusion level of slag with respect to maximizing the chloride resistance has not been reached yet. The common moderate dosage of 50% slag is adopted primarily according to strength considerations [17]. [PERSON] et al. [7] pointed out the time-dependent nature of chloride diffusion. They emphasized that the resistivity to chloride transport by adding slag is enhanced mainly for the periods after 6 months. This is partially related to the relatively slow hydration of slag. Field concretes are normally unsaturated, which plays a crucial role in long-term chloride transport. This issue has been highlighted and elaborated on in recent reports [10; 11; 18].