## 1 Introduction  
Alongside steel, concrete is one of the most important materials in the construction industry, and in general, the durability of concrete structures degrades because of some environmental causes, such as freezing and thawing, steel reinforcement corrosion due to chloride attack and carbonation, and chemical erosion. As such, many studies have been conducted on these causes worldwide [1, 2]. In particular, when porous media, such as concrete, are exposed to a freeze-thaw environment, the water in the pore solution is subjected to repeated freezing and expansion, resulting in expansive cracks in concrete because of the increase in the tensile stress caused by expansive pressure [3]. The mechanism of concrete deterioration due to freezing and thawing has been reported by many studies. The most dominant mechanism reported to date is the glue spall mechanism proposed by [PERSON] and [PERSON] [4], who reported that the tensile stress caused by the difference between the thermal expansion coefficients of ice (generated by freezing) and concrete leads to cracking.  
In addition, many previous studies have reported that the freeze-thaw resistance of concrete is affected by the type and amount of binder. Furthermore, many studies have reported that the concrete containing mineral admixtures possesses excellent frost resistance [5, 6, 7]. In general, mineral admixtures used as binders for concrete include blast furnace slag, fly ash, silica fume, and metakaolin. Among them, blast furnace slag can be classified into water-cooled slag (WS) and air-cooled slag (AS), according to its cooling method [8].  
WS is commonly used as an admixture for concrete, and approximately 30% of its output is used as a concrete binder in South Korea. However, approximately 70% of AS is disposed, and the rest is mostly used as a roadbed material, material for embankment, and silicate fertilizers [1]. Several studies [9; 10] have indicated that AS can reduce the hydration heat and improve the fluidity; owing to these characteristics, its applicability as a concrete admixture has attracted attention. It has been reported that AS can be used as a concrete binder because it is suitable for developing the latent hydraulic property with calcium hydroxide in concrete when used as a concrete admixture, because its chemical composition is similar to that of WS, and it possesses a high basicity [10; 11]. In addition, it is known that cooled AS does not require any special stabilization treatment unlike steel furnace slag because F-CaO and F-MgO hardly exist in AS [1; 5; 9].  
Based on the physical and chemical properties of AS, several studies have been conducted wherein AS was used as a concrete material. In particular, [PERSON] et al. [10] studied the microstructure of the hardened cement matrix containing AS, and [PERSON] et al. [11] attempted to experimentally verify the usability of AS as a substitute for WS. Moreover, the applicability of AS as a concrete binder was investigated in terms of the strength, fluidity, and durability of concrete. On the contrary, [PERSON] and [PERSON] [12] experimentally studied the freeze-thaw resistance and scaling of cement concrete pavements containing AS aggregates. Nevertheless, only a few studies have been conducted on the freeze-thaw resistance of concrete containing AS as a binder.  
In this study, concrete was mixed using AS and WS as binders, and concrete specimens were fabricated to experimentally evaluate the freeze-thaw resistance of hardened concrete, as well as its mechanical performance, such as the compressive and flexural strengths and air-void system. In addition, the microstructure of concrete subjected to freeze-thaw cycles was investigated. The experimental results of this study could provide useful information for selecting optimal binders for concrete structures built in regions experiencing very low temperatures.