## 1 Introduction  
According to the World Steel Association, the global crude steel production in 2019 was 1869.9 million tons, and it has been increasing every year [1]. Large amounts of by-product that have been generated from steelmaking business need to be properly utilized/disposed in order to minimize the environmental impact. Blast furnace slag, generated in a blast furnace that manufactures pig iron from iron ore, is pulverized and successfully recycled as a high value-added supplementary cementitious material (Ground granulated blast-furnace slag, GGBFS) [2, 3, 4, 5, 6, 7]. However, steelmaking slag is mostly used as a low-value-added landfill material such as road pavement and embankment [8, 9]. It should be noted that recycling of such waste materials also reduces the burden for CO2 emission from steelmaking industry [10]. Steelmaking slag usually refers to various types of slags that were generated in the process of refining steel from pig iron. Most common types of steelmaking slags are converter slag and electric arc furnace slag. The amount of production and the chemical composition of those slags can vary depending on the properties of raw materials to be injected, properties (or types) of steel that will be produced, and the processing technology that was applied [11]. In most cases, since the steelmaking slag undergoes a rapid cooling process, there is not enough time for cations such as Ca and Mg present in the slag to be stably fixed inside the silicate structure during the smelting process. This means that some amount of Ca and Mg exists as in the form of free lime (CaO) and periclase (MgO), and thus causes harmful expansion by formation of portlandite (Ca(OH)2) or brucite (Mg(OH)2)when used as aggregates for concrete [11; 12]. Converter slag and electric arc furnace oxidation slag are those that are known to cause harmful expansion, so they are restricted from being utilized as an aggregate for concrete. Previous studies that evaluated the stability of converter slag as concrete aggregate reported that converter slag could be used as an aggregate if the reaction of the expandable material was induced in advance [13; 14]. There was a report that recommends a minimum of three months for aging period in order to be utilized as aggregates for bituminous materials [15]. According to United States Geological Survey (USGS), it is recommended to undergo an aging process of six months to use steelmaking slag as a road-based aggregate. [16; 17]. From the earlier works, three months of aging period was not found to be enough to provide stability of steelmaking slag (combination of electric arc furnace oxidation slag and converter slag) as concrete aggregate [18]. Considering all earlier findings, it is necessary for converter slag to spend six months of aging process in order to avoid any future problem associated with expansive reaction. However, such process takes too long time, and requires additional space for extended storage period. In this work, a method that can process converter slag in shorter period of time is investigated. It is to utilize carbonation by CO\({}_{2}\) in supercritical condition. Since the viscosity and density of supercritical carbon dioxide (SCD) are similar to those of the gaseous state, and the mobility is higher than that of the liquid state, it is known to cause a more aggressive carbonation process [19]. In fact, utilizing carbonation for enhancing properties of recycled aggregate has been an interesting topic for some time [20; 21], but the application of supercritical CO\({}_{2}\) was limited. In earlier works on the reaction of recycled aggregate and waste concrete with SCD [19; 22; 23; 24], it was reported that the calcium hydroxide inside the concrete reacts completely within a short period (within 1 day), and calcium carbonate was also formed to be produced from dissociation of calcium ions present in the C-S-H [19; 22; 23; 24].