## 1 Introduction  
In 2019, the world's crude steel production was 2.196 billion tons [1]. According to the main smelting process, steel slag can be divided into basic oxygen furnace (BOF) slag (also known as converter slag) and electric furnace (EAF) slag [2]. The molten steel slag produced from the steelmaking furnace goes through different means of processing, such as hot steel slag splashing after crushing screening-magnetic separation process technology, fewer steel slag treatment technology, rotary cylinder process technology [3], etc. Steel slag is a by-product of the steel industry [4]; its production is 15-20% of crude steel production and cumulative storage exceeds 1.2 billion tons in China [5]. The accumulation of a large amount of steel slag causes problems such as land occupation, environmental pollution, and the waste of resources [6]. Although the utilization of steel slag has been continuously optimized and improved in recent years, the current utilization rate of steel slag in China is only 22%, which is far behind developed countries such as the United States, Japan, Germany, and France. The utilization rate of steel slag in these countries is close to 100% [7]. Therefore, there is still a void in terms of development in the resource utilization of steel slag in China. The differences in steelmaking raw materials and smelting processes result in different chemical compositions and mineral phase types in steel slag, but most of its main chemical components are composed of calcium oxide (CaO), magnesium oxide (MgO), silicon oxide (SiO\({}_{2}\)), aluminum oxide (Al\({}_{2}\)O\({}_{3}\)), ferrous oxide (FeO), ferric oxide (Fe\({}_{2}\)O\({}_{3}\)), manganese oxide (MnO), free calcium oxide (f-CaO), free magnesium oxide (f-MgO), etc. [8]. Due to the different origins and smelting processes, the percentages of chemical components are also different, but for most steel slags, CaO, MgO, SiO\({}_{2}\), and FeO/Fe\({}_{2}\)O\({}_{3}\) account for the main part. The chemical composition of certain steel slag is shown in Table 1[9]. The main mineral composition of steel slag is dicalcium silicate (C\({}_{2}\)S), tricalcium silicate (C\({}_{3}\)S), dicalcium ferrite (C\({}_{2}\)F), and solid solution (RO). [10]. Steel slag is rich in metal elements, non-metallic elements, and has good structural properties including porous, high density, high hardness, wear resistance, and high compressive strength; thus, it has potential for high value-added applications. Currently, steel slag is mainly used in architecture, agriculture, industry, and other fields. In architecture, steel slag is often used instead of cement to improve the strength of the mixture due to containing the gel composition in steel slag [11], or used to produce steel slag bricks [12], concrete aggregates [13], and asphalt mixtures [14]. It is also used for soil improvement and in chemical fertilizers [15]. In the industrial field, steel slag is not only recycled by steel mills as a co-solvent to replace limestone, dolomite, and other raw materials [16], but also it is often used to extract metals [17]. However, the utilization rate of steel slag in China has not been improved. There may be several reasons: (i) there is a certain amount of iron particles in the steel slag, which leads to its poor grindability; (ii) the hydration activity of steel slag is low, especially the early activity because only a few highly active minerals exist in steel slag [18]; and (iii) the volume stability of steel slag is poor [19] since the high content of f-CaO and f-MgO leads it to undergo hydration reaction to result in volume expansion. However, steel slag used in the field of catalysis is not impacted by the above reasons, and the surplus steel slag will greatly reduce the cost of catalyst raw materials. In recent years, through the research and exploration of steel slag in catalysis, it has been found that the application of steel slag in catalysis has great potential--especially after the modification of steel slag, the catalytic performance is greatly improved.