## 1 Introduction  
Castables are widely used as a lining for the metal line and the bottom of steelmaking ladles; although contact with the liquid slag is not continuous, contact does occur in certain steps of the process [1]. Progressive technological development has made cement castables, i.e., low-cement castable (LCC) and ultra-low-cement castable (ULCC), less often used in today's industry. The above-mentioned castables differ in their CaO content: LCC (1-2.5 wt.%), ULCC (0.2-1 wt.%), and NCC (<0.2 wt.%) [2]. Currently, no-cement castables (NCC) are increasingly used. In comparison, the reactive powders Al\({}_{2}\)O\({}_{3}\), MgO, microsilica, and sols are widely used as binders [3]. Reducing the overall CaO content of the castable to a minimum allows the reduction of porosity, the increase of strength, and an improvment to both refractory and corrosion properties [4]. LCC castables are often supplemented with microsilica. This common phenomenon is due to several factors. One of the main features is the ability to react and form strong bonds, both at low and high temperatures. The use of small amounts of microsilica in castables promotes hydration resistance, e.g., MgO. The porosity of such a castable is also reduced, and the mechanical properties are improved [4; 5; 6]. However, if the CaO content in the castables is too high, low-melting phases (e.g., anorthite: 1553 \({}^{\circ}\)C and gehlenite: 1593 \({}^{\circ}\)C) [7] may form in reaction with SiO\({}_{2}\), which negatively affect the properties of the concretes. Therefore, the use of microsilica is only beneficial in castables with a low CaO content or in castables not containing CaO [8]. Therefore, good technological properties can be proposed as castables composed of corundum as aggregates and a cement-free binder containing fine powders of MgO, Al\({}_{2}\)O\({}_{3}\), and SiO\({}_{2}\). This enables the ability to obtain a resistant material. Therefore, in this study, spinel-forming alumina castables with different amounts of MgOwere tested. The new generation of cement-free castables eliminates the need to use a large amount of mixing water, which increases the material's resistance to high temperatures and reduces the tendency to crack and corrode [9; 10]. Many authors have tested castables by modifying their composition. Analysis of the literature [11; 12; 13; 14; 15] showed that the use of spinel in castables, both spinel-forming and spinel-containing, as well as the content of microsilica in castables, has an impact on the technological solutions of castables. The use of spinel (MgAl\({}_{2}\)O\({}_{4}\)) in castables increases both the slag corrosion resistance and the erosion resistance, and increases the operating temperatures [16]. The slag corrosion resistance of castables containing spinel, as well as the mechanism of the castables' reaction in contact with slag, have been investigated by many authors [17; 18; 19; 20; 21; 22; 23; 24; 25]. Spinel can be added to castables in two ways: as prefabricated spinel or by the in situ matrix reaction of Al\({}_{2}\)O\({}_{3}\) with MgO. The optimal prefabricated spinel content is in arrange from 15 to 30 wt.%. However, a spinel content of between 20 and 25 wt.% is most preferred [16]. Spinel content that is too low increases the rate of corrosion reactions. On the other hand, too much spinel in the material results in deeper penetration of the material by the slag [26; 27]. [PERSON] et al. [28], assessed the corrosion resistance and penetration of alumina spinel castables with different cement content using slag. Based on these studies, it was found that the corrosion of the material increases with the increase of CaO content in the material. By using a prefabricated spinel, the risk of cracking as a result of hydration of free MgO is reduced [29]. Spinel formation, as seen in the in situ reaction, results in the spinel being better dispersed in the matrix than the prefabricated spinel.