## 1 Introduction  
Castables are a wide group of materials, one of the main features of which is the heat treatment process during preparation. The reactions of synthesis and sintering of the materials are activated by high temperatures. Alumina-magnesia spinel is a compound with a wide application in ceramic materials, as well as an important component of alumina spinel refractory castables. The solid-state synthesis of the spinel from oxide powders is the most popular route to obtaining it for an industrial purpose. During this process, the reaction proceeds according to the Wagner model. According to it, the spinel synthesis reaction is a diffusion process (counter-directed diffusion of Mg and Al ions while oxide ions remain on the initial sites), which is activated by the temperature. However, due to a low diffusion coefficient and also the formation of a diffusion-impeding product layer, the process of spinel synthesis is very costly [1]. Moreover, the mixing of the spinel component in the refractories generates problems with obtaining a fully homogeneous distribution in the material. For those reasons, the so-called insitu synthesis of spinel is used. In this kind of process, the spinel is generated from the substrates directly in the refractory castables, during high-temperature heat treatment. However, in this method, the presence of water modifies the course of the synthesis reaction. The starting substrates are not only oxides but also hydroxides of magnesium, and aluminum, and the conditions in the material during the increase in temperature become similar to the hydrothermal ones. This is the difference between the classical solid state synthesis of the spinel and the insitu synthesis of this phase in castables. Although spinel solid state synthesis from oxides is well characterized [1; 2; 3; 4; 5; 6; 7; 8], there is still a lack of knowledge about the insitu synthesis of the spinel in refractory castables, especially at the early stage of such a reaction at the temperature level where water vapor can be present. In refractory castables, oxide powders are used as substrates in spinel synthesis. In contrast to the solid-state reaction in the castables, the synthesis of the spinel takes place in several intermediate stages. In the first step, during the preparation of the castable, the oxide powders hydrate. During the heat treatment, hydraulic compounds undergo parallel dehydration and also rehydration due to high water vapor pressure, after further temperature increase; all these hydraulic phases convert to oxides which react to form the spinel phase. According to [9], this sequence of the reaction influences the properties of the synthesized spinel. The spinel formed insitu in such a process has a lower formation temperature, lower crystal size, and is better dispersed in the castables. This provides outstanding corrosion resistance and improves the high-temperature properties of the castables [9; 10; 11]. According to the MgO-Al\({}_{2}\)O\({}_{3}\)-H\({}_{2}\)O phase system, at low temperatures, characteristic for preparing and curing castables, the reaction of magnesium oxide and alumina oxide with water leads to the formation of magnesium hydroxide, aluminum hydroxide, and Mg/Allayered double hydroxides (LDH) and in the case of the presence of CO\({}_{2}\) in the system also of hydrotalcite. The hydration reaction of Al\({}_{2}\)O\({}_{3}\) in ambient temperature in the refractory castables isusually connected with transition alumina. During the rapid hydration of transition alumina at elevated pH, the main crystalline product is bayerite rather than gibbsite. In these conditions, \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) is inert, but this could change depending on the conditions like pH, temperature and water vapor pressure. Magnesium oxide is transformed to brucite as one stable polymorph of magnesium hydroxide and both MgO and Al\({}_{2}\)O\({}_{3}\) can create DHL or hydrotalcite-like compounds [12; 13].