## 4 Discussion  
This work explains the reactions and the process that occur under hydrothermal conditions in the MgO-Al\({}_{2}\)O\({}_{3}\)-H\({}_{2}\)O system with chelates addition. The aim of the utilization of the hydrothermal conditions was to simulate the conditions that prevail during the first stage of the high temperature heat treatment of the refractory castables up to ca. 500-600 \({}^{\circ}\)C. In this range of temperature, most of the hydraulic phases undergo a decomposition with the creation of ceramic phases. Newly created ceramic phases are often altered in comparison with the original raw materials used in the castables' composition. They have smaller crystallites and are better dispersed and connected with each other, thanks to the synthesis of many new phases like spinel, etc., which takes place at a lower temperature. Chelates like citric and tartaric acid are often used in castables as dispersing agents: from the presented result, it was found that chelates affect the hydration behavior of MgO and Al\({}_{2}\)O\({}_{3}\). According to the XRD analysis results (Figure 2), it can be stated that among other hydraulic phases, samples are also consisting of double magnesium aluminum hydroxide hydrate. However, the thermal analysis shows (Figure 3b) the emission of carbon dioxide in the temperature range of 280-550 \({}^{\circ}\)C is probably connected with the second endothermic effect at ca. 390 \({}^{\circ}\)C. Moreover, the FTIR analysis (Figure 6) shows some amount of carbonate ions in the structure of the samples (wide bands around 1370-1460 cm\({}^{-1}\)). This could indicate hydrotalcite as one of the components of the samples. According to [10; 24], the emission of carbon dioxide in hydrotalcite takes place in the range of 400-580 \({}^{\circ}\)C and the loss of mass connected with that is small and stretched over a long range of temperatures. However, taking all the facts into consideration, it can be stated that the hydrated double phase present in the samples is a mixture of double magnesium aluminum hydroxide hydrate with a poorly crystalline hydrotalcite-like phase. This would explain lack of this phase on the XRD spectra. Based on this, it can be stated that the studied samples are composed of aluminum oxide, aluminum oxide hydrate, boehmite, brucite, and a mixture of poorly crystalline hydrotalcite-like phase and double magnesia alumina hydroxide hydrate. Moreover, in the samples with an addition of chelates, traces of MgO are also present. According to the thermal analysis (Figure 4), FTIR analysis (Figure 6) as well as the XRD analysis results (Figure 5d), the influence of the chelates on the amount of the mixture of hydrotalcite-like phase and double magnesia alumina hydroxide hydrate can be observed. With the addition of acids, the amount of this mixture increases by around 1% (based on the TG curve), which is confirmed by the XRD analysis (Figure 5d), where the intensity of the diffractive lines for the double magnesia and alumina hydroxide hydrate is higher for these samples. Moreover, the FTIR analysis shows a higher relative intensity of the bands from carbonate anion, connected with hydrotalcite-like phase, for the samples with acids additions. Simultaneously, we can observe the XRD spectra traces of MgO and also a decrease in the amount of brucite (Figure 5c) for samples 2 and 3. This difference, based on the TG curve, is around 0.6%, but this is connected both with the decomposition of brucite as well as the mixture of hydrotalcite-like phase and double magnesia alumina hydroxide hydrate. These latter compounds decrease the difference in mass loss between the reference sample and the samples with the additives. Taking all these facts into consideration, it can be stated that the addition of acids increases the content of the double hydroxide and hydrotalcite-like phase, and decreases the amount of brucite. These aspects can be connected with a well-known anti-hydration effect of chelate compounds.