## 4 Discussion
The mechanism of their action is connected with the adsorption and changing of the surface electric charge of ceramic phases as well as the creation of coordination compounds with metal ions preventing the fast precipitation of hydrated phases. During the hydration of MgO, the surface of the grains is protonated by protons from water, after that it creates a complex with -OH groups from water which dissolve in water [30]. This increases the pH and is followed by the dissociation of the citric as well as tartaric acid. Citric acid in pH above 8 is fully dissociated to form C\({}_{6}\)H\({}_{5}\)O\(\gamma^{3-}\), similarly, the tartaric acid in a fully dissociated form has the formula C\({}_{4}\)H\({}_{4}\)O\({}_{6}\)\({}^{2-}\). Both of them have high potential to adsorb on the MgO grains and decrease their hydration; while in the solution, they coordinateMg and Al ions inhibiting their precipitation as hydroxide [14; 16]. This last effect probably promotes the synthesis of the layered hydroxide-like hydrotalcite and double magnesia alumina hydroxide hydrate. This aspect looks similar in the case of alumina, the samples with the addition of chelates have a lower content of boehmite (according to the thermal analysis by around 1.4 and 1.7% for samples 2 and 3, respectively) but higher in aluminum oxide (Figure 5a). This could also be connected with the transformations of aluminum hydroxide and/or aluminum oxide. Based on the conducted analysis, in the studied samples there was no evidence of alumina hydroxide compound, although it was an expected intermediate phase, during the increasein the temperature to the maximum value (240 \({}^{\circ}\)C) assumed in the experiment. According to the different authors, gibbsite transforms to boehmite in temperatures from 130 \({}^{\circ}\)C [20], 150 [19] or 220-230 \({}^{\circ}\)C while to bayerite these were at around 280-300 \({}^{\circ}\)C [22]. Moreover, some authors state that under temperaturexceeding 130-150 \({}^{\circ}\)C, boehmite can be directly created from the alumina as a stable phase [18; 19; 20; 21]. Based on that, it can be concluded that the absence of the aluminum hydroxide is connected with its fast decomposition to boehmite during the experiment (during the increase in temperature up to the assumed 240 \({}^{\circ}\)C), but this conclusion needs further investigation. Nevertheless, this work proves that the presence of chelate compound modified the behavior of the castables during the heat treatment. This is connected with the changes of the content of the hydraulic phases, the increase in the content of hydrotalcite and double magnesium and aluminum hydroxide hydrate, with a simultaneous decrease in brucite and boehmite content. These facts can be used in the better matching of heat treatment conditions during the preparation of the castables. Moreover, some of these compounds like hydrotalcite and double magnesium and aluminum hydroxide hydrate may decrease the synthesis temperature of the spinel phase [9]. This is probably connected with the thermal decomposition of these compounds with the creation of an amorphous, metastable, mixed oxide solution [24]. By controlling the processes in the studied range of temperatures, we can influence the reactions that occur at a higher temperature and finally the properties of the castables.