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Abstract. In this work the synthesis of hercynite from Fe 2 O 3 and Al 2 O 3 powders was carried out by arc-melting method under the protective argon atmosphere. The obtained material was characterized with the use of powder X-ray diffractometry (XRD) and Mössbauer spectroscopy (MS). A Mössbauer effect in hercynite obtained by the arc-melting method indicated the cations distribution in the spinel structure among the tetrahedral and octahedral interstices.

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This work directly links the performance with the phase evolution in the MgO-Al 2 O 3 -SiO 2 -H 2 O system during the hydrothermal treatment. Cement-free refractory binders, considered as alternative to calcium aluminate cements, with the chemical compositions fine-grained mixtures of MgO-Al 2 O 3 , MgO-Al 2 O 3 -SiO 2 , and MgO-SiO 2 reactive powders were subjected conversion from dry mixture to hydrated matrix at ca. 240°C under autogenous water vapor pressure for 56 h. The main purpose of this approach is to simulate the thermal behavior of the hydrated castable matrix belonging to the MgO-Al 2 O 3 -H 2 O, MgO-Al 2 O 3 -SiO 2 -H 2 O, and MgO-SiO 2 -H 2 O systems when exposed to heat treatment of large-format precast monolithic refractories. The phase compositions of the hydrated samples were determined by X-ray diffraction (XRD) technique using CuKα radiation. The FT-IR scans were used to evaluate the functional groups of the hydrated materials. Thermal decomposition mechanism and microstructure were examined by coupled DSC-TG-EGA (MS) and SEM-EDS, respectively. It is shown through presented results that boehmite (AlO(OH)), brucite (Mg(OH) 2 ), and magnesium-and aluminum-layered double hydroxide-like phase ([Mg 6 Al 2 (OH) 18 4.5H 2 O]) were formed via hydrothermal synthesis in the MgO-Al 2 O 3 -H 2 O system. Chrysotile (Mg 3 [Si 2−x O 5 ](OH) 4−4x ) was detected in the MgO-SiO 2 -H 2 O binder system as a main phase and in the MgO(rich)-Al 2 O 3 -SiO 2 -H 2 O binder system as secondary phase. For the sample with the Al 2 O 3 excess, two magnesium aluminum silicate hydroxides ((Mg,Al) 6 (Si,Al) 4 O 10 (OH) 8 , Mg 5 Al 2 Si 3 O 10 (OH) 8 ), together with MgAl(OH) 14 xH 2 O, Mg(OH) 2 , and AlO(OH), were formed in the MgO-Al 2 O 3 (rich)-SiO 2 -H 2 O binder system. Since the type of hydrates contributed to the thermal stability of the binder matrixes, the valuable practical results concern mainly on the optimization of heat treatment process of state-of-the-art CaO-free matrixes being considered as precursors in the low-temperature synthesis of high refractory phases like spinel and forsterite.

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