Influence of hydrothermal conditions on the phase composition of materials from the system MgO-Al2O3-SiO2-H2O

This work directly links the performance with the phase evolution in the MgO-Al2O3-SiO2-H2O 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-Al2O3, MgO-Al2O3-SiO2, and MgO-SiO(2)reactive powders were subjected conversion from dry mixture to hydrated matrix at ca. 240 degrees 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-Al2O3-H2O, MgO-Al2O3-SiO2-H2O, and MgO-SiO2-H2O 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 CuK alpha 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 ([Mg6Al2(OH)(18)4.5H(2)O]) were formed via hydrothermal synthesis in the MgO-Al2O3-H2O system. Chrysotile (Mg-3[Si2-xO5](OH)(4-4x)) was detected in the MgO-SiO2-H2O binder system as a main phase and in the MgO(rich)-Al2O3-SiO2-H2O 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), Mg5Al2Si3O10(OH)(8)), together with MgAl(OH)(14)xH(2)O, Mg(OH)(2), and AlO(OH), were formed in the MgO-Al2O3(rich)-SiO2-H2O 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.