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The hydration of an equimolar mixture of MgO and Al 2 O 3 nano-powders has been proven to be an effective way to synthesize Mg 6 Al 2 CO 3 (OH) 16 ·4H 2 O as a component of a nano-structured matrix and magnesia-alumina spinel precursor for high-performance cement-free corundum-spinel refractory castables. (Mg 3 )-OH-brucite sites (417 • C) formed initially within the magnesia-alumina hydrating blended paste were replaced with (Mg 2 Al)-OH and (Mg 3 )-OH hydrotalcite sites, which were dehydroxylated at 420 • C and 322 • C, respectively. This reorganization was connected with the incorporation of anions and water molecules in the interlayer spacing of hydrotalcite, which was dehydrated at 234 • C. Hence, the thermal decomposition of a nano-structured matrix system containing mainly Mg 6 Al 2 CO 3 (OH) 16 ·4H 2 O consists of a complex sequence of dehydration, dehydroxylation and decarbonization, and this finally leads to the formation of inverse spinel MgAl 2 O 4 and periclase MgO through many intermediate stages containing the mixed tetrahedral-octahedral Al phase and MgO-like structure. Hence, the hydraulic bond that primarily existed was replaced by a ceramic bond at a relatively low temperature, i.e., 700 • C, where a spinel was formed. Important changes in oxygen coordination polyhedra around Al 3+ in the dehydrated-dehydroxylated hydrotalcite occurred between 600 and 1100 • C. instead of micro-scaled particles as magnesia and alumina sources results in an increase in the reactivity of nano-powders [13,14] and their ability to form Mg-Al-CO 3 hydrotalcite-like compounds [8].Due to the special lamellar structure, a new application has been found for hydrotalcite that enables refractory castable manufactures to use reactive nano-and micropowders of alumina (Al 2 O 3 ) and magnesia (MgO) as alternative cementitious materials. Various authors highlighted that materials with "in situ" formed Mg-Al hydrotalcite-like phases exhibit higher mechanical strength of the green body than hydratable alumina-free refractory systems. This advantage is related to the fact that the spinel-like phase can be formed at lower temperatures [15][16][17]. There are many publications about the methods of obtaining hydrotalcite-like phases; it can be obtained through mechanochemical synthesis, sol-gel syntheses or hydrothermal precipitation [8,[18][19][20]. Nevertheless, there are no published data concerning obtaining the hydrotalcite from a mixture of nanometric MgO and Al 2 O 3 oxides. According to the MgO-Al 2 O 3 -H 2 O system at low temperatures, the products of the reaction of magnesium and alumina oxides with water are single or/and double hydroxides. Layered double hydroxides (LDHs) are represented by the general formula [Mg 1−x Al x (OH) 2 ] x+ [(A n− ) x n ·yH 2 O] x− (where A n− is the exchangeable interlayer anion located between two LDH sheets and n-is a charge) [21,22]. The hydrotalcite structure is derived from the structure of brucite and the range of x can be varied, depending on literature sources, between 0.17 and 0.33...

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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.

The reactivities of hydratable alumina, calcium zirconium aluminate and their binary mixtures were investigated by calorimetric method, X-ray diffraction, Fourier transform infrared spectroscopy (FT-IR), differential thermal/thermogravimetric analysis (DTA-TG) and scanning electron microscope observations. The rate of heat evolution illustrates only one distinct exothermic peak in the mixtures containing calcium zirconium aluminate which corresponds the subsequent precipitation of hydrated material mainly in the form of amorphous CaO-Al 2 O 3 -H 2 O phases and crystalline C 4 AH 19 occurs simultaneously after wetting of Ca 7 ZrAl 6 O 18 grains. Nevertheless, coexistence of both unstable calcium aluminate hydrates (CAH 10 , C 2 AH 8 , C 4 AH 19 ) and thermodynamically more stable C 3 AH 6 detected by FT-IR and DTA-TG in the paste cured at 20°C may have originated in the thermally induced partial conversion reaction due to the considerable amount of heat generated by the Ca 7 ZrAl 6 O 18 hydration. It is also found that curing temperature affected the hydration products formed in the hydration products of both Ca 7-ZrAl 6 O 18 and Ca 7 ZrAl 6 O 18 /Al 2 O 3 blends. Reactive alumina influences the hydration behavior of Ca 7 ZrAl 6 O 18 facilitating the nucleation and growth of hydration products, causes characteristic changes in the microstructure of hardened blended pastes and favors recrystallization of dehydrated calcium aluminates.