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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...