In Situ Spinel Formation in a Smart Nano-Structured Matrix for No-Cement Refractory Castables

The hydration of an equimolar mixture of MgO and Al2O3 nano-powders has been proven to be an effective way to synthesize Mg6Al2CO3(OH)16∙4H2O as a component of a nano-structured matrix and magnesia-alumina spinel precursor for high-performance cement-free corundum-spinel refractory castables. (Mg3)–OH–brucite sites (417 °C) formed initially within the magnesia–alumina hydrating blended paste were replaced with (Mg2Al)–OH and (Mg3)–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 Mg6Al2CO3(OH)16∙4H2O consists of a complex sequence of dehydration, dehydroxylation and decarbonization, and this finally leads to the formation of inverse spinel MgAl2O4 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 Al3+ in the dehydrated-dehydroxylated hydrotalcite occurred between 600 and 1100 °C.


Introduction
Cement-free castables (no-cement-content (NCC) castables) have been dynamically developed in recent years because of many reasons: the amount of water required to affect chemical bonding is lower in comparison to the conventional castables; the resulting effective dewatering of castables and both the removal of physically absorbed and chemically bonded water; the absence of lime favorably improves the high temperature properties of castables but they also exhibit outstanding chemical, physical and mechanical properties [1]. The binding systems play a relevant role in workability, dry-out, green mechanical strength, both mechanical and thermomechanical properties and also corrosion resistance. The state-of-the-art nano-scaled alternative binding systems to calcium aluminate cement (CAC) binding have been extensively developed in recent years [2]. The most recent achievements in this field include coagulating binders such as colloidal silica or alumina [1,3], ultrafine-SiO 2 and MgO [4][5][6], ultrafine-Al 2 O 3 and MgO [7][8][9][10] and a suspended colloidal precursor of mullite and spinel [3]. The recent findings from the literature [11] have clearly shown that, e.g., the nanoscale alumina reinforced alumina-spinel matrix castables exhibit a significant improvement in the thermal shock resistance or the mechanical strength of colloidal binder containing castables increases due to higher sinterability of colloidal particles as the temperature increases [12]. Using nano-scaled particles spinel precursor for cement-free corundum-spinel castables. For this purpose, the effect of time on the progress of hydration of nano-MgO-nano-Al 2 O 3 blended paste structure was studied. On the other hand, the spinel formation in a smart nano-structured matrix was analyzed through the thermal transformation of the hydrotalcite-containing blended paste into spinel, being an effective ceramic bonding designed for no-cement corundum-spinel refractory castables. In summary, the combination of nano-powders of MgO and Al 2 O 3 and water provides a cement and CaO-free binding system designed for alumina-based castables. The presented approach illustrates the direction of the chemical reaction within this system, which is slower or faster than within the other system containing, e.g., micropowders or colloidal suspensions, respectively. The Mg-Al layered double hydroxide-like phases formed are considered the main binding agents and precursors for the low-temperature synthesis/formation of the MgAl 2 O 4 spinel phase. We propose state-of-the-art supplementary cementitious materials, representing an environmentally friendly alternative with strong potential for castables, since no CO 2 is emitted during cement-free production processes.

Sample Preparation and Analytical Techniques
The synthesis of the Mg-Al-CO 3 hydrotalcite-like phase was obtained by a curing process of the nano-MgO-nano-Al 2 O 3 blended paste prepared with MgO nano-powder (<50 nm particle size, Aldrich, 99.8%) and Al 2 O 3 nano-powder (13 nm particle size, Aldrich, 99.8%). For this purpose, the dry mix of nano-powders was weighted with the MgO:Al 2 O 3 molar ratio of 1:1, homogenized and mixed with water, keeping the ratio of the weight of water to the weight of dry mix equal to 3. The sample in paste form was sealed in polyethylene bags and cured up to 49 days in a climatic chamber with the relative humidity maintained at 95% and a temperature of 50 • C. Two reference samples were prepared via the hydration of MgO nano-powder or Al 2 O 3 nano-powder under the same curing conditions to achieve hydroxides for 28 days. To identify the blended paste phase changes corresponding to the curing time, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and thermal analysis, i.e., simultaneous thermogravimetry (TG), differential scanning calorimetry (DSC) and evolved gas analysis-mass spectrometry (EGA-MS), were used. Samples for the ex situ measurements were ground in acetone to discontinue the hydration after 24 h, 3 days, 14 days, 28 days and 49 days. The in situ high-temperature XRD (HT-XRD) measurement was applied to evaluate a complex sequence of dehydration, dehydroxylation, decarbonization of the fully-reacted blended paste and formation of a series of metaphases at elevated temperatures. The temperatures were suggested by the thermal analysis results.
As a next step, the fully-reacted (after 49 days of hydration) nano-MgO-nano-Al 2 O 3 blended paste containing synthetic hydrotalcite was heated to 600, 700, 800, 900, 1000 and 1100 • C and maintained at a selected temperature for 10 h. The solid calcination products were characterized by ex situ XRD, FT-IR and 27 Al MAS-NMR (solid-state magic-angle spinning nuclear magnetic resonance) measurements.
Furthermore, sintering of an equimolar mixture of MgO (Acros Organics, 98% MgO) and Al 2 O 3 (Acros Organics, 98% Al 2 O 3 ) micropowders in three-steps with intermediate grinding was applied with success for the synthesis of the stoichiometric spinel MgAl 2 O 4 as reference material. The holding time was 10 h and the applied temperature 1700 • C. This reference material was investigated by NMR, FT-IR and XRD.

Measurement Conditions
The strength of the binder alternative to calcium aluminate cement containing MgO and Al 2 O 3 micropowders was determined using standard tabular alumina, a 1:1 molar mixture of MgO and Al 2 O 3 and water. A basic mixture of mortar was made using the mass proportions of binder (25 mas. %), standard tabular alumina (75 mas. %) and water. The mass proportion of 1 binder and 0.75 water Materials 2020, 13, 1403 4 of 20 (the water/binder mass ratio of 0.75) was applied for this purpose. The reference mortar containing calcium aluminate cement was also prepared with a water/cement mass ratio of 0.5.
The measurements of bending strength (modulus of rupture (MOR)) and cold crushing strength (CCS) of samples were taken after 3 days and 7 days of curing at room temperature. The bending strength was measured by a 3-point bending test using a specimen of 25 × 25 × 120 mm, a span of 75 mm and a test speed of 50 N/s (reference CAC-based mortar) or 10 N/s (MgO-Al 2 O 3 -based mortar). After the mechanical 3-point bending test, half of each sample was placed in the compression testing equipment in order to determine the CCS, and the loading speed was controlled at 2.4 kN/s (reference CAC-based mortar) or 50 N/s (MgO-Al 2 O 3 -based mortar). The thermal insulation properties (thermal conductivity λ, thermal diffusivity a, volumetric heat capacity C v ) of both mortars were measured at room temperature by ISOMET 2114 working on a dynamic measurement principle. For this purpose, the prismatic samples with dimensions of 40 × 40 × 160 mm were cast from a basic mixture of mortars. The heat transport parameters determination was made 3 days after casting the mortar specimens.
Thermal analysis was carried out in airflow (50 mL·min −1 ) at a heating rate of 10 • C·min −1 using a Simultaneous Thermo Analyzer (STA) TG-DSC NETZSCH STA 449F5 Jupiter coupled to QMS 403 D Aëolos (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany). The provided sample mass~20 mg was heated to 1000 • C in a corundum crucible. Alpha-Al 2 O 3 was used as a standard substance.
The in-situ HT-XRD measurement was carried out at temperatures suggested by the thermal analysis results for a 2θ-range of 1-70 • with a step size of 0.016 • and a counting time per step of 4 s. The heating rate was 10 • C·min −1 , and upon reaching the temperature, it was kept constant for 3 h before the measurement at constant temperature (30,100,150,200,250,350,390, 450 and 500-1100 • C with a step of 100 • C) was carried out. The X'Pert Pro (PANalytical, Malvern PANalytical, Malvern, UK) diffractometer was used for this purpose.
The ex situ XRD measurement was carried out at room temperature using an X'Pert ProPANalytical X-ray diffractometer, with Cu Kα radiation, with 0.02 • per step and a time of 3 s per step (2θ-range of 5 • -90 • ). HighScore Plus software (Panalytical) with the PDF-2 database supported by the ICDD (The International Centre for Diffraction Data, Newtown Square, USA) was used for data analysis.
The FT-IR measurements were carried out using a Bruker Vertex 70v FT-IR spectrometer on pelletized ca. 0.5 wt.% powder samples in a KBr matrix. The IR bands were recorded at a spectral range of 400-4000 cm −1 .

Mechanism of Hydrotalcite Formation in the nano-MgO-nano-Al 2 O 3 Blended Paste
The time-dependent hydration behavior of the nano-MgO-nano-Al 2 O 3 blended paste was investigated at the age of 24 h and 3-49 days by the following complementary methods: X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry and thermogravimetry analyses coupled with evolved gas mass spectrometry (DSC-TG-MS). The observations relevant to the specimens tested after each curing period are presented in Figures 1-3. In Figure 1a The given patterns will be useful for further monitoring the compositional changes within the nano-MgO-nano-Al 2 O 3 blended paste. As can be recognized from Figure 1c, magnesium oxide and aluminum oxide nano-powders react almost immediately with water to form a magnesium-aluminum hydrotalcite compound, i.e., Mg 6 Al 2 CO 3 (OH) 16 ·4H 2 O, during the initial 24 h curing period after wet homogenization. It was also Materials 2020, 13, 1403 5 of 20 shown that traces of initially formed Mg(OH) 2 were consumed during hydration and more and more hydrotalcite-like compounds were formed. The time evolution of the main characteristic diffraction lines of Mg 6 Al 2 CO 3 (OH) 16   The progress of hydrotalcite formation in the nano-MgO-nano-Al2O3 blended paste observed by FT-IR method confirmed previous results from XRD. The most characteristic features of the FT-IR spectra of all blended pastes (Figure 2b) is the gradual disappearance of the sharp band at ca. 3701 cm −1 as a result of the consumption of the initially formed, free Mg(OH)2 and the appearance of the broad band in the range of ca. 3000-3700 cm −1 , mainly due to O-H groups present in metal hydroxide layers (hydroxyl stretching region). The interaction between Mg(OH)2 and nano-Al2O3 particles was demonstrated by a reduction in the spectral intensities of the alumina along with the curing time. According to our research, the outline IR spectrum of nanoscale-alumina raw material (Figure 2a) was close to the gamma aluminum oxide (γ-Al2O3) nanoparticles presented in [33]. The wide and unresolved spectrum extending from ca. 450 to 850 cm −1 is typical for a complex and disordered crystallographic structure of nanoscale-alumina. After the initial curing period of 24 h, no band related to Al-OH is seen in the spectrum, indicating no alumina hydroxides were formed, as shown by the comparison of two spectra for blended paste at early hydration age (24 h, Figure 2b) and The progress of hydrotalcite formation in the nano-MgO-nano-Al 2 O 3 blended paste observed by FT-IR method confirmed previous results from XRD. The most characteristic features of the FT-IR spectra of all blended pastes (Figure 2b) is the gradual disappearance of the sharp band at ca. 3701 cm −1 as a result of the consumption of the initially formed, free Mg(OH) 2 and the appearance of the broad band in the range of ca. 3000-3700 cm −1 , mainly due to O-H groups present in metal hydroxide layers (hydroxyl stretching region). The interaction between Mg(OH) 2 and nano-Al 2 O 3 particles was demonstrated by a reduction in the spectral intensities of the alumina along with the curing time. According to our research, the outline IR spectrum of nanoscale-alumina raw material (Figure 2a) was close to the gamma aluminum oxide (γ-Al 2 O 3 ) nanoparticles presented in [33]. The wide and unresolved spectrum extending from ca. 450 to 850 cm −1 is typical for a complex and disordered crystallographic structure of nanoscale-alumina. After the initial curing period of 24 h, no band related to Al-OH is seen in the spectrum, indicating no alumina hydroxides were formed, as shown by the comparison of two spectra for blended paste at early hydration age (24 h, Figure 2b) and hydrated nano-Al 2 O 3 powder ( Figure 2a). Since, in a layered structure of brucite, Mg was partially substituted by Al along with the curing time, the electroneutrality was attained by incorporation of carbonate anions in the interlayer space (ca. 1371 cm −1 ), where water molecules (ca. 1643 cm −1 ) were also located ( Figure 2b). Figure 2c,d presents a deconvoluted FT-IR spectrum of the nano-MgO-nano-Al 2 O 3 blended paste at the age of 49 days in the range of 400-1100 cm −1 (c) and 2700-3800 cm −1 (d). Peak separations were carried out using Gaussian deconvolution. This approach was implemented since the kinetic energy distribution molecules (functional groups) is described by the Gaussian distribution. In other words, Doppler broadening produces a Gaussian line shape due to the Gaussian distribution of molecular velocities. Hence, this approach can be used successfully to study functional groups present in the complex cementitious systems. The positions of infrared bands of hydrotalcite as the main phase formed within this sample are summarized in Table 1. In the region below ca. 1000 cm −1 the IR spectrum of uncalcined Mg-Al-CO 3 -hydrotalcite shows the absorption bands at 550, 776 cm −1 and 605 cm −1 , which correspond to the 'Al'-OH and 'Mg'-OH translation modes, respectively. Another absorption near 449cm −1 is associated with M-O stretching vibrations (M = Mg and Al) in the octahedral host layers, whereas the more complex spectrum of the incompletely unreacted cementitious matrix contains substrates or transitory hydrates [10,37]. Moreover, strong out-of-plane symmetric deformation mode (ν 2 ) and antisymmetric deformation mode (ν 4 ) of hydrotalcite CO 2− 3 ions around 776 and 684 cm −1 , were observed, respectively. The band at 1362cm −1 indicated the antisymmetric stretching vibration (ν 3 ) of carbonate anion [37][38][39][40]. The bending vibration of interlayer water molecules (dO-H) [20,41] occurs as sharp bands at 1628 cm −1 and it can be concluded that interlayer water molecules are hydrogen-bonded to CO 2− 3 interlayer ions [8,10]. The broad and strong band around 3400 cm −1 is associated with the OH-metal vibration. There were the stretching vibrations of hydroxyl -OH groups attached to Mg and to both Mg and Al in brucite-like layers (OH-Mg 3 : 3575 cm −1 and OH-Mg 2 Al: 3454 cm −1 , respectively) [42][43][44]. The band that appears at 3700 cm −1 is associated with the O-H stretching vibration in brucite Mg(OH) 2 . Other bands in this region should be attributed to the CO 2− 3 -H 2 O bridging and H-bonded modes located at 3252 cm −1 and 3069 cm −1 , respectively [8,10].
Materials 2020, 13, x FOR PEER REVIEW 6 of 20 substituted by Al along with the curing time, the electroneutrality was attained by incorporation of carbonate anions in the interlayer space (ca. 1371 cm −1 ), where water molecules (ca. 1643 cm −1 ) were also located ( Figure 2b). Figure 2c,d presents a deconvoluted FT-IR spectrum of the nano-MgO-nano-Al2O3 blended paste at the age of 49 days in the range of 400-1100 cm −1 (c) and 2700-3800 cm −1 (d).
Peak separations were carried out using Gaussian deconvolution. This approach was implemented since the kinetic energy distribution molecules (functional groups) is described by the Gaussian distribution. In other words, Doppler broadening produces a Gaussian line shape due to the Gaussian distribution of molecular velocities. Hence, this approach can be used successfully to study functional groups present in the complex cementitious systems. The positions of infrared bands of hydrotalcite as the main phase formed within this sample are summarized in Table 1. In the region below ca. 1000 cm −1 the IR spectrum of uncalcined Mg-Al-CO3-hydrotalcite shows the absorption bands at 550, 776 cm −1 and 605 cm −1 , which correspond to the 'Al'-OH and 'Mg'-OH translation modes, respectively. Another absorption near 449cm −1 is associated with M-O stretching vibrations (M = Mg and Al) in the octahedral host layers, whereas the more complex spectrum of the incompletely unreacted cementitious matrix contains substrates or transitory hydrates [10,37]. Moreover, strong out-of-plane symmetric deformation mode (ν2) and antisymmetric deformation mode (ν4) of hydrotalcite ions around 776 and 684 cm −1 , were observed, respectively. The band at 1362cm −1 indicated the antisymmetric stretching vibration (ν3) of carbonate anion [37][38][39][40]. The bending vibration of interlayer water molecules (dO-H) [20,41] occurs as sharp bands at 1628 cm −1 and it can be concluded that interlayer water molecules are hydrogen-bonded to interlayer ions [8,10]. The broad and strong band around 3400 cm −1 is associated with the OH-metal vibration. There were the stretching vibrations of hydroxyl -OH groups attached to Mg and to both Mg and Al in brucite-like layers (OH-Mg3: 3575 cm −1 and OH-Mg2Al: 3454 cm −1 , respectively) [42][43][44]. The band that appears at 3700 cm −1 is associated with the O-H stretching vibration in brucite Mg(OH)2. Other bands in this region should be attributed to the -H2O bridging and H-bonded modes located at 3252 cm −1 and 3069 cm −1 , respectively [8,10].  The gases evolved during heating the reference hydrated nano-MgO and nano-Al 2 O 3 powders, and the nano-MgO-nano-Al 2 O 3 blended paste cured between 24 h and 49 days, were analyzed using mass spectrometry (MS) (Figure 3b-d). Evolved gas analysis (EGA) detected an increase in ion current intensity for m/z = 18 (H 2 O + ) produced by the electron impact ionization of water because of two reasons: H 2 O evolution monitored by the detection of fragments at m/z = 18 exhibits a parallel run to the DSC curves having local maximums at similar temperature values, and although the endothermic dehydration/dehydroxylation peaks are more sensitively detected by the MS than DSC curves, the EGA H 2 O profiles were only presented. As can be seen from Figure 3, different types of hydroxyl groups can exist in the nano-MgO-nano-Al 2 O 3 blended paste and the pronounced structural rearrangements involving the hydroxyl groups can occur. This can be further supported by our previous findings [8] and other authors [45] that these four hydroxyl local environments are possible: Mg 3 -OH, Mg 2 Al-OH, MgAl 2 -OH and Al 3 -OH. The nature of the cation-hydroxyl bond and also the distribution of hydroxyl groups varied with hydration time (Figure 3b-d). This rearrangement is evident as a substitution of structural metal cations, like Mg 2+ by cations of higher charge, and Al 3+ in (Mg 3 )-OH-brucite-like sites, which initially becomes dehydroxylated at ca. 417 • C (Figure 3a). Along with the curing time, two hydroxyl local environments, i.e., (Mg 2 Al)-OH-hydrotalcite-like sites (~420 • C) and (Mg 3 )-OH-hydrotalcite-like sites (~322 • C), were progressively formed. Hence, the dehydration and dehydroxylation of hydrotalcite formed as a major product in the nano-MgO-nano-Al 2 O 3 blended paste after 49 days of curing consecutively proceeds through the three main steps (Figure 4) involving the elimination of the interlayer structural water at ca. 234 • C and dehydroxylation overlapping with decarbonization processes at ca. 322 • C ((Mg 3 )-OH-hydrotalcite sites) and 420 • C ((Mg 2 Al)-OH-hydrotalcite sites). Besides, m/z = 18 exhibited an ion intensity peak at ca. 370 • C belonging to a possible residue of magnesium hydroxides ((Mg 3 )-OH-brucite-like sites). Typical DSC-TG heating curves showing hydrotalcite transition are found in the presentation in Figure 4. intensity peak at ca. 370 °C belonging to a possible residue of magnesium hydroxides ((Mg3)-OHbrucite-like sites). Typical DSC-TG heating curves showing hydrotalcite transition are found in the presentation in Figure 4.

Heat Transport Parameters and Mechanical Properties
The results of the thermal conductivity, thermal diffusivity, volume heat capacity of two reference calcium aluminate (CAC)-containing and MgO-Al2O3-containing mortars are summarized in Table 2. All the heat transport parameters of mortars can be observed in the dependence of the applied hydraulic binder. These results represent important parameters for building designers, especially in the case of refractory castables. As can be clearly seen, all the heat transport parameters of MgO-Al2O3-based mortar are generally lower in comparison with the CAC-based mortar. The reduced thermal conductivity of MgO-Al2O3-based mortar cannot be viewed as beneficial since a low thermal conductivity was a barrier for sufficient thermal energy transfer from the materials to the environment. Hence, thermal shock resistance of refractories is increased by increasing the thermal conductivity, the CAC-based mortars should be considered at this stage as more beneficial. Nevertheless, both CAC-based and Mg-Al-based refractory castables after firing at elevated temperatures exceeding 1500 °C require further research.  Table 3 summarizes the mechanical properties of both the MgO-Al2O3-based and the reference CAC-based mortars. The mechanical properties of both cold crushing strength and bending strength of mortars as estimated by the procedure presented in Section 2.2 increased with the increase of the curing time. The presented results show important parameters including the so-called "green mechanical strength" of mortars after casting and curing. As can be clearly seen, both cold crushing strength and bending strength of the MgO-Al2O3-based mortar are generally lower in comparison with the CAC-based mortar. This is understandable as the CAC-based mortar contains mainly

Heat Transport Parameters and Mechanical Properties
The results of the thermal conductivity, thermal diffusivity, volume heat capacity of two reference calcium aluminate (CAC)-containing and MgO-Al 2 O 3 -containing mortars are summarized in Table 2. All the heat transport parameters of mortars can be observed in the dependence of the applied hydraulic binder. These results represent important parameters for building designers, especially in the case of refractory castables. As can be clearly seen, all the heat transport parameters of MgO-Al 2 O 3 -based mortar are generally lower in comparison with the CAC-based mortar. The reduced thermal conductivity of MgO-Al 2 O 3 -based mortar cannot be viewed as beneficial since a low thermal conductivity was a barrier for sufficient thermal energy transfer from the materials to the environment. Hence, thermal shock resistance of refractories is increased by increasing the thermal conductivity, the CAC-based mortars should be considered at this stage as more beneficial. Nevertheless, both CAC-based and Mg-Al-based refractory castables after firing at elevated temperatures exceeding 1500 • C require further research.  Table 3 summarizes the mechanical properties of both the MgO-Al 2 O 3 -based and the reference CAC-based mortars. The mechanical properties of both cold crushing strength and bending strength of mortars as estimated by the procedure presented in Section 2.2 increased with the increase of the curing time. The presented results show important parameters including the so-called "green mechanical strength" of mortars after casting and curing. As can be clearly seen, both cold crushing strength and bending strength of the MgO-Al 2 O 3 -based mortar are generally lower in comparison with the CAC-based mortar. This is understandable as the CAC-based mortar contains mainly hydrates, i.e., compounds, that have in their structures chemically bonded water molecules, whereas the MgO-Al 2 O 3 -based mortar contains mainly an Mg(OH) 2 , M-A-H gel phase and unreacted starting constituents especially at the early stage of hydration and curing. Since the reactions within the MgO-Al 2 O 3 -H 2 O system are much slower than the corresponding reactions within the CaO-Al 2 O 3 (CAC)-H 2 O system, an increase in strength with increasing time is slower as well.

Thermal Decomposition Mechanism of Mg-Al Layered Double Hydroxide as a Magnesia-Alumina Spinel Precursor
The thermal decomposition mechanism of Mg 6 Al 2 CO 3 (OH) 16  The XRD results pointed out the crystalline hydrotalcite as the major phase in the sample hydrated for 49 days (see the XRD pattern in Figure 1). An increase in the calcination temperature to 200 • C does not result in changes in the phase composition of the sample. A significant difference is observed when the temperature reaches 250 • C. At this calcination temperature, the characteristic peak belonging to the Mg-Al hydrotalcite (2θ = 11.6930 • , JCPDS Card No. 00-022-0700) shifts toward a higher 2θ value (13.3934 • ). This shift of the diffraction peak indicates structural changes due to the interlamellar water release. During the dehydration process, loss of crystallinity and shrinkage of the layer lattice dimensions have been observed by other researchers [27,29,46,47]. At ca. 350-390 • C, there were no diffraction peaks belonging to the hydrotalcite while at 700 • C in XRD pattern appeared peaks which are in excellent agreement with simulated XRD pattern of MgAl 2 O 4 (JCPDS Card No. 98-009-1390). The other low intense peaks present at 900 • C or lower temperatures are the major peaks of MgO which are broadened due to poor crystallinity or small crystallites, or both. Moreover, some peaks belonging to the Pt plate were unavoidable.

Ex-situ LT-XRD, FT-IR and NMR Investigations
The ex situ LT-XRD, FT-IR and NMR measurements provided insights into both dehydration/dehydroxylation of hydrotalcite and the mechanism of spinel formation from the thermal decomposition of hydrotalcite. The observations relevant to the specimens tested after heat treatment at 600 to 1100 °C are presented in Figures 6-8. As can be observed in Figure 6a, the high and broad amorphous background of the XRD pattern of hydrotalcite calcined sample and some relatively weak XRD peaks confirm that full dehydration, dehydroxylation and decarbonization processes occurred at 600 °C. The amorphous character of hydrotalcite calcination products, probably spinel nuclei, transition alumina-type structures or Mg(Al)O mixed oxides, was also found in the other works [27,28]. These phases were too poorly crystalline to be detected by XRD. The half-width

Ex-situ LT-XRD, FT-IR and NMR Investigations
The ex situ LT-XRD, FT-IR and NMR measurements provided insights into both dehydration/dehydroxylation of hydrotalcite and the mechanism of spinel formation from the thermal decomposition of hydrotalcite. The observations relevant to the specimens tested after heat treatment at 600 to 1100 • C are presented in Figures 6-8. As can be observed in Figure 6a, the high and broad amorphous background of the XRD pattern of hydrotalcite calcined sample and some relatively weak XRD peaks confirm that full dehydration, dehydroxylation and decarbonization processes occurred at 600 • C. The amorphous character of hydrotalcite calcination products, probably spinel nuclei, transition alumina-type structures or Mg(Al)O mixed oxides, was also found in the other works [27,28]. These phases were too poorly crystalline to be detected by XRD. The half-width at half maximum of the peaks belonging to spinel, indicated by the reference pattern JCPDS Card No. 98-009-1390, is decreasing with the increasing calcination temperature from 700 to 1100 • C (Figure 6b-f). This indicates the significant increase in crystallinity, particle size of the spinel or both. It can be also noted that the amount of spinel phase in the samples increased with an increase in calcination temperature confirmed as an increase in XRD peak intensity. The appearance of crystalline undoped (Al-free) MgO is consistent with other literature sources [30,31].
Materials 2020, 13, x FOR PEER REVIEW 13 of 20 at half maximum of the peaks belonging to spinel, indicated by the reference pattern JCPDS Card No. 98-009-1390, is decreasing with the increasing calcination temperature from 700 to 1100 °C (Figure 6b-f). This indicates the significant increase in crystallinity, particle size of the spinel or both. It can be also noted that the amount of spinel phase in the samples increased with an increase in calcination temperature confirmed as an increase in XRD peak intensity. The appearance of crystalline undoped (Al-free) MgO is consistent with other literature sources [30,31]. As observed in the spectra of Figure 7b, the spectrum of the sample calcined at 700 °C presents characteristic bands of spinel, similar to the reference sample band around 536 and 700 cm −1 ( Figure  7e) [48]. It can be noticed that spectra present a better resolved and more intense spinel structure as the calcination temperature increases (Figure 7a-d). The infrared spectrum of the sample calcined at 600 °C presents a wide, unresolved pattern extending from 380 to 1100 cm −1 , with maximum absorbance at around 530 and 800 cm −1 (Figure 7a). The calcination process at 600 °C induced dehydration, dehydroxylation and decarbonation which lead to the formation of mixed oxides of Al2O3 and MgO or a spinel-like precursors phase. The peaks around 730-630 cm −1 (v4-MgO or Al-O) and around 513 cm −1 (v5-Mg-O or Al-O) can be attributed to the presence of Mg-O and Al-O bands [49]. Additionally, the band at 546 cm −1 is associated with the presence of MgO. The other two bands (612 and 813 cm −1 ) were related to AlO6 and AlO4 stretching, respectively, thus confirming the presence of γ-Al2O3 in the sample [33]. According to another literature source [50], the band at 456 cm −1 was due to the presence of the Mg-O-Al bond that was originally present in the Mg/Al As observed in the spectra of Figure 7b, the spectrum of the sample calcined at 700 • C presents characteristic bands of spinel, similar to the reference sample band around 536 and 700 cm −1 (Figure 7e) [48]. It can be noticed that spectra present a better resolved and more intense spinel structure as the calcination temperature increases (Figure 7a-d). The infrared spectrum of the sample calcined at 600 • C presents a wide, unresolved pattern extending from 380 to 1100 cm −1 , with maximum absorbance at around 530 and 800 cm −1 (Figure 7a [49]. Additionally, the band at 546 cm −1 is associated with the presence of MgO. The other two bands (612 and 813 cm −1 ) were related to AlO 6 and AlO 4 stretching, respectively, thus confirming the presence of γ-Al 2 O 3 in the sample [33]. According to another literature source [50], the band at 456 cm −1 was due to the presence of the Mg-O-Al bond that was originally present in the Mg/Al hydrotalcite. The similarity in the shape of the IR spectrum of the sample calcined at 600 • C to the γ-Al 2 O 3 IR curve may indicate the appearance of the gamma-like structure phase. hydrotalcite. The similarity in the shape of the IR spectrum of the sample calcined at 600 °C to the γ-Al2O3 IR curve may indicate the appearance of the gamma-like structure phase. The 27 Al MAS-NMR spectroscopy was used to characterize cation distribution in the magnesium aluminate spinel and its precursor on an atomic level. Generally, in the spinel structure (AB2O4), the cations can be occupied by tetrahedral and octahedral coordination. In a normal spinel, all divalent ions (Mg 2+ ) are located in A-type sites of the lattice and all trivalent ions (Al 3+ ) are located in B-type sites of the lattice. The distribution of cations over these sites is expressed as (A) tet (B2) oct O4. Inverse spinels exhibit the configuration (B) tet (AB) oct O4, where the A II ions occupy the octahedral voids, whereas one-half of B III ions occupy the tetrahedral voids and the other half occupy octahedral sites. Nevertheless, most spinels show a degree of disorder between these two end members, with formula presented as (A1−xBx) tet (B2−xAx) oct O4, where x is the so-called degree of disorder [51,52]. Figure 8 shows room-temperature 27 Al MAS-NMR spectra of the Mg-Al-CO3 hydrotalcite-like phase formed in the fully hydrated hardened MgO-Al2O3 blended paste after 49 days (a), the Mg-Al layered double hydroxide heat-treated in the temperature range 600-1100 °C (b) and the reference spinel prepared via three-step sintering of equimolar mixtures of MgO and Al2O3 micropowders at 1700 °C (c). The 27 Al MAS-NMR spectrum of the sample before calcination (Figure 8a) shows a relatively narrow resonance peak at a chemical shift, δ, of ~4.0 ppm, which represents the octahedral coordination (0-10 ppm) of Al, Al(OH)6, in the hydrotalcite structure [53]. The trend observed from Figure 8b,c is the The 27 Al MAS-NMR spectroscopy was used to characterize cation distribution in the magnesium aluminate spinel and its precursor on an atomic level. Generally, in the spinel structure (AB 2 O 4 ), the cations can be occupied by tetrahedral and octahedral coordination. In a normal spinel, all divalent ions (Mg 2+ ) are located in A-type sites of the lattice and all trivalent ions (Al 3+ ) are located in B-type sites of the lattice. The distribution of cations over these sites is expressed as (A) tet (B 2 ) oct O 4 . Inverse spinels exhibit the configuration (B) tet (AB) oct O 4 , where the A II ions occupy the octahedral voids, whereas one-half of B III ions occupy the tetrahedral voids and the other half occupy octahedral sites. Nevertheless, most spinels show a degree of disorder between these two end members, with formula presented as ( 4 , where x is the so-called degree of disorder [51,52]. Figure 8 shows room-temperature 27 Al MAS-NMR spectra of the Mg-Al-CO 3 hydrotalcite-like phase formed in the fully hydrated hardened MgO-Al 2 O 3 blended paste after 49 days (a), the Mg-Al layered double hydroxide heat-treated in the temperature range 600-1100 • C (b) and the reference spinel prepared via three-step sintering of equimolar mixtures of MgO and Al 2 O 3 micropowders at 1700 • C (c). The 27 Al MAS-NMR spectrum of the sample before calcination (Figure 8a) shows a relatively narrow resonance peak at a chemical shift, δ, of~4.0 ppm, which represents the octahedral coordination (0-10 ppm) of Al, Al(OH) 6 , in the hydrotalcite structure [53]. The trend observed from Figure 8b,c is the evolution of structural changes of fully dehydrated/dehydroxylated samples during treatment in the temperature range of 600-1100 • C to proceed in the direction of primary spinel crystallization. The interesting observation is that the increase in calcination temperature brings about both a noticeable broadening and a shift (toward more negative chemical shifts) of the AlO 6 spectral line (see Figure 8b), implying a change in the local atomic environments of Al 3+ ions due to some AlO 6 deformation [54]. The spectra were shifted in the direction of the arrow. The spectra of calcined samples also consist of broad and small resonance at ca. 62 ppm (inset figure) in the region characteristic of tetrahedrally coordinated aluminum (40-80 ppm) [27,53,55,56] formed initially at temperatures below 600 • C. The appearance of tetrahedral Al in the fully dehydrated/dehydroxylated samples and its decreasing behavior with increasing calcination temperature from 600 to 1100 • C (Figure 8b,d) seems to have three overlapping reasons; one being the conversion of several possible metastable (transition) aluminas polymorphs (probably γ-Al 2 O 3 or transition alumina-type structure [27,28,54,[56][57][58][59][60] considered also as a mixed tetrahedral-octahedral Al 2 O 3 phase [61] formed initially below 600 • C to the most stable (six-coordinated) corundum phase [59], the second being the migration of Al 3+ ions from the periclase structure probably containing Al 3+ in solid solution (unstable MgO-like phase or periclase Mg(Al)O) [54,[62][63][64][65] or the other being the secondary spinel formation via reaction of a high surface area MgO-Al 2 O 3 mixed oxides [30,47,54,66,67] (or an amorphous mixed-phase oxide of nominal composition MgAlO x [62]) derived from hydrotalcite, in addition to the primary spinel derived from hydrotalcite. Note also that all the transition aluminas contain Al 3+ in both tetrahedral and octahedral sites [68], whereas Al 3+ in the periclase structure can be tetrahedrally [27,65,67,[69][70][71] or octahedrally [54,56,63,[72][73][74] coordinated with oxygen. Simultaneously, the resonance centered at ca. 4 ppm is due to octahedrally coordinated aluminum in spinel, aluminas and periclase structures. Hence, the exact nature of such behaviors is not clear, since it is very difficult to distinguish between MgAl 2 O 4 , Al 2 O 3 and MgO rock salt-like structure, with the Al 3+ cations evenly distributed. Finally, the 27 Al MAS-NMR spectra of sample calcined at 1100 • C (Figure 8b) become more similar in character to the reference spectrum of magnesium aluminate disordered/inverse spinel powder synthesized at 1700 • C (Figure 8c), which exhibits two well-resolved peaks [51] in the region characteristic of tetrahedrally coordinated aluminum (chemical shift δ ≈ 62 ppm) and octahedrally coordinated aluminum (δ ≈ 4 ppm). Thus, a mixture of well-crystallized MgO and MgAl 2 O 4 as a final product of hydrotalcite thermolysis was obtained, as reported earlier by Stanimirova et al. [30,31], Valente et al. [64], Occelli et al. [47] and Auerbach et al. [75].
Materials 2020, 13, x FOR PEER REVIEW 15 of 20 evolution of structural changes of fully dehydrated/dehydroxylated samples during treatment in the temperature range of 600-1100 °C to proceed in the direction of primary spinel crystallization. The interesting observation is that the increase in calcination temperature brings about both a noticeable broadening and a shift (toward more negative chemical shifts) of the AlO6 spectral line (see Figure  8b), implying a change in the local atomic environments of Al 3+ ions due to some AlO6 deformation [54]. The spectra were shifted in the direction of the arrow. The spectra of calcined samples also consist of broad and small resonance at ca. 62 ppm (inset figure) in the region characteristic of tetrahedrally coordinated aluminum (40-80 ppm) [27,53,55,56] formed initially at temperatures below 600 °C. The appearance of tetrahedral Al in the fully dehydrated/dehydroxylated samples and its decreasing behavior with increasing calcination temperature from 600 to 1100 °C (Figure 8b,d) seems to have three overlapping reasons; one being the conversion of several possible metastable (transition) aluminas polymorphs (probably γ-Al2O3 or transition alumina-type structure [27,28,54,[56][57][58][59][60] considered also as a mixed tetrahedral-octahedral Al2O3 phase [61] formed initially below 600 °C to the most stable (six-coordinated) corundum phase [59], the second being the migration of Al 3+ ions from the periclase structure probably containing Al 3+ in solid solution (unstable MgO-like phase or periclase Mg(Al)O) [54,62,[63][64][65] or the other being the secondary spinel formation via reaction of a high surface area MgO-Al2O3 mixed oxides [30,47,54,66,67] (or an amorphous mixed-phase oxide of nominal composition MgAlOx [62]) derived from hydrotalcite, in addition to the primary spinel derived from hydrotalcite. Note also that all the transition aluminas contain Al 3+ in both tetrahedral and octahedral sites [68], whereas Al 3+ in the periclase structure can be tetrahedrally [27,65,67,[69][70][71] or octahedrally [54,56,63,[72][73][74] coordinated with oxygen. Simultaneously, the resonance centered at ca. 4 ppm is due to octahedrally coordinated aluminum in spinel, aluminas and periclase structures. Hence, the exact nature of such behaviors is not clear, since it is very difficult to distinguish between MgAl2O4, Al2O3 and MgO rock salt-like structure, with the Al 3+ cations evenly distributed. Finally, the 27 Al MAS-NMR spectra of sample calcined at 1100 °C ( Figure 8b) become more similar in character to the reference spectrum of magnesium aluminate disordered/inverse spinel powder synthesized at 1700 °C (Figure 8c), which exhibits two wellresolved peaks [51] in the region characteristic of tetrahedrally coordinated aluminum (chemical shift δ ≈ 62 ppm) and octahedrally coordinated aluminum (δ ≈ 4 ppm). Thus, a mixture of well-crystallized MgO and MgAl2O4 as a final product of hydrotalcite thermolysis was obtained, as reported earlier by Stanimirova et al. [30,31], Valente et al. [64], Occelli et al. [47] and Auerbach et al. [75].