Infrared and Raman Spectroscopy of Mullite Ceramics Synthesized from Fly Ash and Kaolin

: Infrared spectroscopy and Raman spectroscopy were used to characterize mullite ceramics prepared from ﬂy ash and kaolin by annealing at 1000 ◦ C, 1100 ◦ C, 1200 ◦ C, and 1300 ◦ C. IR spectroscopy conﬁrmed the presence of SiO 4 tetrahedra and AlO 6 octahedra in samples. The presence of mullite has been conﬁrmed at all temperatures. The presence of quartz has been conﬁrmed up to a temperature of 1100 ◦ C, and the presence of an amorphous form of SiO 2 has been conﬁrmed at temperatures of 1200 ◦ C and 1300 ◦ C. The transformation of quartz into the amorphous form of SiO 2 at temperatures above 1100 ◦ C is assumed. Transformation was performed on the percentage intensity decrease of the bending vibration of Si-O-Si (at about 450 cm − 1 ) and Al-O-Si (at about 550 cm − 1 ). Raman spectroscopy conﬁrmed the presence of mullite at different stages of structural ordering (a well-ordered structure at a temperature of 1100 ◦ C and a disordered structure at a temperature of 1300 ◦ C).


Introduction
Fly ash (FA) is one of the byproducts obtained from burning coal at power plants; other byproducts include, for example, bottom ash and boiler slag.Fly ash is a well-defined inorganic dust composed of spherical glassy particles [1], which are formed as oxidic products of coal combustion from minerals present in coal [2].
Fly ash has a similar chemical and mineralogical composition as traditional ceramic raw materials and is a frequently used ingredient in ceramics [3].Typical architectural ceramics contain three main components: filler (e.g., quartz), binder (e.g., clay), and fluxing agent (e.g., feldspars) [4].Fly ash (as a high-temperature product) does not have the plasticity of a clay binder [5]; therefore, a mixture of fly ash and kaolin is used.Kaolins generally contain kaolinite (about 75% w/w) and accompanying minerals from original rocks (as quartz, muscovite, illite, or feldspar) [6].During thermal treatment, kaolinite transforms directly into mullite at temperatures below 900 From a mineralogical point of view, fly ash contains crystalline and amorphous (glassy) components.The main oxide components of FA are SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, Na 2 O, and K 2 O, and the crystalline phases are mullite (3Al 2 O 3 •2SiO 2 ) and quartz (SiO 2 ) [8].The glassy phases have been characterized as high-silica glass and calcium aluminate glass [1].Both the chemical and mineralogical composition depend on many factors, of which the type of coal burned and the combustion process used are the most significant [9].
The phase composition of ceramic materials is generally studied using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), infrared (IR) spectroscopy, and so on.IR spectroscopy is particularly useful for the analysis of amorphous materials and is also very often used for the analysis of aluminosilicates [10,11] and the mineral composition of coal and coal ash [12][13][14][15][16][17][18][19].The use of Raman spectroscopy for the analysis of aluminosilicates can be complicated by the possible occurrence of fluorescence, which can overlap most of the Raman spectral information [20].Despite this, Raman spectroscopy has been used in some studies for the description of fly ash or mullite ceramics [2,[21][22][23].
The present study deals with analyses of the structural properties of FA and FAkaolin (FA-K) ceramics using IR and Raman spectroscopy.The objective of the study is characterization of the component changes in the spectra of FA and FA-K ceramics annealed from 1000 to 1300 • C by combined use of IR and Raman spectroscopy techniques, which are still insufficiently represented in the literature.

Materials and Sample Preparation
FA and kaolins for this study were identical with those in the previous study [24].FA comes from the combustion of black coal in a coal power station (Dětmarovice Power Station, Dětmarovice, Czech Republic); FA was collected from an electrostatic precipitator.
The three kaolin samples (K1, K2, and K3) from the Czech kaolin deposits were purchased from the company LB Minerals, Ltd. (Horní Bříza, Czech Republic).Two samples (K1 and K2) originate from the kaolin Pilsen Basin, and one sample (K3) originates from the Karlovy Vary region.The elemental composition of the used kaolins and fly ash (in oxide form) is given in Table 1.All three used kaolins contain kaolinite in an average amount of 75% by mass [6] and also some other minerals, in particular quartz, muscovite, and orthoclase [24].
Table 1.The elemental composition (mass %) of fly ash (FA) and kaolin samples (K) [24].Sample preparation was also identical to that in [24].The kaolin samples K1, K2, and K3 and FA were prepared in an FA:K mass ratio of 9:1 for the mixtures (designated FA-K1, FA-K2, and FA-K3).Each mixture was homogenized for 1 h in a bottle at a rotation speed of 40 r•min −1 (Heidolph Reax overhead shaker, REAX 20/4, Merck KGaA, Darmstadt, Germany) and then mixed in an agate planetary ball mill (FRITSCH-Pulverisette 6) for 15 min at a rotation speed of 300 r•min −1 .Samples were next prepared in a slurry with the addition of 20% by mass of distilled water.The slurry was manually pressed into the cubic voids (20 mm × 20 mm × 20 mm) of metal molds.The samples in molds were left for 24 h at room temperature and then dried at 110 • C for 5 h in an oven.Dry samples were taken out of the molds and sintered in an electrical laboratory furnace LH15/13 at a heating ramp 12 • C/min to the desired temperature (of 1000 • C, 1100 • C, 1200 • C, or 1300 • C) and maintained at this temperature for 2 h.The samples were then powdered in an agate mortar with pestle.The list of samples is given in Table 2.

Method (IR and Raman Spectroscopy)
IR spectra of all samples were collected by the potassium bromide pellets technique.Exactly 2.0 mg of sample was ground with 200 mg of dried potassium bromide.This mixture was used to prepare the potassium bromide pellet (diameter: 1 cm).The pellet was pressed by 8 tons for 30 s under vacuum.The infrared spectra were collected using FT-IR spectrometer Nicolet iS50 (Thermo Scientific, Madison, WI, USA) with DTGS detector.The following parameters were used for measurement: spectral region, 4000-400 cm −1 ; spectral resolution, 4 cm −1 ; 64 scans; Happ-Genzel apodization.Regarding the treatment of spectra, a polynomial (second-order) baseline was used, with subtraction of the spectrum of pure potassium bromide.

Description of Sample Annealing Temperature Sample Name
Fly ash -FA 1000 The Raman spectra were measured on the dispersive Raman spectrometer DXR Smar-tRaman (ThermoScientific, Madison, WI, USA) with CCD detector using a backscattering configuration.The wavelength of the excitation laser was 780 nm, the grating 400 lines/mm, the aperture 25 µm, the exposure time 1 s, and the number of exposures 100.An empty sample compartment was used for background measurement.The spectra were corrected for fluorescence by spectral software OMNIC (v.9.2; Thermo Scientific, Madison, WI, USA).

IR Spectroscopy
IR spectra of all prepared samples contain spectral bands that are particularly concentrated in the spectral region below 1500 cm −1 , where positions of the spectral bands of stretching and deformation vibration of Si-O and Al-O bonds are located.
The most intensive band at 1070 cm -1 has two spectral shoulders observed on the side of higher wavenumbers (at 1160 cm −1 ) and on the side of lower wavenumbers (at 910 cm −1 ).The bands at 1070 cm −1 and 1160 cm −1 are due to asymmetrical stretching vibration of the Si-O-Si bond in SiO 4 tetrahedra (transverse and longitudinal mode, respectively).The band at 910 cm −1 is assigned to the deformation vibration of the Al-O bond in AlO 6 octahedra.The bands at 1070 cm −1 and 1160 cm −1 are generally typical for different forms of SiO 2 .The band at 910 cm −1 is characteristic for aluminosilicates or for mullite [32].
The weak doublet at 800 cm −1 and 780 cm −1 and the weak band at 815 cm −1 are associated with the symmetric stretching vibration of Si-O-Si bridges in SiO 4 tetrahedra.The doublet at 800 cm −1 and 780 cm −1 is significantly characteristic for quartz [32]; the band at 815 cm −1 is significant for the amorphous form of SiO 2 [31].The bands confirm the presence of quartz in samples up to a temperature of 1100 • C and its subsequent transformation to the amorphous form of SiO 2 .
Other weak bands are seen at 730 cm −1 and 690 cm −1 .The band at 730 cm −1 is due to deformation vibration of the Al-O in tetrahedra and confirms the substitution of Al for Si in the SiO 4 tetrahedra of mullite [32].The band at 690 cm −1 is due to perpendicular deformation vibration of the Si-O in SiO 4 tetrahedra and is very typical for quartz [33].The band at 730 cm −1 is presented in all spectra; the band 690 cm −1 is presented only in the spectra up to temperature 1100 • C.This fact also confirms the previously mentioned trend that quartz is present in samples only up to a temperature of 1100 • C, and then it changes to the amorphous form of SiO 2 .
The medium intensive bands at about 550 cm −1 and 460 cm −1 are assigned to the bending vibration of Al-O-Si and Si-O-Si, respectively.The vibration at about 550 cm −1 is characteristic for AlO 6 octahedra and is very typical (among others) for aluminosilicates [11] or for mullite [28].The spectral shift (of approx.10 cm −1 ) of both bands (at about 550 cm −1 and 460 cm −1 ) to the lower wavenumbers occurs with increasing temperature of sample treatment.The spectral shift of other bands in the spectra of fly ash samples was not observed.
IR spectra of samples of FA with kaolin K1 (without thermal treatment and annealed at different temperatures) are shown in Figure 2. The IR spectra of samples prepared from FA and the other kaolin (K2 and K3) are almost identical.Compared to the spectra in Figure 1, the spectrum of sample FA-K1 (as well as the spectra of FA-K2 and FA-K3, which are not presented in Figure 2) contains some spectral bands that are not presented in the spectra of annealed samples.The wavenumbers of these spectral bands are labeled by underlined italics font in Figure 2. The following literature sources were used for assignment of the spectral bands: [11,36,37].The most intensive band at 1035 cm −1 has two spectral shoulders: one on the side of higher wavenumbers (at 1110 cm −1 ) and one on the side of lower wavenumbers (at 1010 cm −1 ).The band at 1035 Compared to the spectra in Figure 1, the spectrum of sample FA-K1 (as well as the spectra of FA-K2 and FA-K3, which are not presented in Figure 2) contains some spectral bands that are not presented in the spectra of annealed samples.The wavenumbers of these spectral bands are labeled by underlined italics font in Figure 2. The following literature sources were used for assignment of the spectral bands: [11,36,37].The most intensive band at 1035 cm −1 has two spectral shoulders: one on the side of higher wavenumbers (at 1110 cm −1 ) and one on the side of lower wavenumbers (at 1010 cm −1 ).The band at 1035 cm −1 is due to asymmetrical in-plane stretching vibration of the Si-O in SiO 4 tetrahedra, whereas the shoulder at 1010 cm −1 is associated with a symmetrical mode of the same type of vibration.The shoulder at 1110 cm −1 belongs to the longitudinal mode of stretching vibration of the Si-O in SiO 4 tetrahedra.The weak bands 940 cm −1 and at 915 cm −1 are both assigned to deformation vibration of O-H in the inner hydroxyl groups, and the very weak bands at 755 cm −1 and at 690 cm −1 are due to a perpendicular mode of deformation vibration of the Si-O in SiO 4 tetrahedra.The medium band at 540 cm −1 is associated with bending of the Al-O-Si bond, where Al is octahedrally coordinated with oxygen, whereas Si is tetrahedrally coordinated with oxygen [11].The medium band at 470 cm −1 is due to bending of the Si-O-Si bond of coordinated Si in SiO 4 tetrahedra.The weak shoulder at 430 cm −1 belongs to deformation vibration of the Si-O in SiO 4 tetrahedra.All the abovementioned spectral bands are generally typical for aluminosilicates (especially for clay minerals) [11,36].Moreover, the bands at 940 cm −1 and at 915 cm −1 are very characteristic for kaolinite [11,36].
The rest of the bands in the spectrum of sample FA-K1 were characterized in more detail in the assignment of the spectral bands of FA samples (see Figure 1 description).The spectra of annealed samples FA-K1_1000, FA-K1_1100, FA-K1_1200, and FA-K1_1300 exhibit the same spectral characteristics and spectral trends as the spectra of annealed FA samples.The assignment of all bands presented in the spectra of annealed samples is described above (see the description of spectra of samples containing only FA).
The spectra are presented in so-called "full range" mode in Figures 1 and 2. This mode allows a detailed view of each band in each spectrum but does not consider their real intensity.The intensity of the bands changes depending on the annealing temperature used in the sample preparation.The band intensities dramatically decrease with increasing temperature of the sample treatment.The band at about 450 cm −1 and the band at about 550 cm −1 were used to demonstrate the decrease of intensity of spectral bands in samples annealed at different temperatures.The band at about 450 cm −1 was used as an example for the demonstration of trends in the bands of SiO 4 tetrahedra; the band at about 550 cm −1 was used as example for the demonstration of trends in the bands of AlO 6 octahedra.The percentage decrease (related to the band intensity of selected bands in the sample without thermal treatment) is given in Table 3.
The trend of the percentage decrease of bands' intensities is graphically represented in Figure 3.
The significant decrease of selected band intensities with increasing sample treatment temperature is evident for all samples.The intensity decrease is about 90% at the highest annealing temperature in all samples and in both selected bands.A relatively big difference is visible between the intensity decrease of SiO 4 tetrahedra and that of AlO 6 octahedra in the FA samples.The intensity decrease of the AlO 6 band is bigger than SiO 4 for all annealing temperatures.The decrease of spectral band intensities is relatively similar for both selected bands in the FA-K samples.The decrease of intensity of the SiO 4 tetrahedron band is bigger (than the decrease of intensity of the AlO 6 octahedron band) for samples annealed at a temperature of 1000 • C. At temperatures above 1000 • C, a bigger decrease of bands' intensity is observed for the AlO 6 octahedron band.The trend of the percentage decrease of bands intensities is graphically represent in Figure 3.The decrease of bands' intensities is caused by a transformation of the crystalline state of compounds into an amorphous state.The transformation manifested by decreasing of bands' intensities accompanied by increasing of their width in IR spectra was described in [31,32,35].Due to the weak intensities of selected bands of SiO 4 tetrahedra and AlO 6 octahedra, the increase of spectral width can be much more difficult to express by numerical value (for example, by full width at half height (FWHH)).IR spectra clearly evidence the increase of spectral band width with increasing temperature of sample treatment in Figures 1 and 2.
Based on the above observations, it can be assumed that during thermal sample treatment, a part of the aluminosilicates present in the samples was transformed from the crystalline phase to the amorphous phase.It is also evident that the amount of amorphous phase in samples increased with increasing annealing temperature.

Raman Spectroscopy
Raman spectra (measured in the spectral region 1000-50 cm −1 ) contain higher or lower levels of fluorescence, and therefore software fluorescence correction of the measured spectra was applied.The highest level of fluorescence contains spectra of no annealed samples; Raman spectral bands was completely overlapped by fluorescence in these spectra.The lowest level of fluorescence was in the spectra of samples annealed in 1300 • C. Fluorescence in Raman spectra of aluminosilicates is a common (but very complicated and complex) phenomenon [20].The origin of the fluorescence of aluminosilicates is rarely discussed in the literature, and there is not unified opinion on what causes the fluorescence [20].Nevertheless, several Raman studies of mullite ceramics have been carried out previously [38][39][40][41][42]. Uncorrected Raman spectra of all annealed samples (FA, FA-K1, FA-K2, and FA-K3) are given in the Supplementary Materials Figures S1-S4.
The Raman spectra of the FA and FA-K samples were very similar.Therefore, Raman spectra of FA-K3_1000, FA-K3_1100, FA-K3_1200, and FA-K3_1300 are presented as examples in Figure 4.
both selected bands in the FA-K samples.The decrease of intensity of the SiO4 tetrahedron band is bigger (than the decrease of intensity of the AlO6 octahedron band) for samples annealed at a temperature of 1000 °C.At temperatures above 1000 °C, a bigger decrease of bands intensity is observed for the AlO6 octahedron band.
The decrease of bands intensities is caused by a transformation of the crystalline state of compounds into an amorphous state.The transformation manifested by decreasing of bands intensities accompanied by increasing of their width in IR spectra was described in [31,32,35].Due to the weak intensities of selected bands of SiO4 tetrahedra and AlO6 octahedra, the increase of spectral width can be much more difficult to express by numerical value (for example, by full width at half height (FWHH)).IR spectra clearly evidence the increase of spectral band width with increasing temperature of sample treatment in Figures 1 and 2.
Based on the above observations, it can be assumed that during thermal sample treatment, a part of the aluminosilicates present in the samples was transformed from the crystalline phase to the amorphous phase.It is also evident that the amount of amorphous phase in samples increased with increasing annealing temperature.

Raman Spectroscopy
Raman spectra (measured in the spectral region 1000-50 cm −1 ) contain higher or lower levels of fluorescence, and therefore software fluorescence correction of the measured spectra was applied.The highest level of fluorescence contains spectra of no annealed samples; Raman spectral bands was completely overlapped by fluorescence in these spectra.The lowest level of fluorescence was in the spectra of samples annealed in 1300 °C.Fluorescence in Raman spectra of aluminosilicates is a common (but very complicated and complex) phenomenon [20].The origin of the fluorescence of aluminosilicates is rarely discussed in the literature, and there is not unified opinion on what causes the fluorescence [20].Nevertheless, several Raman studies of mullite ceramics have been carried out previously [38][39][40][41][42]. Uncorrected Raman spectra of all annealed samples (FA, FA-K1, FA-K2, and FA-K3) are given in the Supplementary Materials Figures S1-S4.
The Raman spectra of the FA and FA-K samples were very similar.Therefore, Raman spectra of FA-K3_1000, FA-K3_1100, FA-K3_1200, and FA-K3_1300 are presented as examples in Figure 4.   [38].The strongest band at 415 cm −1 is assigned to the vibration of SiO 4 tetrahedra in crystalline mullite [38,[40][41][42], similar to the band at 300 cm −1 .The presence of an amorphous (glassy) form of mullite can be assumed by the band at 365 cm −1 , which is due to vibration of SiO 4 tetrahedra [38].The band at 230 cm −1 belongs to the lattice vibration of the Al-O in the AlO 6 octahedra of

Minerals 2023, 13 , 864 4 of 11 Figure 1 .
Figure 1.IR spectra of FA, FA_1000, FA_1100, FA_1200, and FA_1300.The spectra of all FA samples are very similar and exhibit spectral bands characteristic for the vibration of the Si-O bond in SiO4 tetrahedra and the Al-O bond (both in AlO4 tetrahedra and AlO6 octahedra) [1,23,25-35].The most intensive band at 1070 cm -1 has two spectral shoulders observed on the side

Figure 3 .
Figure 3.The percentual decrease of band intensity of bending vibration mode of SiO4 tetrahed and AlO6 octahedra with increasing annealing temperature of samples.

Figure 3 .
Figure 3.The percentual decrease of band intensity of bending vibration mode of SiO 4 tetrahedra and AlO 6 octahedra with increasing annealing temperature of samples.

Figure 4 .
Figure 4. Raman spectra of samples FA-K3_1000, FA-K3_1100, FA-K3_1200, and FA-K3_1300.The intensities of Raman bands change with the annealing temperature of the sample.The bands' intensities increase up to a temperature of 1100 • C, and then they dramatically decrease.No spectral bands are seen in the spectra of the sample annealed at 1300 • C. All spectral bands presented in Figure 4 are due to the lattice vibration of the Si-O and Al-O bonds of mullite.The band at 615 cm −1 is due to the lattice vibration of the Al-O in mullite tetrahedra, in which Si was substituted by Al[38].The strongest band at 415 cm −1 is assigned to the vibration of SiO 4 tetrahedra in crystalline mullite[38,[40][41][42], similar to the band at 300 cm −1 .The presence of an amorphous (glassy) form of mullite can be assumed by the band at 365 cm −1 , which is due to vibration of SiO 4 tetrahedra[38].The band at 230 cm −1 belongs to the lattice vibration of the Al-O in the AlO 6 octahedra of

Table 2 .
List of samples.

Table 3 .
Absorbances and intensity decrease percentage of the selected bands.