Effects of Kaolin Additives in Fly Ash on Sintering and Properties of Mullite Ceramics

The effective utilization of fly ash (FA) as a raw material for ceramics production is performed on the FA-kaolin mixtures containing kaolins 10% by mass. The mixtures in comparison with FA and three raw kaolins were annealed to mullite ceramics at temperatures of 1000, 1100, 1200 and 1300 °C. The main aims were to contribute to the discussion on the effect of impurity of Na,K-feldspars in kaolins and Fe2O3 in FA on sintering procedure, porous ceramics properties and mullite structural properties. The phases were characterized using X-ray diffraction and thermogravimetry DTA/TGA methods. Mercury intrusion porosimetry was used for characterization of porosity of ceramic samples. Results evidenced the influence of feldspars in kaolins and Fe2O3 in FA on the sintering temperatures and properties of mullite ceramics. The fully FA-based ceramic sintered at 1100 °C exhibited post-sintering properties of bulk density 2.1 g/cm3; compressive strength 77.5 MPa; and porosity, 2% in comparison with the FA/kaolin-based ceramics properties of bulk density 2.2 g/cm3; compressive strength, 60–65 MPa; and porosity from 9.3 to 16.4% influenced by Na,K-feldspars. The best structural and mechanical characteristics were found for the FAK3 sample, supported by the high content of kaolinite and orthoclase in the kaolin K3 additive. The FAK3 annealed at 1100 °C exhibited good compressive strength of 87.6 MPa at a porosity of 10.6% and density of 2.24 g/cm3 and annealed at 1300 °C the compressive strength of 41.3 MPa at a porosity of 19.2% and density of 1.93 g/cm3.


Introduction
Fly ash (FA) properties depend on the coal source, the method of combustion of power plants, storage, etc. For example, burning in pulverized-coal combustion boiler, 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 ) [1]. FA and traditional ceramic raw materials have similar chemical and mineralogical compositions and therefore, make FA a promising ingredient in ceramics [2]. Architectural ceramics are typically prepared using a triaxial formulation of quartz (filler, 5-30 mass%), clay (binder, 30-60 mass%), and feldspar (fluxing agent, 15-40 mass%) [3]. Quartz as a filler can be replaced by FA because of the proper filling properties. Effect of substitution of fly ash for quartz in triaxial kaolin-quartz-feldspar system demonstrated higher density and flexural strength [4,5]. However, FA as a high-temperature product cannot provide plasticity like a clay binder [6]. FA in the range of 5-40 mass% replacing clay improved the bending strength, abrasion resistance, and hardness of porcelainized stoneware tiles but worsened the bending strength of the green compacts [7]. FA containing abundant alkali and alkaline earth metals can partially substitute for feldspar to promote melting, especially in the preparation of ceramic tiles [8,9]. Building mullite-based ceramics can be produced from the recycling of conventional coal combustion ash and clay in the initial mixture, acting as This paper focuses on the use of coal ash (FA) and raw kaolins additive in the production of mullite ceramics. FA and kaolins are classified as mullite ceramic precursors. FA as a high-temperature product cannot provide plasticity, while clays in ceramic mixtures act as a binder and plasticizer. The objectives of the study are focused on the characterization of ceramics prepared from the three mullite precursors: (1) FA, (2) raw kaolins, and (3) FA-kaolin additive 10% by mass and annealed at temperatures of 1000, 1100 1200 and 1300 • C, which, according to the literature, were assigned to the formation of the monophasic mullite, the diphasic gel of mullite and the Al-Si spinel phase and the mullite.
The solution is focused on the effect of impurities in kaolins and fly ash on the formation of phases, porosity and strength of ceramics and crystal chemical properties of mullite. The main aim was to document and explain the effect of impurity of Na, K and Fe bounded in Na,K-and K-feldspars in kaolins and Fe 2 O 3 in FA on sintering procedure, porous ceramics properties and mullite structure crystallizing at 1000 and 1300 • C.

Materials
Fly ash (the sample marked FA) was obtained from the combustion of black coal captured on an electrostatic precipitator in the powder station (Czech Republic). The three kaolin samples from the Czech kaolin deposits were purchased from the LB Minerals, Ltd., Horní Bříza, Czech Republic. Two samples originating from the kaolin Pilsen Basin were marked K1 and K2 and one sample from the Karlovy Vary region was marked K3. These kaolins generally contain kaolinite in an average amount of 75% by mass and the original minerals, in particular quartz, muscovite/illite and feldspars from their original rocks [30,31].

Sample Preparation
The kaolin samples K1, K2 and K3 and FA were prepared in the FA:K mass ratio of 9:1 to the mixtures designated FA/K1, FA/K2 and FA/K3. Each mixture was homogenized for 1 h in bottle at the rotate speed of 40 r·min −1 (Heidolph Reax overhead shaker, REAX 20/4, Merck KGaA, Darmstadt, Germany) and then mixed in the agate planetary ball mill (FRITSCH-Pulverisette 6) for 15 min at the rotate speed of 300 r·min −1 . Samples for sintering were prepared into a slurry with the addition of 20% by mass of distilled water. The slurry was manually pressed into 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 the heating ramp 12 • C/min to the desired temperature of 1000, 1100, 1200, 1300 • C and maintained at this temperature for 2 h. The sintering temperature of the ceramic sample is indicated at the end of the designation (e.g., FA/K1-1000).

Methods
Elemental analysis of metal oxide concentrations was performed using a SPECTRO XEPOS energy dispersive X-ray fluorescence (ED-XRF) spectrometer (Spectro Analytical Instruments, Kleve, Germany). Mineral phases were characterized by X-ray powder diffraction (XRD) using XRD patterns (Rigaku SmartLab diffractometer, Rigaku Corporation, Tokyo, Japan) under CoK α radiation at 40 kV and 40 mA, at 15 r·min −1 , a step size of 0.01 • . The thermal analysis of kaolins powders and FA/K mixtures dried at 110 • C was performed on the Simultaneous DTA/TGA SDT 650 system (TA Instruments, New Castle, DE, USA) in an air dynamic atmosphere with a flow rate of 0.1 L min −1 at a heating rate of 10 • C min −1 over the range 25-1400 • C. The porosity of the ceramic samples was measured using a mercury intrusion porosimeter AutoPore IV 9500 (Micromeritics Instrument Corporation, Norcross, GA, USA). The compressive strength (CS) parameters of ceramic samples were measured using the SERVO-PLUS EVOLUTION hydraulic concrete compression machine 2400 kN (MATEST S.p.A., Treviolo (BG), Italy).

Shrinkage, Porosity and Compressive Strength
The base shrinkage (BS) ratio of the cubes from the size of the length of 2.0 cm × 2.0 cm × 2.0 cm after annealing at 1000, 1100, 1200 and 1300 • C was calculated according to Equation (5) [35]: BS is the shrinkage after annealing, Los is the original length, and Ld is the length after firing. BS values in Table 4 are an averaged length changes of the five cubes of each sample.

Effect of Feldspars in Kaolins on Thermal Decomposition
Thermal changes during the progress of sintering were monitored from 25 to 1400 • C ( Figure 4). DTA curve of FA revealed the two endotherms maxima at about 1288 and 1310 • C. These maxima of FA/K1, FA/K2 and FA/K3 were at about 1165 and 1250 • C. The exothermic peaks maxima at all samples was at 1335 • C (Figure 4a). The endotherm was assigned to the melting and maximum to the crystallization of mullite [27]. The mass loss at 1100 • C in FA and kaolin K3 was similar (3.0%) in comparison with K1 and K2 (5.2 and 6.5%), (Figure 4b). The effect of individual basic oxides, CaO, Fe 2 O 3 , Na 2 O and K 2 O and the ratio of Base/Acid, SiO 2 /AlO 3 and CaO/Fe 2 O 3 on ash fusibility has been widely discussed [36]. Generally, the melting temperature in FA was increased by SiO 2 and Al 2 O 3 . The basic oxides K 2 O or Na 2 O and feldspar minerals containing K or Na form lower melting point and improve fusibility. Intensity of the feldspar peaks on XRD pattern decreased slightly due to decrease in crystallinity when the temperature was higher than 1000 • C and finally disappeared at 1300 • C, similarly, as it has been observed in [37]. The mixtures of kaolins K1 and K2 and FA produced on the TG curves in comparison with FA about twice the mass loss of FA/K1 and more than three times greater mass loss of FA/K2.
The reaction of CaO with the silica led to the formation of aluminosilicates and limited formation of a liquid phase during sintering [38]. At a temperature of 1100 • C, intensity of peaks of anorthite increased (Figure 2a) due to the reaction of CaO from FA with a decomposition product of kaolinite, according to the reaction shown in Equation (6): The fact that all the natural feldspars can be expressed in terms of the three end members, orthoclase (K), albite (Na) and anorthite (Ca) was declared long ago, e.g., [37]. Moreover, as there is very little solid solution between orthoclase and anorthite it is convenient to consider the chemical relationships of the feldspar group in terms of two major series, the alkali feldspars and the plagioclases. Structural change of feldspar in kaolin samples K1 and K2 at 1000 • C ( Table 2) observed on the intensive endotherms on the TG curves and mass loss probably supported reactions with CaO in FA/K1 and FA/K2 (Figure 2). The basic oxides K2O or Na2O and feldspar minerals containing K or Na form lower melting point and improve fusibility. Intensity of the feldspar peaks on XRD pattern decreased slightly due to decrease in crystallinity when the temperature was higher than 1000 °C and finally disappeared at 1300 °C, similarly, as it has been observed in [37]. The mixtures of kaolins K1 and K2 and FA produced on the TG curves in comparison with FA about twice the mass loss of FA/K1 and more than three times greater mass loss of FA/K2.  The reaction of CaO with the silica led to the formation of aluminosilicates and limited formation of a liquid phase during sintering [38]. At a temperature of 1100 °C, intensity of peaks of anorthite increased (Figure 2a) due to the reaction of CaO from FA with a decomposition product of kaolinite, according to the reaction shown in Equation (6): The fact that all the natural feldspars can be expressed in terms of the three end members, orthoclase (K), albite (Na) and anorthite (Ca) was declared long ago, e.g., [37]. Moreover, as there is very little solid solution between orthoclase and anorthite it is convenient to consider the chemical relationships of the feldspar group in terms of two major series, the alkali feldspars and the plagioclases. Structural change of feldspar in kaolin samples K1 and K2 at 1000 °C (Table 2) observed on the intensive endotherms on the TG curves and mass loss probably supported reactions with CaO in FA/K1 and FA/K2 (Figure 2).

Effect of Fe2O3 on Mullite Structure
The ash fusion system in the pseudo-ternary system Al2O3-SiO2-Base (FeO + CaO + K2O) [39] allowed to determine about 5 mass% solid solution limit of Fe2O3 in mullite structure sintered at 1300 °C [24]. Considering the presence of Fe2O3 in FA and kaolins (Table 1) as well as hematite (Table 3), substitution of Fe 3+ for Al 3+ in the mullite structure can be assumed. Lattice parameters of mullites were obtained from the Rietveld analysis of the XRD patterns. The expansion of lattice parameters in primary mullites at 1000 °C and secondary mullites at 1300 °C was taken as an indicator for the iron dissolution within the structure ( Figure 5). Relations between the lattice parameters a and b of mullites sintered from kaolin samples and from FA and FA/K mixtures at 1000 and 1300 °C

Effect of Fe 2 O 3 on Mullite Structure
The ash fusion system in the pseudo-ternary system Al 2 O 3 -SiO 2 -Base (FeO + CaO + K 2 O) [39] allowed to determine about 5 mass% solid solution limit of Fe 2 O 3 in mullite structure sintered at 1300 • C [24]. Considering the presence of Fe 2 O 3 in FA and kaolins (Table 1) as well as hematite ( Table 3), substitution of Fe 3+ for Al 3+ in the mullite structure can be assumed. Lattice parameters of mullites were obtained from the Rietveld analysis of the XRD patterns. The expansion of lattice parameters in primary mullites at 1000 • C and secondary mullites at 1300 • C was taken as an indicator for the iron dissolution within the structure ( Figure 5). Relations between the lattice parameters a and b of mullites sintered from kaolin samples and from FA and FA/K mixtures at 1000 and 1300 • C (Figure 5a

Effect of Kaolins on Porosity, Shrinkage, and Compressive Strength
Porosity of samples was formed by pores in the unimodal, bimodal and multimodal pores size distribution at annealed temperatures 1000, 1100, 1200 and 1300 °C ( Figure 6) as follows: The samples annealed at 1000 °C show: (I) unimodal pore size distribution with the pores size region at about 0.83 µm in FA-1000 and FA/K3-1000; 1.24 µm in FA/K1-1000 The sintering FA and kaolin aids shifted the pore sizes toward smaller values. This phenomenon may be ascribed to the generation of more liquid phase during the sintering processes when kaolins were added.
The shrinkage at the temperatures of 1000, 1100, and 1200 °C (Figure 7) can also be accompanied by the formation of crystalline phases during sintering. The thermal shrinkage at 1100 ° ( Figure 7a) and porosity (Figure 7b), and bulk density at 1200-1300 °C (Figure 7c) were attributed to a mullitization-crystal-growth induced volume expansion [41].
According to the literature, the process of vitrification started at sintering temperatures between 1050 and 1100 °C and the alkali metal oxides and alkaline earth metal oxides supported the emergence of secondary mullite at the lower temperature [42].
Average value of shrinkage BS = 30.0 ± 1.0% calculated for post-sintered cubes at 1100 °C (Figure 7a, Table 4) corresponds to the porosity 1.9% in FA-1100, 9.3 and 10.6% The samples annealed at 1300 • C show: (II) bimodal pore size distribution with the pores size in the two regions: 0.008 and 60.6 µm in FA-1300; 0.01 and 24.8 µm in FA/K3-1300; and (III) multimodal pore size distribution region pores smaller than 0.05 and the pores size of 33.8 µm in FA/K1-1300; a more pronounced multimodal pore size distribution in FA/K2-1300 from the region 0.006 to 0.13 µm to the region of 40.0 µm.
The sintering FA and kaolin aids shifted the pore sizes toward smaller values. This phenomenon may be ascribed to the generation of more liquid phase during the sintering processes when kaolins were added.
The shrinkage at the temperatures of 1000, 1100, and 1200 • C (Figure 7) can also be accompanied by the formation of crystalline phases during sintering. The thermal shrinkage at 1100 • (Figure 7a) and porosity (Figure 7b), and bulk density at 1200-1300 • C (Figure 7c) were attributed to a mullitization-crystal-growth induced volume expansion [41]. Microstructure of ceramic samples at sintering temperatures can be characterized by the porosity and apparent density in the relation with the compressive strength (Figure 8). Ceramic samples at sintering temperatures 1100 °C showed minimum porosity (Figure 7b) at high density ( Figure 7c) and exhibited compressive strength higher than 50 MPa (Figure 8). FA-1100 achieved a highest compressive strength of 100.1 MPa at a porosity of 1.9% and density of 2.11 g/cm 3 (Table 4). However, FA-1300 reached only the smallest values of 9.3 MPa at a porosity 29.2% and density of 1.82 g/cm 3 (Figure 8a, Table  4). The FAK3-1100 exhibited good compressive strength of 87.6 MPa at a porosity of According to the literature, the process of vitrification started at sintering temperatures between 1050 and 1100 • C and the alkali metal oxides and alkaline earth metal oxides supported the emergence of secondary mullite at the lower temperature [42].
Microstructure of ceramic samples at sintering temperatures can be characterized by the porosity and apparent density in the relation with the compressive strength ( Figure 8). Ceramic samples at sintering temperatures 1100 • C showed minimum porosity (Figure 7b) at high density ( Figure 7c) and exhibited compressive strength higher than 50 MPa (Figure 8). FA-1100 achieved a highest compressive strength of 100.1 MPa at a porosity of 1.9% and density of 2.11 g/cm 3 (Table 4). However, FA-1300 reached only the smallest values of 9.3 MPa at a porosity 29.2% and density of 1.82 g/cm 3 (Figure 8a, Table 4). The The glassy phase at 1100 °C, resulting from the transformation of kaolinite and impurities such as K2O can influence the reactions and growth of mullite grains [43].
Samples sintered at 1300 °C in comparison to their samples sintered at 1100 and 1200 °C showed volume expansion, higher porosity and reduced bulk density (Table 4). In the literature, this phenomenon was explained on the basis of a detailed study of the sintering process [44]. The liquid phase generates at sintering temperature a certain value of capillary pressure at the contact points of the particles and binds the fine granules together, causing an increase in shrinkage. The gas in the closed pores exerts pressure on the pore walls, which is unfavorable for the densification of the sample. These closed pores expand as the pressure increases with the sintering temperature. When the sintering temperature exceeded a certain temperature (1300 °C) the increased content of liquid phase and released gases from decomposition reactions increase the apparent porosity at the expense of the bulk density decreased.

Conclusions
The ceramics were prepared from the FA, three raw kaolins, and FA-kaolin additive 10% by mass by annealing at temperatures of 1000, 1100 1200 and 1300 °C.
Kaolin samples contained kaolinite from 85.5% to 77% by mass, quartz, muscovite The glassy phase at 1100 • C, resulting from the transformation of kaolinite and impurities such as K 2 O can influence the reactions and growth of mullite grains [43].
Samples sintered at 1300 • C in comparison to their samples sintered at 1100 and 1200 • C showed volume expansion, higher porosity and reduced bulk density (Table 4). In the literature, this phenomenon was explained on the basis of a detailed study of the sintering process [44]. The liquid phase generates at sintering temperature a certain value of capillary pressure at the contact points of the particles and binds the fine granules together, causing an increase in shrinkage. The gas in the closed pores exerts pressure on the pore walls, which is unfavorable for the densification of the sample. These closed pores expand as the pressure increases with the sintering temperature. When the sintering temperature exceeded a certain temperature (1300 • C) the increased content of liquid phase and released gases from decomposition reactions increase the apparent porosity at the expense of the bulk density decreased.

Conclusions
The ceramics were prepared from the FA, three raw kaolins, and FA-kaolin additive 10% by mass by annealing at temperatures of 1000, 1100 1200 and 1300 • C.
Kaolin samples contained kaolinite from 85.5% to 77% by mass, quartz, muscovite and K-and K,Na-orthoclase. Kaolins containing small amount of Fe 2 O 3 (about 1 mass%) and Na,K feldspars were sintered at 1300 • C to the mullite with the negligible substitution of Fe 3+ for Al 3+ in the structure. This phenomenon in mullite structures crystallizing in ceramics annealed from mixtures of these kaolins with fly ash was verified on the literary data.
Ceramics annealed at temperatures 1000 and 1100 • C showed unimodal pores size distribution and porosity slightly higher, supported by the kaolins containing Na,K-feldspars. Annealing at temperatures 1200 • C and 1300 • C produced in ceramics bimodal pore size distribution in FA and FA/K3 (containing K-feldspar) in comparison with multimodal pore size distribution in FA/K1 and FA/K2 (containing Na,K-feldspar).
Ceramic samples produced from kaolins additive containing Na,K feldspars in comparison with kaolins additive containing K-feldspar showed lower shrinkage values corresponding to the compressive strength.
Microstructure of ceramic samples at sintering temperatures was characterized by the porosity and apparent density in the relation with the compressive strength. Ceramic samples at sintering temperature 1100 • C showed minimum porosity, high density and exhibited compressive strength higher than 50 MPa. At sintering temperature 1300 • C porosity increases at the expense of density and resistance to the compressive strength. The best structural and mechanical characteristics were found for FAK3 sample, supported by the high content of kaolinite and orthoclase in kaolin K3 additive. The FAK3 annealed at-1100 • C exhibited good compressive strength of 87.6 MPa at a porosity of 10.6% and density of 2.24 g/cm 3  Data Availability Statement: All data supporting reported results are performed in this paper.