Study of the illite-kaolinite-CaCO3 mixtures during firing

The raw materials used in traditional ceramics include highly plastic clay minerals such as illite or kaolinite. Non-plastic materials are usually added to the mixtures, which affect their workability and sintering process. The formation of a new crystalline phase (e.g. anorthite) is often observed in mixtures that contain a large amount of CaO. The improvement of final properties of ceramic materials is attributed to the formation of anorthite. The aim of this paper is to study the firing of anorthite ceramics in the system illite-kaolinite-CaCO3. Four sets of samples with different illite:kaolinite ratio (8:2, 6:4, 4:6, 2:8), and with a constant addition of CaCO3 are produced. The samples were subjected to the thermal and structural analyses. A significant mass loss is observed for all studied samples during the process of dehydroxylation and decomposition of CaCO3. An exothermic peak around 950 °C, which is connected with the formation of anorthite, is observed during differential thermal analysis. Above this temperature, no significant length changes are recorded. The presence of anorthite is confirmed by XRD analysis. The most formation of anorthite is observed in the illite-kaolinite sample with ratio 8:2.


Introduction
Natural plastic raw materials such as illitic clay and kaolin together with fluxes and fillers represent the main component in the production of traditional ceramics [1].In addition, waste materials are also added to the mixtures for the production of traditional ceramics.A positive influence on the properties of the final ceramic products (porosity, mechanical strength, Young's modulus of elasticity, bulk density, and coefficient of thermal conductivity) by waste materials with high CaO content are presented in [2][3][4][5].The positive properties are attributed to the formation of the mineral anorthite in final product.Anorthite (CaAl2Si2O8) belongs to the plagioclase group.Crystallization of anorthite in the microstructure increases the strength, chemical stability and affects the thermal expansion of the material [6].For this reason, the crystallization of anorthite in a ceramic body is a desirable process.In the scientific field, there are relatively few works devoted to the study of illite-based anorthite ceramics.The reason may be that it is not possible to achieve the amount of Al2O3, SiO2, and CaO in the stoichiometric requirements of anorthite with illite.
During the firing of ceramic materials based on kaolin or illitic clay, several processes occur, such as the releasing of physically bound water from pores and surfaces of crystals (approximately up to 250 °C) and the dehydroxylation of kaolinite and illite (from 400 °C to 700 °C) [7,8].The dehydroxylation of kaolinite proceeds in one step.Kaolinite is transformed into metakaolinite and a shrinkage occurs [7].On the other hand, the dehydroxylation of illite proceeds in two steps and an expansion occurs [8].At temperatures around 925 °C, the crystalline lattice collapses and the deformed Al(O, OH)6 layers transform into Al-Si spinel and amorphous SiO2.Mullite is formed at temperatures above 1050 °C and vitrification is observed at temperatures around 980 °C [9].
By modifying the composition of green mixtures, it is possible to influence the microstructure and phase composition in the firing process.The high content of CaO in ceramics mixtures shifts the phase composition of the sherd to the area of wollastonite-anorthite-gehlenite compatibility.At a higher temperature (>1100 °C), gehlenite reacts with released silicon dioxide (formed after the decomposition of kaolinite or illite) to form anorthite and wollastonite [1].
The aim of this study is to investigate the crystallization of anorthite in samples made from illitic clay, kaolin, and CaCO3 and discover if illitic clay is suitable material for the production of anorthite ceramics.The investigation is performed by differential thermal analysis (DTA), thermogravimetry (TGA), thermodilatometry (TDA), and XRD analysis.

Samples and measurement methods
To study the processes during firing, the samples made of illitic clay and kaolin in mass ratio of 8:2, 6:4, 4:6, and 2:8 were used.According to the stoichiometric ratio of anorthite, the laboratory CaCO3 was added to the mixtures.Illitic clay was from Füzérradvány in the north-east of Hungary in the Tokaj region and Sedlec kaolin with kaolinite content more than 90 mass% was used.The composition of the experimental mixtures is shown in table 1.From each mixtures, 500 g of dry raw materials were homogenized in planetary ball mill (Retsch PM100) for 5 min with an interval of 2 min and 200 rpm.Subsequently, deionized water (190 ± 10 g) was added in mixtures.The samples were extruded from the plastic mixture and the prismatic samples were obtained.Then, the samples dried at room temperature for approximately 7 days, which the samples reached equilibrium moisture content.Then the samples were stored in plastic bags to ensure the same moisture content of samples before each measurement.The processes taking place in the samples during thermal treatment were investigated by differential thermal analysis and thermogravimetry simultaneously using apparatus Derivatograph 1100° [10].The temperature interval from room temperature up to 1050 °C with a heating rate of 5 °C/min and static atmosphere was applied.
X-ray diffraction was used to determine the mineralogical composition.The measurement took place on vertical powder θ-θ diffractometer D8 Discover, Bruker AXS, Germany.The samples investigated by X-ray diffraction were fired at temperature of 1150 °C.
Thermal expansion was measured with horizontal push-rod dilatometer [11].The temperature interval from room temperature up to 1150 °C with a heating rate of 5 °C/min and from 1150 °C down to room temperature with a cooling rate of 5 °C/min was used.

Results and discussion
The results from DTA analysis are shown in figure 1a).In the temperature interval from 25 °C to 200 °C, an endothermic reaction, which was associated with the release of physically bound water from pores and surface of crystals, occurred.This reaction had two steps and it was connected with mass loss (figure 1b)).The smallest mass loss (1.07%) was in the sample with the highest content of kaolin (I2K8).The mass loss increased with the amount of illitic clay.This was attributed to the properties of illitic clay.Illitic clay is able to absorb more water than kaolin [8].The highest relative mass loss (2.05%) was measured for the sample I8K2.A small exothermic reaction in the temperature interval from 290 °C to 320 °C was attributed to the combustion of organic impurities.The mass loss was negligible.
In the temperature interval from 450 °C to 730 °C, a significant endothermic reaction was observed, which was caused by the dehydroxylation of kaolinite and illite.The dehydroxylation of illite runs in two steps [13], what was clearly visible in samples with the higher content of illitic clay (I8K2 and I6K4).The dehydroxylation process was accompanied by a sharp decrease in mass [5,9].With the higher content of kaolinite, the mass loss increased.Next endothermic reaction was observed in the temperature interval from 750 °C to 870 °C.This reaction was attributed to the decomposition of CaCO3 into CaO and CO2.The mass loss increased with the amount of CaCO3.
Finally, a significant exothermic peak was observed from the temperature of 950 °C.It was the most pronounced in samples with the highest content of kaolin.As the amount of kaolin in the samples increased, a shift of the peak to higher sintering temperatures was observed.This reaction was not accompanied by mass loss.It was attributed to the formation of new crystalline phases, such as anorthite.For samples I2K8 and I4K6, the peak was partially overlapped.This was attributed to the formation of gehlenite, which was formed from metakaolinite and calcite.The formation of Al-spinel was suppressed by the formation of anorthite, as it was evidenced in the results of XRD analysis (table 2).The final mass loss reached 16.60%, 17.71%, 19.61%, and 21.08% for I8K2, I6K4, I4K6, and I2K8, respectively.The results showed that final mass loss increased with the content of kaolin.Clay minerals such as kaolinite and illite are layered and exhibit anisotropy [13].Therefore, the samples were measured in the axial (figure 2a)) and also transversal direction (figure 2b)).The standard thermal expansion of studied samples occurred up to 450 °C instead of temperature interval from room temperatures up to 170 °C, where releasing of physically bound water caused slight decreasing of an expansion.
Above 450 °C, the dehydroxylation of kaolinite and illite occurs.Samples with the highest content of illitic clay expand during the dehydroxylation [8].On the other hand, samples with the highest amount of kaolin contract during the dehydroxylation [7].The increase of dimension in illitic samples can be also caused by the modification of low-temperature α-quartz to high-temperature β-quartz [12].It is visible in figure 2 that samples with higher content of illitic clay have the highest expansion and the samples contract with decreasing of illitic clay.
Above the temperature of 880 °C, all studied samples shrink rapidly due to sintering [13].Next, slight expansion of dimensions occurs after temperature 950 °C.This expansion is attributed to the formation of new crystalline phases in the samples (gehlenite and anorthite) [1].Then, during cooling a) b) the standard shrinkage occurred in all samples.Only around 573 °C, there is visible a small drop due to the β-α quartz transition [12].After a firing, the greatest relative shrinkage (2.99% in the axial direction and 5.60% in the transversal direction) was obtained for the I2K8 sample.With the addition of illitic clay in the samples, the final shrinkage decreased.For sample I4K6 was the relative shrinkage in the axial direction 1.29% and 2.92% in the transversal direction.The relative shrinkage for sample I6K4 was 0.44% in the axial and 1.80% in the transversal direction.And finally, the smallest shrinkage was obtained for the sample I8K2 (0.08% in the axial direction and 1.15% in the transversal direction).The results of the mineralogical composition of samples fired at 1150 °C determined by X-ray diffraction are in table 2. In samples, there was found mainly anorthite and Ca-feldspar, then gehlenite, quartz, mullite, wollastonite, and leucite.Anorthite was determined in all studied samples and its amount varied from 24% to 39%.Ca-feldspar, which differs from anorthite only in one crystalline lattice parameter c [14], was determined in samples.Its amount varied from 16% to 46%.The amount of anorthite decreased with kaolin content in samples.For Ca-feldspar content, it was opposite.Its amount increased with kaolin content.This was not valid for the sample I2K8, where the result was unexpected and with no explanation.Next, there was found gehlenite (the content increased with kaolin content), quartz (the content decreased with kaolin content), and wollastonite (the content decreased with kaolin content).Leucite was presented only in samples with the high illitic clay content (I8K2, I6K4) and the sample with the highest kaolin content contained also mullite (8%).

Conclusions
Four mixtures with different ratio of input raw materials (kaolin, illitic clay, and CaCO3) were investigated during the firing using DTA, TGA, TDA, and XRD analysis.An endothermic reaction, attributed to the dehydroxylation, was observed for all samples.The dehydroxylation in samples with a higher content of illitic clay (I8K2 and I6K4) had a two-step.The magnitude of peak increased with kaolin content.At 950 °C, a significant exothermic reaction was observed, which corresponded to the formation of new phases, such as anorthite and gehlenite.
The significant mass loss was observed for all samples up to the temperature of 890 °C.This mass loss was attributed to the releasing of physically bound water, dehydroxylation of illite and kaolinite, and decomposition of CaCO3.The results showed that final mass loss in samples increased with the content of kaolin.
Based on the results of XRD analysis, the most anorthite was observed (crystalized) in the sample with the highest content of illitic clay (I8K2).
Thermodilatometric analysis showed a significant anisotropy in all samples.Thermal shrinkage in transversal direction was almost two times higher than in axial direction.At the temperature of 1050 °C, a sintering and dimensional changes stopped in all samples.Also, in the case of samples with a high content of illitic clay, the small influence of quartz transition during heating and cooling was observed.
The obtained results indicate that (despite the fact that researchers focus mainly on kaolin-based anorthite ceramics) illitic clay appears to be also the suitable material for the production of anorthite ceramics.

Figure 1 .
Figure 1.DTA curves (a) and mass changes (b) of the measured samples.

Figure 2 .
Figure 2. Thermal expansion of the measured samples in axial (a) and transversal direction (b).

Table 1 .
The composition of experimental mixtures [in mass%]

Table 2 .
The mineralogical composition of samples fired at 1150 °C in %.