Electrical conductivity and thermal analyses studies of phase evolution in the illite – CaCO3 system

Applied Clay Science 178 (2019) 105140

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research paper

Electrical conductivity and thermal analyses studies of phase evolution in the illite – CaCO3 system

T

⁎

Štefan Csákia,c, , Tomáš Húlanb, Ján Ondruškab, Igor Štubňab, Viera Trnovcováb, František Lukáča,c, Patrik Dobroňa a

Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic Department of Physics, Constantine the Philosopher University in Nitra, Tr. A. Hlinku 1, 949 74 Nitra, Slovak Republic c Institute of Plasma Physics, The Czech Academy of Sciences, Za Slovankou 3, 182 00 Prague, Czech Republic b

A R T I C LE I N FO

A B S T R A C T

Keywords: Crystallization AC conductivity Loss tangent Anorthite Leucite

Mixture of a purified illitic clay (> 90 wt% of illite, Northeastern Hungary) and CaCO3, in laboratory grade, was studied with respect to the high-temperature crystallization of anorthite and leucite mineral phases. The prepared samples were subjected to thermal analyses – differential scanning calorimetry (DSC) and thermodilatometry (DIL), and microstructure was characterized by scanning electron microscopy. The phase composition of the samples after firing was determined by the help of XRD measurement. Among anorthite and leucite (which were present as dominant mineral phases), gehlenite was found in the fired samples. Crystallization temperatures were determined from DSC peaks (940 °C and 1070 °C for gehlenite/anorthite and leucite crystallization, respectively). The samples exhibited a steep contraction as the low-viscosity glassy phase appeared and the anorthite/gehlenite started to crystallize. The crystallization temperatures were confirmed by the temperature dependence of the loss tangent, which exhibited two maxima at 940 °C and 1070 °C. Thus, during crystallization, the contribution of the ions to the conductivity was hindered by their cooperative migration to occupy new sites.

1. Introduction Ceramic materials containing CaO have been produced for centuries (Heimann, 2010; Hoard, 1995), however, originally, CaO has not been added intentionally. It is included in the raw materials naturally, often as a constituent of illitic clays. An advantage of using the calcareous clays is that a dense ceramic bodies could be produced at temperatures considerably below 1100 °C. The presence of Ca-carbonates in ceramic mixtures acts as a flux and also lowers the sintering temperature (Heimann, 2010). On the other hand, high content of carbonates can limit the extent of vitrification due to lower content of silicates at temperatures above 1000 °C (Cultrone et al., 2001). It was reported in many papers that the optimal addition of CaO improved mechanical strength (Monteiro et al., 2004; Sokolář et al., 2014; Vodova et al., 2014), decreased porosity (Vieira et al., 2004; Vodova et al., 2014), lowered the firing shrinkage (Monteiro et al., 2004; Sokolář et al., 2014; Vodova et al., 2014), and lowered the firing temperature. These positive effects of additives are more pronounced with increasing content of CaO and they are often connected with a formation of mineral anorthite (CaO·Al2O3·2SiO2). This anorthite formation is frequently expoited in ⁎

kaolinite-based ceramics. While the manufacturers in the past relied on empirical experiences, current research of an CaO influence on the ceramic materials is focused on the analysis of microstructure, phase composition, and resulting properties. Ceramic clays often contain illite as the main mineralogical phase. More recently, illite is also attractive in more advanced applications, such as production of geopolymers (Dietel et al., 2017; Hu et al., 2017). Although the ceramic mixtures containing illite and CaCO3 are important for the ceramic industry, no systematic study on the illite – CaCO3 system, which does not contain other phases, has been carried out. This is one of the objectives of this study. It can be stated that little is known about the transformations on the interface of carbonates and phylosilicates. There is no clear understanding of mechanism of reactants transport (diffusion, viscous flow, others…) and processes take place during the crystallization of new mineralogical phases (reactions in solid state, melt crystallization, others…). To contribute to the clarification of these issues, the phase transformations in two illitic clays (one rich in carbonates) were compared in (Cultrone et al., 2001). It was found that the presence of carbonates affected textural and mineralogical composition at the

Corresponding author at: Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic. E-mail address: [email protected]ff.cuni.cz (Š. Csáki).

https://doi.org/10.1016/j.clay.2019.105140 Received 1 March 2019; Received in revised form 8 May 2019; Accepted 21 May 2019 Available online 28 May 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

Applied Clay Science 178 (2019) 105140

Š. Csáki, et al.

temperatures between 700 °C and 800 °C. The authors concluded that formation of gehlenite, wollastonite, diopside and anorthite occurred on the carbonate-silicate sites by combined viscous flow and reactiondiffusion process. Besides the standard methods employed in ceramic research such as thermodilatometry (DIL), differential scanning calorimetry (DSC), measurement of Young's modulus at elevated temperatures, and scanning electron microscopy (SEM), X-ray diffraction (XRD), etc.), the measurement of alternating current (AC) electrical conductivity at elevated temperatures proves to be a suitable tool for a study of processes occuring during thermal treatment of ceramic materials. This experimental method not only provides information about the conduction mechanism and the dominant charge carriers, but also helps to designate an optimal firing procedure (Csáki et al., 2017; Kriaa et al., 2014; Kubliha et al., 2016; Ondruška et al., 2015). It was found that electrical conductivity of illitic clays is mostly carried by H+, OH−, Na+, K+ and Ca2+ ions. The H+ and OH– ions are dominant charge carriers during the removal of the physically bound water (up to 250 °C) and during dehydroxylation (450–750 °C). Moreover, the presence of Ca2+ ions leads to an increase in the conductivity at high temperatures. Previous research revealed that the electric conduction in illite-based ceramic materials occurs via the hopping mechanism (Csáki et al., 2018). The goal of the present paper is to provide a detailed description of processes which occur during heating of the purified illitic clay - CaCO3 mixture. The amount of CaCO3 in experimental mixtures was chosen to be optimal for anorthite formation during thermal treatment, which is an approach often exploited in kaolinite-based ceramics, but no such efforts were made on illite-based ceramics. In addition to auxiliary thermal analyses, the electrical properties of the mixture were thoroughly investigated. Hence, the dominant charge carriers and the conductivity mechanism could be determined. Emphasis is given on the influence of crystallization processes on AC conductivity, which is a rather unexplored phenomenon. Thus, the results of the present study may serve as a basis for crystallization studies of different phases using electrical conductivity measurements. In addition, the obtained results can be of high importance for the ceramic industry, particularly for preparation of illite-based anorthite ceramics.

100

3.0 80

2.5 2.0

60

1.5 40

1.0 20

0.5 0.0 10-1

Normalized particle amount (%)

Relative quantity of particles (%)

3.5

0

100

101

102

103

Equivalent particle diameter ( m) Fig. 1. Particle size distribution of the purified illitic clay. Table 1 Chemical composition of purified illitic clay in wt%. LOI – lost on ignition. SiO2

Al2O3

K2O

Fe2O3

CaO

TiO2

LOI

51.22

32.30

8.92

0.78

0.64

0.14

6.00

The AC conductivity was measured on disc samples (∅ 22 × 2.5 mm) at 17 different frequencies (ranging from 40 Hz to 4 MHz). The sample was placed between two platinum plate electrodes, and the measurements of capacitance and parallel equivalent resistance were carried out using the Tegam 3550 LCR meter in a static air atmosphere. The other parameters – AC conductivity, loss tangent, and real part of the complex permittivity – were then calculated from the obtained results. The heating regime for all analyses was as follows: 1. linear heating from the room temperature to 780 °C at the rate of 25 °C/min 2. keeping at 780 °C for 20 min 3. linear heating from 780 °C to 1150 °C at the rate of 5 °C/min

2. Experimental procedure Illitic clay, mined in north-eastern Hungary, was used as a base material. The choice of illitic clay from this locality is justified by its purity and because of several studies, which were performed on this material (Csáki et al., 2018; Húlan et al., 2017; Knapek et al., 2016; Ondruška et al., 2018a; Ondruška et al., 2018b). The clay was crushed, milled and further purified by sedimentation prior to sample preparation. The as-prepared material contained > 90 wt% of illite, ~3 wt% of quartz, traces of montmorillonite and orthoclase. Most of the particles had equivalent spherical diameter around 2 μm (Fig. 1). The chemical composition of the clay is shown in Table 1. A CaCO3 calcite powder, in laboratory grade (supplied by Centralchem, s.r.o.), was added to the clay, so that the mixture contained 25 wt% of CaCO3 and 75 wt% of dry clay. The overall chemical composition of prepared mixture lay in the anorthite region of ternary equilibrium CaO – Al2O3 – SiO2 diagram. The input materials of relatively high purity were chosen to create a simple system without considerable amount of impurities. Samples for DSC, DIL, and AC conductivity measurements were prepared by an addition of 30 wt% of distilled water into the powder mixture to obtain a gel for its subsequent casting into the gypsum molds. Afterwards, the samples were dried freely on air for a week. Samples for DSC had a cylindrical shape and the mass of ~25 mg. The DSC analysis was carried out in protective Ar atmosphere. Thermodilatometry was carried out on cuboid samples 8 × 8 × 25 mm on the Netzsch DIL 402C thermodilatometer in protective N2 atmosphere.

The phase composition was determined using a powder X-ray diffraction (PXRD) method. The measurements were carried out on the vertical powder θ-θ diffractometer D8 Discover (Bruker AXS, Germany), using CuKα radiation with the NiKβ filter. The diffracted beam was detected by 1D detector LynxEye. The phase identification was done using X'Pert high score software, which accessed the PDF-2 database of crystalline phases. Quantitative Rietveld refinement was performed using the TOPAS V5 software. 3. Results and discussion 3.1. Thermal and structural analyses During heating, initially, the physically bound water (PBW) was removed from the samples. The PBW is bound to the grain surface in several layers and its removal takes place in more steps (Levitt and Young, 2007; Osipov, 2012; Saarenketo, 1998). This process is represented by an endothermic peak on the DSC curve, having a maximum rate at 130 °C (Fig. 2). In the first step, the adsorbed water was evaporated. Afterwards, the water molecules were removed from the interlayer space. However, the individual steps are not recognizable on the DSC curve. More information can be obtained from the derivative of the heat flow curve with respect to temperature (Fig. 2). Temperatures 2

Applied Clay Science 178 (2019) 105140

Š. Csáki, et al.

-0.1

dDSC (mW/mg/°C)

Heat flow (mW/mg)

At 830 °C, an endothermic reaction was observed on the DSC curve (Fig. 2), which is related to the decomposition of CaCO3. The reaction is described by the formula

Isothermal heating

0.0

-0.2 -0.3

CaCO3 (solid) → CaO(solid) + CO2 (gas) The sample exhibited a continuous expansion up to ~900 °C caused by the expansion of the illite crystals (Wang et al., 2017) and the CaCO3 decomposition. The CO2 gas, produced during the decomposition, remained partially trapped in the softened structure and contributed to expansion of the sample. After the CaCO3 decomposition, the presence of the all base oxides (SiO2, Al2O3, and CaO) for anorthite crystallization was guaranteed. However, as the anorthite stoichiometry was non-stoichiometric, at higher temperatures, other mineral phases were formed. At 940 °C, a sharp exothermic peak represents the crystallization of anorthite and its intermediate phase gehlenite (Traoré et al., 2003). The exothermic peak at 1070 °C is linked to the crystallization of leucite (KAlSi2O6), which was possible due to the potassium contained in the illitic clay. The presence of these mineral phases was confirmed by XRD analysis (Fig. 5). Applying Rietveld refinement on the XRD data, the quantity of the mineral phases was estimated as follows: anorthite 51 wt%, leucite 26 wt%, and gehlenite 18 wt%. The vitrification and the following crystallization of anorthite led to a significant linear contraction of the sample (1.5%) up to 950 °C. Thus, the sintering is assisted by viscous flow. The highest rate of the contraction was observed at 930 °C, what is in agreement with the DSC peak temperature of the anorthite crystallization. Starting from ~950 °C, the sample dimensions remained almost unchanged. A slight expansion, observed above 1050 °C, can be linked to the crystallization of the leucite. The overall contraction of the sample reached only 0.03%. The microstructure of the sample heated at 1150 °C exhibited big pores (Fig. 4). The formation of the pores can be related to the evolution of CO2 gas, what causes voids in the softened microstructure, and in turn, increases the porosity.

-0.4 -0.5 0

300

600

780 900

1200

Temperature (°C) Fig. 2. Differential scanning calorimetry of the mixture. Blue curve: derivative of the DSC signal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Relative expansion (%)

0.1 1.5 0.0 1.0 -0.1 -0.2

0.5

-0.3 0.0

Expansion rate (% min-1)

0.2

2.0

Isothermal heating 0

300

600

780 900

-0.4 1200

3.2. Electrical properties of the mixture during heating

Temperature (°C)

The removal of the PBW significantly influenced the AC conductivity (Fig. 6). As the H2O molecules leaved the sample with increasing temperature, the AC conductivity decreased up to reaching its minimum value at ~240 °C. In this temperature region, H+ and OH– ions are the dominant charge carriers. The ions originate from the dissociation of water molecules (Csáki et al., 2018). In addition, H3O+ ions (originated from the self-ionization of water molecules) also contribute to the conduction in the samples. As the PBW was removed, the number of mobile charge carriers decreased, what led to a minimum value in the conductivity. With increasing temperature, the conductivity slightly increased owing to the increasing but still relatively low mobility of free alkali ions (mainly K+) and the relatively low number of available sites for ion hopping. The increase became more pronounced as the temperature reached ~360 °C. The contribution of alkali ions to AC conductivity increased due to higher thermal energy. Noteworthy, Ca2+ ions might also be involved in the conduction process. However, due to the relatively large size of these ions, their contribution is much lower than that of K+ ions (Ondruška et al., 2015). The dehydroxylation regime occurred at ~450 °C contributed to the conductivity by mobile OH– and H+ ions. The two steps of the illite dehydroxylation were not distinguishable on the conductivity curve. However, a drop in the conductivity was observed after finishing the dehydroxylation. This behavior can be explain by the depletion of the H+ and OH– source (Csáki et al., 2018) and/or by decomposition of CaCO3. After keeping the samples for 20 min at 780 °C, an increase in the conductivity with rising temperature was observed (Fig. 6). This is related to the increasing number of mobile charge carriers, which can contribute to the conduction process. The Arrhenius-like increase in the

Fig. 3. Thermodilatometry of the mixture. Blue curve: Expansion rate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

for the removal of the individual PBW layers were obtained from the derivative of the DSC curve as follows: 170 °C and 240 °C for the adsorbed water and interlayer water, respectively. These values correlate well with those determined from the derivative of the dilatometry curve (Fig. 3). During the removal of the PBW, a slight thermal expansion of the sample was observed (Fig. 3). In the temperature interval from 450 °C to 750 °C, the OH– groups are removed from the illite (Gualtieri and Ferrari, 2006). The process (dehydroxylation) was proceeded in two steps, which are represented by two endothermic peaks (630 °C and 730 °C, respectively) on the DSC curve (Fig. 2). During this process, octahedral OH– groups diffuse through the expanded tetrahedron ring to the interlayer space. Afterwards, the 2D diffusion in the interlayer space of these groups leads to their removal from the illite structure (Gualtieri and Ferrari, 2006). Contrary to kaolinite, the illite lattice does not collapse after the dehydroxylation. On the DIL curve, the process was represented by a significant expansion of the sample. After the dehydroxylation, illite continued to expand. The onset of an endothermic peak at ~750 °C represents the start of the CaCO3 decomposition (Galan et al., 2013). For this reason, the samples were kept at 780 °C for 20 min. A rather dense microstructure was observed after the isothermal heating (Fig. 4), what indicates that the thermal decomposition of CaCO3 was not finished. 3

Applied Clay Science 178 (2019) 105140

Š. Csáki, et al.

Fig. 4. SEM images of the sample after the isothermal treatment (left) and after firing at 1150 °C (right).

Sqrt(Counts)

viscosity glassy phase, because the conductivity is very sensitive even to small amounts (Wang and Xiao, 2002). The low-viscosity glassy phase allows the development of continuous conduction pathways, what increase the overall conductivity (Lerdprom et al., 2017). Anorthite and gehlenite crystallization occurred at ~1000 °C, which slowed down the increase of the AC conductivity. As the ions resting in their potential wells have to undergo defined displacement during the crystallization, they are not able to participate in the conduction mechanism. A similar trend was observed at the leucite crystallization. The frequency dependence of the conductivity allows to determine the dominant conduction mechanism (Blumenthal et al., 1974; Raju, 2003). At low frequencies, the long-range hopping of the charge carriers has the major contribution to the overall conductivity. However, as the frequency increased, the short-range interactions (between the nearest neighbors) became important. The frequency dependences of the conductivity obeyed Jonscher's universal power law (Jonscher, 1983).

5

10

15

20

25

30

35

40

2 (°) Fig. 5. XRD analysis of the sample fired at 1150 °C (● – Anorthite (ICDD #00041-1486), ♦ – Leucite (ICDD #00-038-1423), ∇ – gehlenite (ICDD #01-0751677)).

σAC = σ0 + Aω s ,

conductivity between 780 and 900 °C allowed the determination of conduction activation energies (CAE). The CAE decreased with increasing frequency from 1.37 eV (44 Hz) to 0.32 (4 MHz). In temperature region above 780 °C, the dominant charge carriers are alkali K+ ions complemented by Ca2+ ions. Above 900 °C, the AC conductivity increased rapidly. This can be related to the appearance of the low-

where σAC is the AC conductivity, σ0 is the frequency independent (DC-like) conductivity A is the frequency factor, ω is the angular frequency, and s is the frequency exponent. The parameter s remained below 1 (Table 2), which suggested that the conduction occurred by the hopping mechanism (Dutta and De, 2007; Entürk et al., 2016; Kriaa et al., 2014). The real part of the complex permittivity (Fig. 7) decreased with

(1)

30 0

12 00 90 0 78 0 60 0

Temperature (°C) 10-2 10-2

Isothermal heating Conductivity (S m 1)

Conductivity (S m 1)

10-3 10-4 -5

10

10-6

10-3

10-4

10-7 750

10-8 0.6

0.9

1.2

1.5

1.8

2.1

2.4

900

1050

1200

Temperature (°C)

2.7

1000/T (K 1) Fig. 6. Temperature dependence of the AC conductivity of the mixture (at 1 kHz). Right curve: evolution of the AC conductivity after the isothermal heat treatment. 4

Applied Clay Science 178 (2019) 105140

Š. Csáki, et al.

crystallization of gehlenite and anorthite mineral phases. The drop in the value of the real part of the complex permittivity starting at 1015 °C, with peak temperature 1065 °C, represented the crystallization of leucite. The reason for this can be ascribed to the arranged (or cooperative) movement of ions during the crystallization process, which restricts (hinders) their contribution to the conduction mechanism. The temperature dependence of the loss tangent (Fig. 8) exhibits three maxima. The first one was observed during the illite dehydroxylation, where the dominant charge carriers, OH– groups, are removed from the structure. The second, at ~940 °C, and the third one at 1070 °C, indicate that the losses are the highest at these temperatures. This can be related to the arranged movement of ions during crystallization, which restricts their free movement and contribution to the AC conductivity.

Table 2 Values of the frequency independent conductivity σ0 and the frequency exponent s at different temperatures. σ0/S m−1

s

300 500 780 900 1000

1.18 × 10−7 6.26 × 10−5 5.68 × 10−5 2.35 × 10−4 2.63 × 10−3

0.89 0.59 0.72 0.63 0.48

Real part of the complex permittivity

Temperature/°C

104

Isothermal heating 103

4. Conclusion Mixture of an illitic clay (Northeastern Hungary) and CaCO3 was studied using differential scanning calorimetry (DSC) and thermodilatometry (DIL). To better understand the processes occurring during the heating stage of the firing, AC electrical conductivity of the mixtures was in-situ measured. The main conclusions can be summarized as follows:

102

101

• During heating, two crystallization peaks were observed on the DSC 100 0

300

600

780 900

1200

Temperature (°C)

• •

Fig. 7. Temperature dependence of the real part of the complex permittivity.

36

Isothermal heating

Loss tangent

30

• •

24 18 12

•

6 0 0

300

600

780 900

curve. The first one, at ~ 950 °C, is related to the gehlenite and anorthite crystallization. The second peak at 1070 °C is ascribed to the crystallization of leucite. The presence of the mineral phases was confirmed by XRD. The overall linear expansion of the samples only reached 0.03%, which is favorable for the ceramic industry. SEM micrographs revealed a porous microstructure after the keeping the samples for 20 min at 780 °C, which suggested (a partial) decomposition of the CaCO3. The sample structure after firing was dense and homogeneous. The measurement of electrical conductivity can be used for a study of crystallization processes. Electrical conductivity exhibited a sudden increase in its value prior to the crystallization and it was followed by a deceleration. This can be related to the defined displacement of ions during the crystallization, which reduces their contribution to the overall conductivity. The temperature dependency of the loss tangent confirmed the high losses during crystallization. These are related to the arranged motion of the ions to occupy their new sites in the newly forming crystal lattice.

Acknowledgment

1200

Temperature (°C)

This work has been supported by the Czech Science Foundation (grant No. 17-16772S).

Fig. 8. Temperature dependence of the loss tangent.

References increasing temperature up to 240 °C due to the removal of the PBW. As the alkali ions were not mobile enough to follow the changes of the applied field, the real part of the complex permittivity reached its minimum value. This was in good agreement with results found for clays (~5) with low moisture content (Saarenketo, 1998). However, as the temperature increased, the real part of the complex permittivity also rose due to an increasing number of ions becoming available to follow the changes of the field. The increasing tendency remained until the dehydroxylation was finished. As the OH– source was depleted, the real part of the conductivity decreased, probably because of the start of the CaCO3 decomposition. A steep increase in the real part of the complex permittivity represented the appearance of the low-viscosity glassy phase with the

Blumenthal, R.N., Seitz, M.A., Tallan, N.M., 1974. Electrical conductivity in ceramics and glass. In: Experimental Techniques, Electrical Conductivity in Ceramics and Glass. M. Dekker, New York. Csáki, S., Trnovcová, V., Štubňa, I., Ondruška, J., Sunitrová, I., Vozár, L., Dobroň, P., 2017. AC Conductivity of an Illitic Clay with Zeolite Addition after Firing at Different Temperatures. American Institute of Physics Inchttps://doi.org/10.1063/1.4994488. Csáki, Š., Ondruška, J., Trnovcová, V., Štubňa, I., Dobroň, P., Vozár, L., 2018. Temperature dependence of the AC conductivity of illitic clay. Appl. Clay Sci. 157, 19–23. https://doi.org/10.1016/j.clay.2018.02.026. Cultrone, G., Rodriguez-Navarro, C., Sebastian, E., Cazalla, O., De La Torre, M.J., 2001. Carbonate and silicate phase reactions during ceramic firing. Eur. J. Mineral. 13, 621–634. https://doi.org/10.1127/0935-1221/2001/0013-0621. Dietel, J., Warr, L.N., Bertmer, M., Steudel, A., Grathoff, G.H., Emmerich, K., 2017. The importance of specific surface area in the geopolymerization of heated illitic clay. Appl. Clay Sci. 139, 99–107. https://doi.org/10.1016/j.clay.2017.01.001.

5

Applied Clay Science 178 (2019) 105140

Š. Csáki, et al.

Water Science, Second edition. CRC Press, pp. 1136–1139. https://doi.org/10.1081/ E-EWS2-120010269. Monteiro, S.N., Peçanha, L.A., Vieira, C.M.F., 2004. Reformulation of roofing tiles body with addition of granite waste from sawing operations. J. Eur. Ceram. Soc. 24, 2349–2356. https://doi.org/10.1016/S0955-2219(03)00638-1. Ondruška, J., Trnovcová, V., Štubňa, I., Podoba, R., 2015. DC Conductivity of ceramics with calcite waste in the temperature range 20 – 1050°C. Ceramics-Silikáty 59, 176–180. Ondruška, J., Trnovcová, V., Štubňa, I., Bačík, P., 2018a. Depolarization currents in illite. J. Therm. Anal. Calorim. 131, 2285–2289. https://doi.org/10.1007/s10973-0176862-7. Ondruška, J., Trnovcová, V., Štubňa, I., Csáki, Š., Vozár, L., 2018b. Influence of texture on DC conductivity and dimensional changes of kaolin and illitic clay. Ceram. Int. 45, 2425–2431. https://doi.org/10.1016/j.ceramint.2018.10.162. Osipov, V.I., 2012. Nanofilms of adsorbed water in clay: mechanism of formation and properties. Water Res. 39, 709–721. https://doi.org/10.1134/S009780781207010X. Raju, G.G., 2003. Dielectrics in Electric Fields. CRC Press. Saarenketo, T., 1998. Electrical properties of water in clay and silty soils. J. Appl. Geophys. 40, 73–88. https://doi.org/10.1016/S0926-9851(98)00017-2. Sokolář, R., Vodová, L., Šveda, M., 2014. Limestone sludge in the brick body. In: Advanced Materials Research. Trans Tech Publications Ltd, pp. 158–161. https://doi. org/10.4028/www.scientific.net/AMR.1000.158. Traoré, K., Kabré, T.S., Blanchart, P., 2003. Gehlenite and anorthite crystallisation from kaolinite and calcite mix. Ceram. Int. 29, 377–383. https://doi.org/10.1016/S02728842(02)00148-7. Vieira, C.M.F., Soares, T.M., Sánchez, R., Monteiro, S.N., 2004. Incorporation of granite waste in red ceramics. Mater. Sci. Eng. A 373, 115–121. https://doi.org/10.1016/j. msea.2003.12.038. Vodova, L., Sokolar, R., Hroudova, J., 2014. The effect of CaO Addition on Mechanical Properties of Ceramic Tiles. Int. J. Civil, Environ. Struct. Constr. Archit. Eng. 8, 717–720. Wang, X., Xiao, P., 2002. Characterisation of clay sintering process using impedance spectroscopy. J. Eur. Ceram. Soc. 22, 471–478. https://doi.org/10.1016/S09552219(01)00335-1. Wang, G., Wang, H., Zhang, N., 2017. In situ high temperature X-ray diffraction study of illite. Appl. Clay Sci. 146, 254–263. https://doi.org/10.1016/j.clay.2017.06.006.

Dutta, K., De, S.K., 2007. Optical and electrical characterization of polyaniline-silicon dioxide nanocomposite. Phys. Lett. Sect. A Gen. At. Solid State Phys. 361, 141–145. https://doi.org/10.1016/j.physleta.2006.09.025. Entürk, E., Akçakale, N., Okutan, M., 2016. Conductivity mechanism of mica powders inlaid elastomer based materials. J. Alloys Compd. 686, 43–47. https://doi.org/10. 1016/j.jallcom.2016.05.291. Galan, I., Glasser, F.P., Andrade, C., 2013. Calcium carbonate decomposition. J. Therm. Anal. Calorim. 111, 1197–1202. https://doi.org/10.1007/s10973-012-2290-x. Gualtieri, A.F., Ferrari, S., 2006. Kinetics of illite dehydroxylation. Phys. Chem. Miner. 33, 490–501. https://doi.org/10.1007/s00269-006-0092-z. Heimann, R.B., 2010. Classic and Advanced Ceramics: From Fundamentals to Applications. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. https:// doi.org/10.1002/9783527630172. Hoard, R.J., 1995. A Materials-science approach to undrstanding Limestone Temper Pottery from the Midwest. J. Archaeol. Sci. 22, 823–832. Hu, N., Bernsmeier, D., Grathoff, G.H., Warr, L.N., 2017. The influence of alkali activator type, curing temperature and gibbsite on the geopolymerization of an interstratified illite-smectite rich clay from Friedland. Appl. Clay Sci. 135, 386–393. https://doi. org/10.1016/j.clay.2016.10.021. Húlan, T., Trník, A., Medveď, I., 2017. Kinetics of thermal expansion of illite-based ceramics in the dehydroxylation region during heating. J. Therm. Anal. Calorim. 127, 1–8. https://doi.org/10.1007/s10973-016-5873-0. Jonscher, A.K., 1983. Dielectric Relaxation in Solids. Chelsea Dielectrics Press Limited. Knapek, M., Húlan, T., Minárik, P., Dobroň, P., Štubňa, I., Stráská, J., Chmelík, F., 2016. Study of microcracking in illite-based ceramics during firing. J. Eur. Ceram. Soc. 36, 221–226. https://doi.org/10.1016/j.jeurceramsoc.2015.09.004. Kriaa, A., Hajji, M., Jamoussi, F., Hamzaoui, a.H., 2014. Electrical conductivity of 1 : 1 and 2 : 1 clay minerals. Surf. Eng. Appl. Electrochem. 50, 84–94. https://doi.org/10. 3103/S1068375514010104. Kubliha, M., Trnovcová, V., Ondruška, J., Štubňa, I., Bošák, O., Kaljuvee, T., Bačík, P., 2016. DC conductivity of illitic clay after various firing. J. Therm. Anal. Calorim. 124, 81–86. https://doi.org/10.1007/s10973-015-5129-4. Lerdprom, W., Li, C., Jayaseelan, D.D., Skinner, S.J., Lee, W.E., 2017. Temperature dependence of electrical conductivity of a green porcelain mixture. J. Eur. Ceram. Soc. 37, 343–349. https://doi.org/10.1016/j.jeurceramsoc.2016.08.019. Levitt, D.G., Young, M.H., 2007. Soils: Hygroscopic water content. In: Encyclopedia of

6