Influence of firing temperature and mineralogical composition on bending strength and porosity of ceramic tile bodies

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Applied Clay Science 42 (2008) 266 – 271 www.elsevier.com/locate/clay

Influence of firing temperature and mineralogical composition on bending strength and porosity of ceramic tile bodies M.M. Jordan a,⁎, M.A. Montero a , S. Meseguer b , T. Sanfeliu b a

Department of Agrochemistry and Environment, University Miguel Hernández, Elche. Avda. de la Universidad s/n. 03202 ELCHE (Alicante), Spain b Unit of Applied Mineralogy, Department of Experimental Sciences, University Jaume I, Campus de Riu Sec s/n. 12080 Castellón, Spain Received 23 February 2006; received in revised form 10 January 2008; accepted 14 January 2008 Available online 26 January 2008

Abstract This study is focussed on the behaviour of ceramic clays from the area around Castellon (Spain) having a large ceramic industry. Test samples have been prepared by extrusion and firing in the range of 800–1150 °C. Analysis of the fired samples was carried out by X-ray diffraction. The results from the study of mineralogical transformations show the persistence of illite up to at least 900 °C. From the destruction of illite, an intermediate phase between spinel and hercynite originates. Mixtures of illitic clays containing CaCO3 form gehlenite, wollastonite and anorthitic plagioclases from 950 °C onwards. Quartz and hematites are present in samples poor in CaCO3. The optimum firing temperature for each sample has been established. The maximum bending strength of each ceramic body has been determined. Porosity-strength relations have been established. © 2008 Elsevier B.V. All rights reserved. Keywords: Ceramic clays; Tiles; Bending strength; Porosity; Technological behaviour

1. Introduction During firing of phyllosilicates and accompanying minerals like quartz, feldspar, calcite, dolomite and hematite, a series of transformations occurs which determine the final properties of the ceramic products. Through the ceramic process, these crystalline phases, once they exceed their stability limits, partially decompose and simultaneously others are being formed. An instantaneous destruction of the pre-existing structure does not occur (Jordán et al., 1999). The process of firing in ceramic kilns has been studied extensively and reported in the literature. Calcareous clays or marl were the subject of studies by Peters and Jenny (1973), Peters and Iberg (1978) and Pollifrone and Ravaglioli (1973). Capel et al. (1985) studied the formation of gehlenite and calcic plagioclase in calcareous clays. González-García et al. (1990) verified the formation of gehlenite and anorthite phases upon firing clays which were originally composed of illite, kaolinite, quartz and calcite. Jordán et al. (1993, 1994, 1995) studied Cretaceous clays from ⁎ Corresponding author. Tel.: +34 966658416; fax: +34 966658340. E-mail address: [email protected] (M.M. Jordan). 0169-1317/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2008.01.005

Castellón (Spain) and their behaviour when subjected to rapid firing. Wagh et al. (1993) proposed a model for the dependence of bending strength on the porosity in polycrystalline ceramic materials. This model is based on the simplification of considering the microstructure of ceramic products as a continuous network of solid cylinders with porous channels between them. The equations proposed for the model are: E ¼ Eo ð1  PÞm

ð1Þ

rf ¼ ro ð1  PÞmþ0:5

ð2Þ

where E, σf represent Young's modulus for porous materials, Eo and σo are the respective values for the free pore matrix, P is the porosity and the exponent m is function of the characteristics of the cylinders which model the microstructure of the solid. Although the model describes the irregular forms and the sizes of the pores in the ceramic bodies, certain observations in reference to the applicability and utility of this approximation can be made. The equations for this model have the advantage of being mathematically simple to work with. Other equations, however, include only empirical formulas, considered simply

M.M. Jordan et al. / Applied Clay Science 42 (2008) 266–271

for the goodness of fit. Comparison with experimental data has so far shown an excessive deviation between the predictions of these other equations and the data. A simplification of Eq. (1) can be carried out, considering, for example, the dependence of the porosity on bending strength. Using Hook's Law, we get the equation: e ¼ eo ð1  PÞ0:5 :

ð3Þ

This Eq. (3) predicts the invariable dependence of the bending strength on the porosity. The slope of the mechanical-porosityresistance should be the same for all ceramic products and porous structures. This generalisation is in disagreement with the results of many experiments. The studies carried out by Datta and Mukhopadhyay (1988) are aimed at highlighting this discrepancy between the theoretical and experimental results. These studies brought together 107 experimental results from the literature over a wide range of porosity and concluded that the exponent varies according to the nature of the porosity. The problem is that the exponent m of the Eqs. (1) and (2) has no clear physical connection with the real structure of the porosity. The authors failed to take into account the dependence of m on the characteristics of the pores, such as their form and orientation and fail to mention how the value m was derived from the measurements of the parameters related to the porosity. The pores in a real system are defined as spheroids with an effective axial ratio (z/x) and an orientation of the direction of the applied force (αp) which are the mean values for the system under consideration. With this spheroidal characterisation of the pores, the equation derived from the dependence of porosity on Young's modulus has been proposed: rf ¼ ro ð1  PÞk

ð4Þ

For the study of the resistance to fracture of porous materials, the following equation has been proposed: h i   
z 1=3  z 2 2 2=3 E ¼ Eo 1  1:21 : 1þ  1 Cos ap P x x ð5Þ where the exponent K is related to the factor of concentration of the force caused by the pores (Kim and Lee, 2007). It is interesting to compare this equation with the model proposed by Wagh et al. (1993), as it predicts a similar tendency in the variation of the resistance to fracture with the porosity, given by the model of porosity, but with a different physical significance for the exponent. Exponent K in Eq. (4) is related to the factor of concentration of the force (stress) of the pores, and depends on the form and the orientation of the pores and on the Poisson ratio of the material. It can be calculated using the theory of three-dimensional elasticity for spheroidal cavities (Boccaccini, 1994). If there are no preferential orientations, αp = 54° can be used for the random orientation of the pores, and the Poisson's constant v = 0.20 for ceramic materials. In this way K ¼ f xz, which can take on values of xz ¼ 0:3  1 depending on the type of material.

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If we take xz ¼ 0:8, αp = 54 °C, then K = 2.2, whereas the experimental data gives an adjusted value for m = 2.14. We can therefore deduce that K is approximately equal to m. It can be stated that the spheroidal model adapts itself much better to the experimental data than the Wagh model. The model proposed by Wagh et al. (1993) is limited in its applicability for the adjustment of experimental data and has no predictive character. Given the fact that the different mathematical models presented are in state of constant evolution, because no valid model that can adjust theoretical data to experimental data for ceramic bodies has yet to be found, the study of these two parameters has been carried out independently. For this reason, only qualitative relationships have been obtained between both of the parameters for the particular mineral phases present (Maiya et al., 1993). The focus of this study, therefore, has been on the analysis of bending strength of ceramic tile bodies fired at different temperatures to establish relationships between both parameters. Reactions involving gaseous phases are frequent in the firing process of the clays, and it can be shown that the kinetics of many of reactions of oxidation and decomposition are influenced strongly by the permeability of the porous solid to the gases. In the process of heating the ceramic tile body, the gases present in the interior of the initially open pores escape, and this produces a gaseous flow from the interior of the tile body to the recesses of the atmosphere of the kiln. This gaseous flow basically depends on the permeability of the solid (Amorós et al., 1992). Several authors have shown that the porosity of ceramic pieces is an important factor in their resistance to freezing temperatures (Wagh et al., 1991). Also, the porosity of a ceramic body is intimately related to its bending strength, as we mentioned above. On the other hand, the mechanical properties of ceramic pieces are very important in determining their utilisation and application for any specific function (Zweben, 1991). 2. Materials and methods Five outcrops of Tertiary-age clays (series T1, T2, T3, T4 and T5) and five outcrops of Cretaceous clays (series C1, C2, C2, C4 and C5) which are used in the formulation of ceramic pastes have been selected. Mine samples were sieved using a sieve of 63 μm. Mixtures with each of the compositions were prepared in proportions of 65% solid dry and 35% water with 1% deflocculating agent (sodium pyrophosphate). Test pieces were approximately 100 mm in length and 10 mm in diameter. The free water content was subsequently eliminated through infra-red heating at a temperature of 65 °C for 3 days. The pieces were finally heated to 800, 850, 900,950, 1000, 1050, 1100 and 1150 °C and kept at the maximum temperature for 35 min. The fractionation method modified according to Sanfeliu (1991) was used in preparations for mineralogical analyses. Oriented clay samples (normal, heated to 550 °C for 2 h and treated with ethylene glycol for 2 h) were studied. X-ray spectra were recorded using a Siemens D-500 diffractometer with Bragg–Bretano geometry. Chemical analysis of raw materials was carried out by XRF using the conventional techniques. A mineralogical analysis of fired samples was carried out by XRD. The semi-quantitative analysis was carried out using the programmes EVA and EDQ following Chung (1974). Once the bodies had been fired at the indicated temperatures, the corresponding tests were carried out in order to determine their bending strengths. The tests for bending strength were done in an INSTRON 1341 with a digital control system, applying tension at four points separated two by two by 0.04 m (l2) and 0.02 m (l1) respectively, in order to obtain a representative average statistical value.

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Table 1 Chemical analysis (% by weight, dry) of Tertiary-age series Sample

SiO2

Al2O3

Fe2O3

K2O

CaO

MgO

TiO2

MnO

P2O5

T1 T2 T3 T4 T5

49.07 59.00 48.71 50.10 40.02

17.43 12.74 15.00 15.00 14.80

6.10 3.75 4.98 4.64 4.75

3.65 2.86 3.14 3.43 3.20

16.95 18.38 18.66 19.57 27.75

2.24 1.94 1.50 1.57 1.88

0.65 0.52 0.64 0.64 0.54

0.03 0.03 0.03 0.05 0.04

0.17 0.14 0.13 0.16 0.14

The velocity of the piston was set at v = 0.022 mm/s, the minimum charge limit was 1000 KN and the minimum position limit was set at 0.0 mm. The limit of protection was 20.0 mm. From the bending strength data obtained, the modulus of bending strength (σ) was calculated, according to the expression: r¼

Pðl 2  l1 Þ : pr 3

ð6Þ

The distribution of the size of the pores of each of the fired tile bodies was determined by mercury porisimetry. Mercury porisimetry is based on the introduction of mercury, a non-wetting liquid, which has a surface tension T and an angle of contact C, in the interior of pores with radius R (Lorici and Brusa, 1991). The calculation of the applied pressure is carried out using Washburn's equation, which relates the applied pressure and the radius of the capillaries: P¼

2T CosC : R

ð7Þ

The equation supposes that all of the pores are cylindrical, non-circuitous and that they are of the same length and size. Even if this is not true, the results are obtained in terms of equivalent cylindrical diameters. A PORESIZER 9320 V1.01 mercury penetrometer was used, with 10.79 μL/pF constant and 130° angle of contact. The measuring system used measures the variation in the volume of mercury that penetrates into the pores of the fired ceramic tile bodies when the pressure is increased. An increase in the pressure indicates that the mercury is able to occupy the smaller size pores (Lucarelli and Venturi, 1995). The total volume of intrusion (VT), the total pore area (ST), the average pore radius (r), as well as the apparent density of the samples (φap) was calculated from the intrusion curves for each of the fired tile bodies. The value of the surface area of the pores was determined supposing they were cylindrical. The following equation was used: ST ¼

PX max Po

4DVi di

ð8Þ

in which the maximum and minimum pressure reached with the mercury porosimeter is Po and Pi respectively, ΔVi is the volume of mercury introduced between two consecutive measurements of the pressures Pi and Pi + 1 and di is the average diameter between pressures Pi and Pi + 1. The open porosity of the fired samples (ε) is calculated from the expression: e ¼ VT  uap

ð9Þ

where VT is the total volume of mercury intrusion per unit of mass and φap is the apparent density of the fired piece.

On the other hand, the laws of Darcy and Poiseuille have been used to obtain the equation which allows for the calculation of the coefficient of permeability. This coefficient is calculated assuming the hypothesis that the fired tile bodies are formed by a system of non-circuitous cylindrical pores of the same length and radius, which is in effect a simplification and idealisation of the porous system. For the calculation of the coefficient of permeability, the following equation was used: Kp ¼

r2  e : 8

ð10Þ

3. Results and discussion 3.1. Mineralogy and chemistry of ceramic clays Calcite and quartz are the predominant phases in the Tertiary clays. In all of the samples analysed, the constant presence of feldspars, plagioclase and orthoclase, the latter in the lowest proportion, was observed. The clay fraction is made up principally of chlorite and kaolinite; while the presence of quartz, gypsum and, in general, traces of calcite and dolomite were also noted. Montmorillonite was detected in some samples. The chemical analysis of the raw material (Table 1) shows a high CaO content. Iron varies between 3.75 and 6.10%. The alkaline metals (Na and K) are in the range of 0.48–0.79% and 2.80–3.65% respectively. The Al varies between 12.74 and 17.43%. The semi-quantitative analysis of Cretaceous clays (Table 2) revealed a great similarity in the mineralogical composition. The clays have a definite illitic character (40–54%), a quartz content of approximately 30% and chlorite and kaolinite together comprising about 10%. The average content of hematite is around 5%. Potassic feldspar is more abundant than plagioclase although it never exceeds 7%. These materials can be defined from a mineralogical point of view as illitic clays with a high sand content. The mineralogy of the sand fraction is principally composed of quartz followed by illite agglomerates which even surpass the quartz content in some samples, and also in the fine sand fraction. The mineralogical analysis of the finer fraction has shown illite as a principal component as well as traces of kaolinite. Chlorite was not found in the samples analysed. This raw materials can therefore be designated as illitic clays. The chemical analysis of the Cretaceous raw material (Table 3) shows a low CaO content. Fe2O3 varies between 4.37 and 9%. The alkaline metals are in the range of 2.95–4.84 respectively. Al2O3 varies between 14.68 and 18.37%.

Table 2 Semi-quantitative mineralogical analysis (values in %) Sample

Q

Cc

Do

M+I

C+K

Hm

FdK

Plg

C1 C2 C3 C4 C5

33 35 27 29 32

5 3 b1 6 4

b1 – – b1 –

42 46 54 43 40

8 10 9 13 9

5 4 5 6 6

b1 – 4 b1 7

5 2 – b1 2

Legend: Q (quartz), Cc (calcite), Do (dolomite), M + I (muscovite + illite), C + K (chlorite + kaolinite), Hm (hematite), Fdk (K-feldspars) y Plg (plagioclases).

Table 3 Chemical analysis (% by weight, dry) of Cretaceous series %

SiO2

Al2O3

Fe2O3

K2 O

CaO

MgO

TiO2

MnO

P2O3

C1 C2 C3 C3 C5

65.96 55.05 61.25 59.72 56.00

14.68 17.34 18.09 18.37 17.20

4.37 5.52 4.86 6.68 9.00

4.24 3.87 4.84 3.44 2.95

2.18 4.03 1.64 1.35 5.05

1.28 1.64 1.18 1.65 1.32

0.76 0.71 0.83 0.86 0.66

0.04 0.05 0.02 0.07 0.08

0.11 0.19 0.11 0.12 0.12

M.M. Jordan et al. / Applied Clay Science 42 (2008) 266–271

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Table 4 High temperature minerals in the studied raw materials (in number of counts, Cps) T (°C)

Q

CaO

800 950 1050 1100 1150

3504 1766 1620 1098 709

177 197 175

Plg 426 512 546 719

Au

Gh

Hm

768 856 674

881 645 328 210

374 183 130 120 130

S

W

265

579

I

Cc

Or

304

256 290

429 446 388 416 449

710

Key: Q = Quartz; Plg = Plagioclase; Au = Augite; Gh = Gehlenite; Hm = Hematite; S = Spinel; W = Wollastonite; I = Illite; Cc = Calcite; Or = Potassic Feldspar.

3.2. Mineralogy of the fired samples Ceramic bodies have a texture similar to a rock with a pyroclastic flow. According to the classical petrographic classification these materials have a rhyolitic–rhyodacitic composition (Jordán et al., 1999). Microcrystalline growth rims (probably formed at the expense of glass) can be seen on the internal edge of the vesicles. This glass phase is decanted from a microcrystalline mass to a porous vesiculased zone which tends to be filled up with glass. Occasionally growths of epitaxic microcrystals occur in this vitreous mass. The persistence of illite is observed up to at least 900 °C. From the destruction of illite an intermediate phase between spinel (MgO·Al2O3) and hercynite (FeO·Al2O3) originates. The samples which are rich in illite form hercynite. Enstatite is formed at 1000 °C in the series poor in carbonate, and remains in this series up to a temperature of 1150 °C. It is thus clear that enstatite is preferentially formed in the samples which had a high chlorite content. In samples of the Tertiary-aged outcrops, gehlenite is formed at a temperature of 950 °C. This phase is present up to 1050 °C but is not detected at higher temperatures In Tertiary-age series, gehlenite coexists with anorthite at a temperature of 1100 °C (Table 4). Anorthite is formed at 1050 °C and reaches its maximum at 1100 °C. At higher temperatures other phases such as mullite are formed as well as an abundant amorphous phase. 3.3. Bending strength and porosity tests

Fig. 1. Variation of bending strength modulus σ (MPa) for five compositions as function of firing temperature. Legend: C1 (Traigera); C2 (Zucaina); C3 (Traiguera); C4 (Morella) and C5 (Cervera).

temperature the bending strength remains practically constant for the series T1, T2 and C4. Bending strength depends on the crystalline phase content and is directly related to the percentage of calcium carbonate and calcic silico-aluminates between 950–1050 °C. The presence of crystalline phases in the ceramic matrix provides the piece with a high fired mechanical strength. The greatest bending strength for the bodies fired at 950 °C was seen in the series T1 (10.72 MPa), T2 (10.59 MPa) and C4 (10.29 MPa), with minor values in the series C1 (7.88 MPa) and C2 (5.18 MPa). Only samples of the series C2 and C1 showed an appreciable improvement in their mechanical behaviour when they reached 1150 °C (Fig. 1). The values for the coefficient of permeability (Kp) and the average pore radius (r) of the fired pieces increased strongly with the temperature until they reached a maximum, to later

The bending strength calculated for each temperature and composition applying Eq. (7) is shown in Table 5. An increase in the resistance to bending was noted in all of the series, and this was related to the increase in the maturing temperature and to the reduction of porosity. This increase was more pronounced from 1000 °C, with a very pronounced slope of the straight line that joins the point corresponding to 1050 °C with the point at 1100 °C. From the point of this maturing Table 5 Bending strength σ (MPa) vs firing temperature for ceramic tile bodies T (°C)

900

950

1000

1050

1100

1150

T1 T2 T3 T4 T5

7.83 4.31 10.36 8.86 9.79

7.88 5.18 10.72 10.29 10.59

11.28 6.33 11.77 12.12 12.00

13.82 11.29 13.55 15.32 14.96

17.86 13.47 16.34 21.23 19.57

22.25 18.46 10.07 22.00 20.55

Fig. 2. Evolution of bending strength σ (MPa) and porosity (ε) vs temperature in T2 fired ceramic bodies. Legend: Bending strength (●); Porosity (♦).

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Fig. 3. Variation in the average pore size radius with the firing temperature for an average composition (series C4).

progressively decrease as the temperature increased. As can be noted, the jump in the resistance to bending (σ) corresponds to the decrease in porosity (ε). The distribution of the porosity, after extrusion, is not homogeneous because bending strength of the ceramic bodies has been determined precisely in the mid zone. Fig. 2 shows the relationship between bending strength and porosity. The relationship between mineralogy of the raw materials and phase changes taking place during their sintering under different conditions have been examined (Daskshama et al., 1992). Between 900 and 1000 °C a sintering process takes place, which consists in the aggregate compaction of particles. This process is not complete, so the ceramic tile bodies are still quite porous. Towards 1000 °C, as can be seen in the graphs r = f(T), the larger pores are seen to increase (between 1 and 10 μm). This phenomenon coincides with the destruction of illites, chlorites and their re-crystallisation into quartz and spinel (hercinyte type) principally. Between 1050 and 1100 °C a considerable decrease in the porosity occurs, coinciding with the beginning of vitrification.

A considerable decrease in the average radius of the pores therefore also begins. At 1150 °C the tile bodies become earthenware and the pores close, and the porosity decreases significantly (Fig. 3). For the compositions studied, the variations in the average pore size diameter is mainly due to two factors that are produced simultaneously but with opposite effects. These effects are developed in the pieces in relation to the degree in which their porosity diminishes due to an increase in the content in liquid phase and to a decrease in their viscosity. On the other hand, as a consequence of the micro-structural heterogeneity of the raw tile bodies, with the increase in the firing temperature we find a progressive elimination of the smaller pores, which bring about differential contractions among the different micro-regions of the tile body and as a consequence the average pore size diameter increases. On the other hand, as the liquid phase content increases and its viscosity decreases, besides producing the reduction in the porosity of the tile bodies, the pre-existent capillary system is partially blocked (Amorós et al., 1992), which reduces the interconnected porosity even more. As a result of these two conflicting effects, curves (average pore radius vs. temperature) with a maximum can be observed (Fig. 4). The reduction of porosity is linked to the increase in the bending

Table 6 Parameters that describe the porous texture of the ceramic tile bodies Sample T (C)

VT (ml/g)

ST φ φap ε (m2/g) (g/ml) (g/ml)

Kp·10− 16 r·10− 8 (m) (m)

C1

0.152 0.143 0.110 0.071 0.130 0.152 0.162 0.144 0.141 0.112 0.081 0.045 0.158 0.146 0.134 0.104 0.058 0.037 – 0.186 0.175 0.149 0.113 0.069 0.186 0.188 0.174 0.143 0.094 0.0113

2.73 2.07 1.79 1.73 2.18 2.73 2.90 1.79 2.02 0.95 0.84 1.56 5.462 3.502 1.928 1.914 1.334 1.754 – 3.477 2.945 1.839 1.829 4.871 4.72 3.43 2.96 2.27 2.44 2.694

6.09 8.91 5.46 1.51 6.02 6.09 6.71 12.22 9.05 20.17 9.69 0.48 1.24 2.46 6.38 3.26 1.27 0.20 – 4.80 5.70 9.53 4.47 0.15 2.56 4.99 5.45 5.47 1.45 –

C2

C4

T1

T2

Fig. 4. Variation in the average pore size radius with the firing temperature for an average composition (series C2).

900 950 1000 1050 1100 900 900 950 1000 1050 1100 1150 900 950 1000 1050 1100 1150 900 950 1000 1050 1100 1150 900 950 1000 1050 1100 1150

1.866 1.895 2.025 2.181 1.946 1.866 1.837 1.889 1.914 2.016 2.148 2.264 1.885 1.918 1.950 2.105 2.279 2.352 1.772 1.795 1.847 1.926 2.047 2.158 1.779 1.773 1.821 1.920 2.082 2.111

2.604 2.601 2.601 2.579 2.605 2.603 2.620 2.600 2.623 2.603 2.599 2.522 2.685 2.669 2.644 2.697 2.628 2.577 1.778 2.699 2.728 2.707 2.667 2.534 2.659 2.660 2.664 2.649 2.589 2.163

0.395 0.373 0.288 0.182 0.339 0.395 0.427 0.376 0.370 0.291 0.210 0.115 0.297 0.281 0.262 0.219 0.133 0.087 – 0.335 0.323 0.288 0.232 0.149 0.331 0.333 0.316 0.275 0.196 0.028

11.11 13.83 12.32 8.16 11.93 11.11 11.21 16.13 13.97 23.54 19.19 5.78 5.75 8.37 13.95 10.90 8.75 4.24 – 10.72 11.88 16.27 12.41 2.83 7.87 10.95 11.74 12.61 7.70 0.84

Legend: T (temperature); VT (total Hg volume that penetrates into the pores); ST (specific surface); φ (density); φap (apparent density); ε (open porosity); Kp (coefficient of permeability); r (average pore radius).

M.M. Jordan et al. / Applied Clay Science 42 (2008) 266–271

strength of the clay matrix at the same time as the thermal expansion of the ceramic piece increases. What is especially striking is the evolution of the average pore size radius when the firing temperature was increased in series C1; here we were able to observe the unusual presence of two maxima (Fig. 4). The first maximum occurred at 950 °C and the second at 1050 °C. From 1050 °C on we witnessed a rapid decrease in the average pore size radius. The decrease in the average pore size when going from 950 °C to 1000 °C is difficult to explain, although it could be attributed to the production, in this interval of temperatures, of volumetric changes subsequent to polymorphous changes which readjust to available spaces; we were unable to find any similar behaviour in the literature consulted. We also tried to account for this behaviour by attributing it to the lack of homogeneity in the raw tile bodies. The increase seen in the average pore size radius of the ceramic piece with the reduction in porosity becomes more marked as the microstructure of the raw piece becomes less uniform. From 1000 °C upward in some pieces or from 1050 °C in others, high levels of sintering are reached, and this becomes evident with the fast decrease in the permeable porosity. As far as the density of the pieces (fired ceramic bodies) is concerned, we were able to observe a normal effect of progressive increase as the firing temperature was increased, together with a decrease in the quantity of pores due to the greater vitreous phase content because the sintering reaction is completed. The increase in the apparent density of the material (φap) is a result of the intensive reduction in open porosity (Table 6). Even though the sintering reaction has been completed, the possible discrepancies can be attributed to the large number of different sizes of internal pores that the material contains. 4. Conclusions The ceramic materials studied can be defined as illitic clays with a high sand content. Kaolinite and chlorite can be present together with illite. The coarse fraction is composed of quartz and illite agglomerates. The results from the study of mineralogical transformations show the persistence of illite up to at least 900 °C. An intermediate phase between spinel and hercynite originates from the destruction of illite. Mixtures of illitic clays containing CaCO3 form gehlenite, wollastonite and anorthitic plagioclases from 950 °C onwards. Samples poor in CaCO3 give a very simple mineralogical composition (quartz and hematite). The relation between porosity and the parameters that describe the porous texture of the ceramic tile bodies studied is complicated because of the fact that these have many pores with a complex and irregular spatial ordination, with a broad distribution of forms and sizes, and as such they do not fit into established empirical equations. The bending strength values (σ) obtained for each of the compositions fired at 1100 °C can be situated within the interval for those materials used for pavements and/or floor coverings (16–22 MPa), but are somewhat inferior to the required specifications for earthenware (34–35 MPa). The values obtained

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