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Firing transformations of Chilean clays for the manufacture of ceramic tile bodies
Applied Clay Science 51 (2011) 147–150
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Firing transformations of Chilean clays for the manufacture of ceramic tile bodies F. Pardo a, S. Meseguer a, M.M. Jordán b,⁎, T. Sanfeliu a, I. González c a b c
Unit of Applied Mineralogy, Department of Agrarian Sciences and Environment, University Jaume I, Campus de Riu Sec s/n., 12080 Castellón, Spain Department of Agrochemistry and Environment, University Miguel Hernández, Elche. Avda. de la Universidad s/n., 03202 Elche, Alicante, Spain Department of Industry, Metropolitan University of Technology, Avda. José Predro Alexandre s/n., Macul, Santiago, Chile
a r t i c l e
i n f o
Article history: Received 30 July 2010 Received in revised form 19 November 2010 Accepted 20 November 2010 Available online 25 November 2010 Keywords: Chilean clays Firing Mineral transformation Bending strength
a b s t r a c t This contribution is focused on the study of the mineralogical changes occurring in the ceramic body after heating ceramic clays. Chile has an important local ceramic industry. Five deposits of clays with industrial applications were studied. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L), Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP). The samples were heated to 830, 975, 1080 and 1160 °C keeping at the maximum temperature for 35 min. The bending strength of each ceramic body was determined at 1100 °C. Mineralogical analysis of the fired samples was carried out by X-ray diffraction. The SVTT contained quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite; the LC clays quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/ muscovite and talc; the LH clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite; the MP clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite and wollastonite and the L clays quartz, microcline and mullite. The persistence of illite at at least 900 °C was observed for LC and LH. SVTT and LH showed the required specifications for earthenware. The L clays were refractory clays with very low bending strength. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During firing, a series of transformations occurs, which determine the final properties of the ceramic products (González-García et al., 1990; Jordán et al., 1999, 2001). During the ceramic process, once the crystalline structures of minerals exceed their stability limits, they are partially decomposed while simultaneously others are being formed. The destruction of the pre-existing structure does not occur instantaneously (Jordán et al., 1999). The knowledge of the origin, diagenesis and physicochemical composition of the clays is essential when sketching out suitable compositions required for ceramic production (Meseguer et al., 2009; Sanfeliu and Jordan, 2009). Upon firing, the minerals in the clay bodies undergo chemical and structural modifications deeply transforming the original clayey materials. The high temperature, low-pressure mineral transformations are mainly influenced by the chemical and mineralogical compositions of the original clay, its grain-size distribution, the maximum heating temperature, heating rate, duration of firing and kiln redox atmosphere (Maggeti, 1982; Moropoulou et al., 1995). Firing in ceramic kilns was extensively reported in the literature. González-García et al. (1990) verified the presence of gehlenite and anorthite phases in fired clays which were originally composed of illite, kaolinite, quartz and calcite. Jordán et al. (1993, 1994, 1995, 1999)
⁎ Corresponding author. E-mail address: [email protected] (M.M. Jordán). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.11.022
studied Cretaceous clays from Castellón, Spain and their behaviour when subjected to fast firing. Jordán et al. (2009) studied the firing transformations of non-calcareous Permo-Triassic clays used in the manufacture of ceramic tile bodies in the Spanish ceramic industry. The relationship between the mineralogical composition of the raw materials and the phase changes during sintering was examined (Daskshama et al., 1992; Jordán et al., 1999, 2009). Between 900 and 1000 °C, the sintering process consists in the aggregation compacting of particles. As this process is not complete, the ceramic tile bodies are quite porous. The reduction of water absorption is linked to the increase in the bending strength of the clay matrix and the thermal expansion of the ceramic piece increases. Above 1000 °C for some samples and above 1050 °C for other samples, high levels of sintering are reached, indicated by the fast decrease of water absorption. Towards 1000 °C, the larger pores increase (between 1 and 10 μm). This phenomenon coincides with the destruction of illites, chlorites and their re-crystallisation into quartz and spinel (Jordán et al., 2008). Several authors checked that the porosity of ceramic pieces is an important factor in their resistance to freezing temperatures (Wagh et al., 1993). The mechanical properties of ceramic pieces are very important in determining their application for any specific function (Boccaccini, 1994; Zweben, 1991). Also, the porosity of a ceramic body is related to its bending strength. Wagh et al. (1993) proposed relations between bending strength and porosity in polycrystalline ceramic materials. The equations proposed for the model were reported by Jordán et al. (2008). The applicability of this model is limited to the availability of experimental
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F. Pardo et al. / Applied Clay Science 51 (2011) 147–150
data and has no predictive character. For this reason, only qualitative relationships among bending strength, porosity and mineral phases present in the ceramic bodies were obtained by Maiya et al. (1993). Ceramic clays are one of the most complicated ceramic systems because of the very complex relationship between the behaviour of minerals during the ceramic processing and the transformations during heating. A major challenge is to predict the phase transformations in silicate ceramics, since complex relationships occur between the structural characteristics of the fired products and the physical properties. Clay minerals undergo a complex path of thermal transformations during heating. The focus of this study, therefore, was the analysis of the mineralogical changes of the ceramic bodies after heating industrial clays from Chile, and to evaluate the firing transformations in terms of crystalline phases.
Table 2 Mineralogical transformations of LC clays during heating at 830–975–1080–1160 °C. Mineral phase
LC 830 °C
LC 975 °C
LC 1080 °C
LC 1160 °C
Quartz Mullite Spinel Microcline Albite Anorthite Hematite Diopside Enstatite Illite/muscovite Talc
+++ − − + ++++ ++ + − (+) (+) (+)
+++ (+) − + ++++ +++ + − + − (+)
+++ (+) + (+) +++ +++ + − + − −
++ (+) ++ (+) ++ ++++ + (+) + − −
Legend: ++++ (N20%); +++ (N 15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
2. Materials and methods Five deposits of Chilean clays which can be used in the formulation of ceramic pastes were selected. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L) in the VI Region of Chile, and Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP) from the IV Region of Chile. The geological background of the mineralogical and chemical compositions of these clays was described by Meseguer et al. (2010). The clays are kaolinic–illitic clays with a high quartz content. Chlorite can also be present. Thermodilatometric analysis and the relationship between linear contraction and water absorption were discussed by Meseguer et al. (2010). Eight samples of each clay deposit were collected. They were ovendried at 110 °C until constant mass and then ground with a hammer mill to null residue in the 630 μm control sieve, following the normal practice in ceramic laboratories. A representative sample of each deposit was selected for firing tests. To simulate industrial pressing conditions, the clays were moistened by hand, mixed and sieved (b1 mm) until homogeneous materials with 6% water were obtained. They were left to rest for 24 h and then pressed (0.3 MPa, 80 × 40× 5 mm) by a laboratory press. The pieces were heated to 830, 975, 1080 and 1160 °C, keeping at the maximum temperature for 35 min. Analysis of the fired samples was carried out by X-ray diffraction (XRD) using the Siemens D-5000 diffractometer, CuKα radiation. The bending strength of fired bodies was measured at 1100 °C following standard methods with the Grabbrielli Crap 424 with a digital control system. 3. Results and discussion 3.1. Mineralogical composition of the fired samples The mineralogical transformations are reported in Tables 1 to 5. The XRD reflection at 10 Å corresponded to dehydroxilated mica (illite/muscovite). The reflection of sample LC was reduced with increasing firing temperature above 830 °C and disappeared at 975 °C
Table 3 Mineralogical transformations of LH clays during heating at 830–975–1080–1160 °C. Mineral phase
LH 830 °C
LH 975 °C
LH 1080 °C
LH 1160 °C
Quartz Cristobalite Microcline Albite Anorthite Hematite Diopside Augite Illite
++++ − ++ +++ + + + − +
++++ − ++ ++ ++ + + − (+)
+++ (+) + − +++ + + (+) −
+++ + + − ++++ + (+) (+) −
Legend: ++++ (N20%); +++ (N15%); ++ (N10%); + (N5%); (+) present (b5%); and − not present.
(Table 2). The persistence of illite was observed up to at least 975 °C for LH (Table 3). Spinel was formed in the SVTT and LC series (Tables 1 and 2). Diopside in the MP series formed, probably from decomposition of the Mg rich chlorite (Table 4). Enstatite appeared at 830 °C in the series LC (Table 2) and at 1080 °C in the SVTT series (Table 1), and was stable up to the temperature of 1160 °C. Thus enstatite is preferably formed in samples with higher chlorite contents. Anorthite was formed at 830 °C and reached the maximum amount at 1160 °C in the LC, LH and MP series (Tables 2–4). In the SVTT series (Table 1), anorthite appeared at 830–980 °C but disappeared at higher temperatures. Wollastonite was present in the MP series (Table 4). Usually, wollastonite and gehlenite are considered intermediate compounds which become unstable in the presence of SiO2, forming anorthite (Jordán et al., 2001; Trindade et al., 2009). Quartz decomposed between 830 and 1180 °C. The decomposed and disappearing phases contribute to the formation of a vitreous phase, up to 830 °C. Besides hematite formation, new crystalline phases appeared. At 1160 °C mullite was formed in the series LC (Table 2).
Table 4 Mineralogical transformations of MP clays during heating at 830–975–1080–1160 °C. Table 1 Mineralogical transformations of SVTT clays during heating at 830–975–1080–1160 °C. Mineral phase
SVTT 830 °C
SVTT 980 °C
SVTT 1080 °C
SVTT 1160 °C
Quartz Spinel Cristobalite Microcline Albite Anorthite Hematite Enstatite
++++ (+) (+) +++ ++++ + + −
++++ (+) (+) ++ ++ + + −
+++ (+) ++ + − − ++ +
+++ (+) ++ − − − ++ +
Legend: ++++ (N20%); +++ (N15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
Mineral phase
MP 830 °C
MP 975 °C
MP 1080 °C
MP 1160 °C
Quartz Cristobalite Microcline Albite Anorthite Hematite Diopside Gehlenite Enstatite Wollastonite
+++ − + ++++ ++ ++ (+) (+) (+) (+)
+++ − + +++ +++ ++ (+) (+) (+) (+)
+++ − + − ++++ ++ + (+) + (+)
++ ++ + − ++++ ++ + (+) ++ (+)
Legend: ++++ (N20%); +++ (N 15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
F. Pardo et al. / Applied Clay Science 51 (2011) 147–150 Table 5 Mineralogical transformations of L clays during heating at 830–975–1080–1160 °C. Mineral phase
L 830 °C
L 975 °C
L 1080 °C
L 1160 °C
Quartz Mullite Microcline
++++ − ++
++++ − ++
++++ − +
++++ (+) +
Legend: ++++ (N20%); +++ (N15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
After firing to the highest temperature (1160 °C), the material became body amorphous with residual grains of quartz and neoformated hematite and other high temperature mineral phases (mainly quartz, cristobalite and hematite in SVTT, anorthite, albite, quartz and spinel in LC, quartz and anorthite in LH, quartz, enstatite and hematite in MP and quartz with microcline in L clays). Quartz was a residual phase, i.e. a component of the raw clay that did not suffer chemical transformations during the firing stage. On the contrary, plagioclase, hematite, mullite and other silicates were formed during firing. Mullite was formed from metakaolinite.Variables affecting mullite formation include kaolinite content and behaviour during heating, possible occurrence of melted phases and impurities like Fe2O3. The presence of mullite was only observed in the LC and L series (Tables 2 and 5). In the MP and LH series, mullite was not present because of the high content of CaO (6–7%), decomposed clay minerals could combine with CaO, yielding anorthite instead of mullite (Trindade et al., 2009). 3.2. Bending strength The bending strength measured at 1100 °C is shown in Table 6. The high level of deviation (Kim and Lee, 2007; Lorici and Brusa, 1991), made it necessary to carry out a high number of determinations, between 25 and 50 for each test. The discrepancies were mainly due to errors that depend on the dimensions of the piece, on existent surface micro-fissures, as well as other experimental errors. Bending strength for the LC and MP could not be measured due to the curved geometry of ceramic bodies obtained after heating. Mechanical properties are closely related to the structure of the mullite network. The process itself influences the kinetic of structural transformation and particularly the density of the compacted powder and the firing cycle. In calcareous clays, the bending strength depends on the content of crystalline phases and is directly related to the content of calcium carbonate and calcium silico-aluminates (Jordán et al., 2008). In general, the presence of crystalline phases in the ceramic matrix provides the fired piece with a high mechanical strength. The water absorption reduction (Meseguer et al., 2010) is linked to the increase in the bending strength of the clay matrix. The values of the SVTT and LH samples comply with the requirements of the European Standard EN 14411:2004, which proposes bending strength values, as determined by the European Standard EN ISO 10545-4, N15 MPa and N20 MPa for covering and pavement ceramic tiles, respectively. The highest bending strength of bodies fired at 1100 °C was observed in the LH series (97 MPa). This value is not usual for conventional ceramic products, so it is probably related to the
Table 6 Bending strength σ(MPa) of tiles vs. firing temperature for ceramic tile bodies. Sample
Sintered 1100 °C
SVTT LC LH MP L
31 ± 5 – 97 ± 12 – 4 ± 0.5
149
simultaneous presence of anorthite/dipside and the low porosity of the ceramic matrix. However, the L series showed a clear refractory behaviour with bending strength values b5 MPa at 1100 °C. The results for the L industrial ceramic bodies showed no possibility of complying with the technical specifications, even at temperatures as high as 1100 °C. The reason is the intrinsic refractory nature of these kaolinitic clays. It is not possible to comply with the norms for floor tiles by exclusively using raw materials in which the porosity cannot be reduced to adequate values. Even for long firing cycles, the refractoriness of kaolinite clays hinders the closing of pores up to the maximum temperatures that can be used by the industrial furnaces.
4. Conclusions This study of the mineralogical transformation induced by firing potential ceramic raw materials available in the IV and VI Regions of Chile may contribute to developing the ceramic industry in these regions of this country, focusing on the manufacture of conventional ceramic products with possible applications as tiles (pavements and coverings in construction). The firing behaviour properties and the bending strength of the ceramic bodies of the Chilean clays indicated that the clays can be used in floor tile fabrication. The study of mineralogical transformations shows the presence of i) quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite for the SVTT samples; ii) quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/muscovite and talc for the LC samples; iii) quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite for the LH samples; iv) quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite for the MP samples and v) quartz, microcline and mullite for the L samples. In view of the bending strength values obtained, we can state that only the SVTT and LH series can be situated within the required specifications for earthenware. The bending strength values for the LC and MP series were not valid due to the curved test probes at this temperature. The L series have a clear refractory behaviour with bending strength values b5 MPa at 1100 °C. Actually, this is not only a technical problem but also a scientific verification. The highest temperatures reached by the local industrial furnaces are still not enough to promote the sintering mechanisms for sufficient closure of pores in kaolinitic clayey ceramics. A substantial reformulation of the body composition that could include larger amounts of alkaline oxides and lower loss on ignition could be a proper solution.
Acknowledgements The authors would like to express their gratitude to the Generalitat Valenciana (Project code: AE07/12) and Prof. G. Lagaly, editor of Applied Clay Science, for help in the discussion and additional comments.
References Boccaccini, A.R., 1994. Comment on dependence of ceramic fracture properties on porosity. Journal of Materials Letters 13, 1035–1037. Daskshama, V., Mohan, B.V., Lalithambika, M., Nair, C.G.R., 1992. Sintering studies on plastic clays. Ceramics International 18 (6), 359–364. EN 14411:2004. 2004. Ceramic tiles. Definitions, classification, characteristics and making. EN ISO 10545-4. 2004. Ceramic tiles. Part 4. Determination of modulus of rupture and bending strength. González-García, F., Romero-Acosta, V., García Ramos, G., González Rodríguez, M., 1990. Firing transformations of mixtures of clays containing illite, kaolinite and calcium carbonate used by ornamental tile industries. Applied Clay Science 5, 361–375. Jordán, M.M., Sanfeliu, T., de la Fuente, C., Ballbé, E., 1993. Ceramic interest Weald clays from Castellon. Third Euro-Ceramics 2, 947–952 Faenza Editrice Iberica. Jordán, M.M., Boix, A., De la Fuente, C., Ballbé, E., Queralt, I., Sanfeliu, T., 1994. Mineralogy and ceramic properties of wealdian cretaceous clays from Castellon. Boletín de la Sociedad Española de Mineralogía 17, 135–143.
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Jordán, M.M., Boix, A., Sanfeliu, T., de la Fuente, C., 1995. The mineralogy of cretaceous clays in Castellon and their application in the ceramic industry. International Ceramics Journal 10, 25–29. Jordán, M.M., Boix, A., Sanfeliu, T., de la Fuente, C., 1999. Firing transformations of cretaceous clays used in the manufacturing of ceramic tiles. Applied Clay Science 14, 225–234. Jordán, M.M., Sanfeliu, T., De la Fuente, C., 2001. Firing transformations of tertiary clays used in the manufacturing of ceramic tile bodies. Applied Clay Science 20, 87–95. Jordán, M.M., Montero, M.A., Meseguer, S., Sanfeliu, T., 2008. Influence of firing temperature and mineralogical composition on bending strength and porosity of ceramic tile bodies. Applied Clay Science 42, 266–271. Jordán, M.M., Martín-Martín, J.D., Sanfeliu, T., Gómez-Gras, D., De la Fuente, C., 2009. Mineralogy and firing transformations of permo-triassic clays used in the manufacturing of ceramic tile bodies. Applied Clay Science 44, 173–177. Kim, J.W., Lee, D.G., 2007. Measurement of residual stresses in thick composite cylinders by the radial-cut-cylinder-bending method. Composite Structures 77 (4), 444–456. Lorici, L., Brusa, A., 1991. Porous and vitrified single-fired tiles. Ceramic Engineering and Science Proceedings 12 (1–2), 183–221. Maggeti, M., 1982. Phase analysis and its significance for technology and origin. In: Olin, J.S. (Ed.), Archaeological Ceramics. Smithsonian Institution Press, pp. 121–133.
Maiya, P.S., Russell, R., Poa, D.S., Wagh, A.S., Poeppel, R.B., 1993. Microstructural characterization, mechanical-properties, and corrosion-resistance of the CaO–TiO2 system. Journal of Materials Research 8 (6), 1387–1393. Meseguer, S., Jordan, M.M., Sanfeliu, T., 2009. Use of mine spoils from Teruel coal mining district. Environmental Geology 56, 845–853. Meseguer, S., Pardo, F., Jordan, M.M., Sanfeliu, T., González, I., 2010. Ceramic behaviour of five Chilean clays which can be used in the manufacture of ceramic tile bodies. Applied Clay Science 47, 372–377. Moropoulou, A., Bakolas, A., Bisbikou, K., 1995. Thermal analysis as a method of characterizing ancient ceramic technologies. Thermochimica Acta 2570, 743–753. Sanfeliu, T., Jordan, M.M., 2009. Geological and environmental management of ceramic clay quarries: a review. Environmental Geology 57, 1613–1618. Trindade, M.J., Dias, M.I., Coroado, J., Rocha, F., 2009. Mineralogical transformations of calcareous rich clays with firing: a comparative study between calcite and dolomite rich clays from Algarve, Portugal. Applied Clay Science 42, 345–355. Wagh, A.S., Singh, P., Poeppel, R.B., 1993. Dependence of ceramic fracture properties on porosity. Journal Material Science 28, 3589. Zweben, C., 1991. Ceramic matrix composites — mechanical properties and test methods. Ceramic Engineering and Science Proceedings 12 (1–2), 409–503.
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Firing transformations of Chilean clays for the manufacture of ceramic tile bodies F. Pardo a, S. Meseguer a, M.M. Jordán b,⁎, T. Sanfeliu a, I. González c a b c
Unit of Applied Mineralogy, Department of Agrarian Sciences and Environment, University Jaume I, Campus de Riu Sec s/n., 12080 Castellón, Spain Department of Agrochemistry and Environment, University Miguel Hernández, Elche. Avda. de la Universidad s/n., 03202 Elche, Alicante, Spain Department of Industry, Metropolitan University of Technology, Avda. José Predro Alexandre s/n., Macul, Santiago, Chile
a r t i c l e
i n f o
Article history: Received 30 July 2010 Received in revised form 19 November 2010 Accepted 20 November 2010 Available online 25 November 2010 Keywords: Chilean clays Firing Mineral transformation Bending strength
a b s t r a c t This contribution is focused on the study of the mineralogical changes occurring in the ceramic body after heating ceramic clays. Chile has an important local ceramic industry. Five deposits of clays with industrial applications were studied. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L), Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP). The samples were heated to 830, 975, 1080 and 1160 °C keeping at the maximum temperature for 35 min. The bending strength of each ceramic body was determined at 1100 °C. Mineralogical analysis of the fired samples was carried out by X-ray diffraction. The SVTT contained quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite; the LC clays quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/ muscovite and talc; the LH clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite; the MP clays quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite and wollastonite and the L clays quartz, microcline and mullite. The persistence of illite at at least 900 °C was observed for LC and LH. SVTT and LH showed the required specifications for earthenware. The L clays were refractory clays with very low bending strength. © 2010 Elsevier B.V. All rights reserved.
1. Introduction During firing, a series of transformations occurs, which determine the final properties of the ceramic products (González-García et al., 1990; Jordán et al., 1999, 2001). During the ceramic process, once the crystalline structures of minerals exceed their stability limits, they are partially decomposed while simultaneously others are being formed. The destruction of the pre-existing structure does not occur instantaneously (Jordán et al., 1999). The knowledge of the origin, diagenesis and physicochemical composition of the clays is essential when sketching out suitable compositions required for ceramic production (Meseguer et al., 2009; Sanfeliu and Jordan, 2009). Upon firing, the minerals in the clay bodies undergo chemical and structural modifications deeply transforming the original clayey materials. The high temperature, low-pressure mineral transformations are mainly influenced by the chemical and mineralogical compositions of the original clay, its grain-size distribution, the maximum heating temperature, heating rate, duration of firing and kiln redox atmosphere (Maggeti, 1982; Moropoulou et al., 1995). Firing in ceramic kilns was extensively reported in the literature. González-García et al. (1990) verified the presence of gehlenite and anorthite phases in fired clays which were originally composed of illite, kaolinite, quartz and calcite. Jordán et al. (1993, 1994, 1995, 1999)
⁎ Corresponding author. E-mail address: [email protected] (M.M. Jordán). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.11.022
studied Cretaceous clays from Castellón, Spain and their behaviour when subjected to fast firing. Jordán et al. (2009) studied the firing transformations of non-calcareous Permo-Triassic clays used in the manufacture of ceramic tile bodies in the Spanish ceramic industry. The relationship between the mineralogical composition of the raw materials and the phase changes during sintering was examined (Daskshama et al., 1992; Jordán et al., 1999, 2009). Between 900 and 1000 °C, the sintering process consists in the aggregation compacting of particles. As this process is not complete, the ceramic tile bodies are quite porous. The reduction of water absorption is linked to the increase in the bending strength of the clay matrix and the thermal expansion of the ceramic piece increases. Above 1000 °C for some samples and above 1050 °C for other samples, high levels of sintering are reached, indicated by the fast decrease of water absorption. Towards 1000 °C, the larger pores increase (between 1 and 10 μm). This phenomenon coincides with the destruction of illites, chlorites and their re-crystallisation into quartz and spinel (Jordán et al., 2008). Several authors checked that the porosity of ceramic pieces is an important factor in their resistance to freezing temperatures (Wagh et al., 1993). The mechanical properties of ceramic pieces are very important in determining their application for any specific function (Boccaccini, 1994; Zweben, 1991). Also, the porosity of a ceramic body is related to its bending strength. Wagh et al. (1993) proposed relations between bending strength and porosity in polycrystalline ceramic materials. The equations proposed for the model were reported by Jordán et al. (2008). The applicability of this model is limited to the availability of experimental
148
F. Pardo et al. / Applied Clay Science 51 (2011) 147–150
data and has no predictive character. For this reason, only qualitative relationships among bending strength, porosity and mineral phases present in the ceramic bodies were obtained by Maiya et al. (1993). Ceramic clays are one of the most complicated ceramic systems because of the very complex relationship between the behaviour of minerals during the ceramic processing and the transformations during heating. A major challenge is to predict the phase transformations in silicate ceramics, since complex relationships occur between the structural characteristics of the fired products and the physical properties. Clay minerals undergo a complex path of thermal transformations during heating. The focus of this study, therefore, was the analysis of the mineralogical changes of the ceramic bodies after heating industrial clays from Chile, and to evaluate the firing transformations in terms of crystalline phases.
Table 2 Mineralogical transformations of LC clays during heating at 830–975–1080–1160 °C. Mineral phase
LC 830 °C
LC 975 °C
LC 1080 °C
LC 1160 °C
Quartz Mullite Spinel Microcline Albite Anorthite Hematite Diopside Enstatite Illite/muscovite Talc
+++ − − + ++++ ++ + − (+) (+) (+)
+++ (+) − + ++++ +++ + − + − (+)
+++ (+) + (+) +++ +++ + − + − −
++ (+) ++ (+) ++ ++++ + (+) + − −
Legend: ++++ (N20%); +++ (N 15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
2. Materials and methods Five deposits of Chilean clays which can be used in the formulation of ceramic pastes were selected. The clays came from San Vicente de Tagua-Tagua (SVTT), Litueche (L) in the VI Region of Chile, and Las Compañías-Río Elqui (LC), La Herradura-Coquimbo (LH) and Monte Patria-Coquimbo (MP) from the IV Region of Chile. The geological background of the mineralogical and chemical compositions of these clays was described by Meseguer et al. (2010). The clays are kaolinic–illitic clays with a high quartz content. Chlorite can also be present. Thermodilatometric analysis and the relationship between linear contraction and water absorption were discussed by Meseguer et al. (2010). Eight samples of each clay deposit were collected. They were ovendried at 110 °C until constant mass and then ground with a hammer mill to null residue in the 630 μm control sieve, following the normal practice in ceramic laboratories. A representative sample of each deposit was selected for firing tests. To simulate industrial pressing conditions, the clays were moistened by hand, mixed and sieved (b1 mm) until homogeneous materials with 6% water were obtained. They were left to rest for 24 h and then pressed (0.3 MPa, 80 × 40× 5 mm) by a laboratory press. The pieces were heated to 830, 975, 1080 and 1160 °C, keeping at the maximum temperature for 35 min. Analysis of the fired samples was carried out by X-ray diffraction (XRD) using the Siemens D-5000 diffractometer, CuKα radiation. The bending strength of fired bodies was measured at 1100 °C following standard methods with the Grabbrielli Crap 424 with a digital control system. 3. Results and discussion 3.1. Mineralogical composition of the fired samples The mineralogical transformations are reported in Tables 1 to 5. The XRD reflection at 10 Å corresponded to dehydroxilated mica (illite/muscovite). The reflection of sample LC was reduced with increasing firing temperature above 830 °C and disappeared at 975 °C
Table 3 Mineralogical transformations of LH clays during heating at 830–975–1080–1160 °C. Mineral phase
LH 830 °C
LH 975 °C
LH 1080 °C
LH 1160 °C
Quartz Cristobalite Microcline Albite Anorthite Hematite Diopside Augite Illite
++++ − ++ +++ + + + − +
++++ − ++ ++ ++ + + − (+)
+++ (+) + − +++ + + (+) −
+++ + + − ++++ + (+) (+) −
Legend: ++++ (N20%); +++ (N15%); ++ (N10%); + (N5%); (+) present (b5%); and − not present.
(Table 2). The persistence of illite was observed up to at least 975 °C for LH (Table 3). Spinel was formed in the SVTT and LC series (Tables 1 and 2). Diopside in the MP series formed, probably from decomposition of the Mg rich chlorite (Table 4). Enstatite appeared at 830 °C in the series LC (Table 2) and at 1080 °C in the SVTT series (Table 1), and was stable up to the temperature of 1160 °C. Thus enstatite is preferably formed in samples with higher chlorite contents. Anorthite was formed at 830 °C and reached the maximum amount at 1160 °C in the LC, LH and MP series (Tables 2–4). In the SVTT series (Table 1), anorthite appeared at 830–980 °C but disappeared at higher temperatures. Wollastonite was present in the MP series (Table 4). Usually, wollastonite and gehlenite are considered intermediate compounds which become unstable in the presence of SiO2, forming anorthite (Jordán et al., 2001; Trindade et al., 2009). Quartz decomposed between 830 and 1180 °C. The decomposed and disappearing phases contribute to the formation of a vitreous phase, up to 830 °C. Besides hematite formation, new crystalline phases appeared. At 1160 °C mullite was formed in the series LC (Table 2).
Table 4 Mineralogical transformations of MP clays during heating at 830–975–1080–1160 °C. Table 1 Mineralogical transformations of SVTT clays during heating at 830–975–1080–1160 °C. Mineral phase
SVTT 830 °C
SVTT 980 °C
SVTT 1080 °C
SVTT 1160 °C
Quartz Spinel Cristobalite Microcline Albite Anorthite Hematite Enstatite
++++ (+) (+) +++ ++++ + + −
++++ (+) (+) ++ ++ + + −
+++ (+) ++ + − − ++ +
+++ (+) ++ − − − ++ +
Legend: ++++ (N20%); +++ (N15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
Mineral phase
MP 830 °C
MP 975 °C
MP 1080 °C
MP 1160 °C
Quartz Cristobalite Microcline Albite Anorthite Hematite Diopside Gehlenite Enstatite Wollastonite
+++ − + ++++ ++ ++ (+) (+) (+) (+)
+++ − + +++ +++ ++ (+) (+) (+) (+)
+++ − + − ++++ ++ + (+) + (+)
++ ++ + − ++++ ++ + (+) ++ (+)
Legend: ++++ (N20%); +++ (N 15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
F. Pardo et al. / Applied Clay Science 51 (2011) 147–150 Table 5 Mineralogical transformations of L clays during heating at 830–975–1080–1160 °C. Mineral phase
L 830 °C
L 975 °C
L 1080 °C
L 1160 °C
Quartz Mullite Microcline
++++ − ++
++++ − ++
++++ − +
++++ (+) +
Legend: ++++ (N20%); +++ (N15%); ++ (N 10%); + (N 5%); (+) present (b5%); and − not present.
After firing to the highest temperature (1160 °C), the material became body amorphous with residual grains of quartz and neoformated hematite and other high temperature mineral phases (mainly quartz, cristobalite and hematite in SVTT, anorthite, albite, quartz and spinel in LC, quartz and anorthite in LH, quartz, enstatite and hematite in MP and quartz with microcline in L clays). Quartz was a residual phase, i.e. a component of the raw clay that did not suffer chemical transformations during the firing stage. On the contrary, plagioclase, hematite, mullite and other silicates were formed during firing. Mullite was formed from metakaolinite.Variables affecting mullite formation include kaolinite content and behaviour during heating, possible occurrence of melted phases and impurities like Fe2O3. The presence of mullite was only observed in the LC and L series (Tables 2 and 5). In the MP and LH series, mullite was not present because of the high content of CaO (6–7%), decomposed clay minerals could combine with CaO, yielding anorthite instead of mullite (Trindade et al., 2009). 3.2. Bending strength The bending strength measured at 1100 °C is shown in Table 6. The high level of deviation (Kim and Lee, 2007; Lorici and Brusa, 1991), made it necessary to carry out a high number of determinations, between 25 and 50 for each test. The discrepancies were mainly due to errors that depend on the dimensions of the piece, on existent surface micro-fissures, as well as other experimental errors. Bending strength for the LC and MP could not be measured due to the curved geometry of ceramic bodies obtained after heating. Mechanical properties are closely related to the structure of the mullite network. The process itself influences the kinetic of structural transformation and particularly the density of the compacted powder and the firing cycle. In calcareous clays, the bending strength depends on the content of crystalline phases and is directly related to the content of calcium carbonate and calcium silico-aluminates (Jordán et al., 2008). In general, the presence of crystalline phases in the ceramic matrix provides the fired piece with a high mechanical strength. The water absorption reduction (Meseguer et al., 2010) is linked to the increase in the bending strength of the clay matrix. The values of the SVTT and LH samples comply with the requirements of the European Standard EN 14411:2004, which proposes bending strength values, as determined by the European Standard EN ISO 10545-4, N15 MPa and N20 MPa for covering and pavement ceramic tiles, respectively. The highest bending strength of bodies fired at 1100 °C was observed in the LH series (97 MPa). This value is not usual for conventional ceramic products, so it is probably related to the
Table 6 Bending strength σ(MPa) of tiles vs. firing temperature for ceramic tile bodies. Sample
Sintered 1100 °C
SVTT LC LH MP L
31 ± 5 – 97 ± 12 – 4 ± 0.5
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simultaneous presence of anorthite/dipside and the low porosity of the ceramic matrix. However, the L series showed a clear refractory behaviour with bending strength values b5 MPa at 1100 °C. The results for the L industrial ceramic bodies showed no possibility of complying with the technical specifications, even at temperatures as high as 1100 °C. The reason is the intrinsic refractory nature of these kaolinitic clays. It is not possible to comply with the norms for floor tiles by exclusively using raw materials in which the porosity cannot be reduced to adequate values. Even for long firing cycles, the refractoriness of kaolinite clays hinders the closing of pores up to the maximum temperatures that can be used by the industrial furnaces.
4. Conclusions This study of the mineralogical transformation induced by firing potential ceramic raw materials available in the IV and VI Regions of Chile may contribute to developing the ceramic industry in these regions of this country, focusing on the manufacture of conventional ceramic products with possible applications as tiles (pavements and coverings in construction). The firing behaviour properties and the bending strength of the ceramic bodies of the Chilean clays indicated that the clays can be used in floor tile fabrication. The study of mineralogical transformations shows the presence of i) quartz, spinel, cristobalite, microcline, albite, anorthite, hematite and enstatite for the SVTT samples; ii) quartz, mullite, spinel, microcline, albite, anorthite, hematite, diopside, enstatite, illite/muscovite and talc for the LC samples; iii) quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, illite and augite for the LH samples; iv) quartz, cristobalite, microcline, albite, anorthite, hematite, diopside, gehlenite, enstatite for the MP samples and v) quartz, microcline and mullite for the L samples. In view of the bending strength values obtained, we can state that only the SVTT and LH series can be situated within the required specifications for earthenware. The bending strength values for the LC and MP series were not valid due to the curved test probes at this temperature. The L series have a clear refractory behaviour with bending strength values b5 MPa at 1100 °C. Actually, this is not only a technical problem but also a scientific verification. The highest temperatures reached by the local industrial furnaces are still not enough to promote the sintering mechanisms for sufficient closure of pores in kaolinitic clayey ceramics. A substantial reformulation of the body composition that could include larger amounts of alkaline oxides and lower loss on ignition could be a proper solution.
Acknowledgements The authors would like to express their gratitude to the Generalitat Valenciana (Project code: AE07/12) and Prof. G. Lagaly, editor of Applied Clay Science, for help in the discussion and additional comments.
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