Firing transformations of mixtures of clays containing illite, kaolinite and calcium carbonate used by ornamental tile industries

Applied Clay Science, 5 ( 1990 ) 361-375

361

Elsevier Science Publishers B.V., Amsterdam

Firing transformations of mixtures of clays containing illite, kaolinite and calcium carbonate used by ornamental tile industries F. Gonz~llez-Garcia, V. Romero-Acosta, G. Garcia-Ramos and M. Gonz~tlez-Rodriguez Department of lnorganic Chemistry, Faculty of Chemistry, Universityof Seville, 41012 Seville, Spain (Received March 3, 1990; accepted after revision July 12, 1990)

ABSTRACT Gonzfilez-Garcia, F., Romero-Acosta, V., Garcia-Ramos, G. and Gonz~ilez-Rodriguez, M., 1990. Firing transformations of mixtures of clays containing illite, kaolinite and calcium carbonate used by ornamental tile industries. Appl. Clay Sci., 5:361-375. In this work were studied the firing transformations undergone by mixtures of illitic-kaolinitic clays containing CaCOa, free quarts, iron oxides and other impurities, with other kaolinitic and illitic clays also including some impurities but no CaCO3. Such mixtures yielded no mullite on heating from room temperature to 950 and 1100 ° C. This process involves the formation of calcium silicates such as wollastonite in medium to high proportions or calcium aluminosilicates such as gehlenite and anorthitic plagioclases. The proportion in which the last were obtained increased considerably with the firing temperature; on the other hand, the proportion of wollastonite only increased moderately and that of gehlenite decreased with increasing temperature. Some thermodynamic considerations on the formation of these crystalline phases and the absence of mullite were made. Such phases influence the technical properties of the fired products.

INTRODUCTION

The ornamental tile industry has a long tradition in Spain, and in Seville in particular, where it was started in the middle of the 13th or beginning of the 14th century; it reached its height in the 16th century and is still preserved and in continuous search for new manufacturing methods and procedures. A major problem facing tile manufacturers lies in selecting suitable raw materials for the making of pastes ensuring the high quality demanded from the fired pieces (e.g. moderate contraction on drying and firing, appropriate porosity for perfect adhesion of the enamel, an adequate expansion coefficient, a high resistance to bending, etc. ). The blue marls from the Tertiary (Miocene) extracted from the Guadalquivir valley near Seville (SW Spain ) are usual ingredients of such pastes. 0169-1317/90/$03.50

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F GONZALEZ-GARCiA ET AL.

These marls typically contain illite, kaolinite, some smectites, free quartz, iron oxides, sparse feldspars and large amounts of CaCO3. Their high plasticity and contraction on drying can be reduced by mixing with other sandier, kaolinitic or illitic materials from the Quaternary or the Palaeozoic also occurring in the area. The composition of these mixtures, which has so far been established only empirically, can be determined more accurately by physicochemical studies of the starting materials and their fired ware, and the transformatioris involved in the firing process. Mixtures of clays and CaCO3 have been found to form calcium silicates and aluminosilicates such as wollastonite (fl-CaSiO3), larnite (Ca2SiO4), gehlenite (Ca2A12SiOT) and anorthitic plagioclases (CaAI2Si208). By using DTA, Grim et al. ( 1945 ) found gehlenite to be formed in the reaction between kaolinite and CaCO3. This process was later studied by Mackenzie and Rahmann (1987) and Mackenzie et al. (1988a,b) by applying DTA, TGA and XRD to kaolinite-calcite mixtures; they found the endothermal peak due to the decomposition of CaCO3 to split into two or three, and o~'-Ca~SiO4, gehlenite and probably anorthitic plagioclases to be formed at temperatures slightly higher than that of the beginning of the exothermal peak of kaolinite. The formation ofgehlenite and the plagioclases was also observed by Alarc6n et al. ( 1978 ) and Barahona et al. ( 1985 ) on heating natural calcareous clays, and by Capel et al. (1985) and Gonz~ilez-Vilches et al. (1985) on heating archaeological ceramic pieces containing CaCO3 and free quartz. Gonz~ilez-Garcia et al. ( 1988 ) found clays from the Guadalquivir valley containing illite, kaolinite, smectite, CaCO3, free quartz and iron oxides to form wollastonite, larnite, gehlenite and calcium plagioclases on heating at 950, 1020 and 1100°C. One of the clays used in this work was sample 21. These authors (1990) also found clays from the same area containing 3050% CaCO3 to form wollastonite, gehlenite and anorthitic plagioclases from 800 ° C onwards. This paper reports a study of the firing transformations undergone by mixtures of blue marls from the Tertiary (Miocene), represented by sample 21, with other kaolinitic or illitic clays from the Quaternary and the Palaeozoic in the Guadalquivir valley aimed at determining their utility as raw materials for manufacturing ornamental tiles, as well as the influence of the transformations involved on the properties of the fired ware. EXPERIMENTAL

Materials All the clays studies were sedimentary materials from the Guadalquivir valley near Seville (SW Spain). Some of them are commonly used in manufacturing ornamental tiles.

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Sample 21 was a blue marl from the Tertiary (Miocene), extracted from a deposit in Santiponce, 10 km north of Seville. Its properties and firing transformations were studied by Gonz~ilez-Garcia et al. (1988), from which the data used here were obtained. It consisted of illite, kaolinite, some smectite, free quartz, iron oxides, sparse feldspars and 25% CaCO3. Sample 22 was extracted from a deposit of the Early Quaternary in Dos Hermanas, about 8 km southwest of Seville. It contained abundant illite, sparse kaolinite, abundant free quartz and hydrated iron oxides, but no CaCO3. Sample 26 was a reddish sandy-clayey material from the Early Quaternary in the Guadalquivir valley, extracted from a deposit about 3 km south of Seville. It contained abundant kaolinite, some illite, sparse smectite, free quartz and hydrated iron oxides, but no CaCO3. Sample 31 was a highly weathered brown shale from the Paleozoic extracted from a deposit in Villanueva del Rio, about 35 km NE of Seville. It contained abundant kaolinite, an intermediate proportion ofillite, sparse free quartz and hydrated iron oxides, but no CaCO3. Sample 34 was extracted from a permic-carboniferous Palaeozoic deposit in the banks of the Viar River, about 25 km northeast of Seville. It contained abundant illite, sparse kaolinite, free quartz and hydrated iron oxides, but no CaCO3. The samples were ground manually with a roller and sifted to a particle size less than 2 mm. They were again sifted in an aqueous medium to < 0.12 mm. From the sifted materials were prepared samples M-l, M-3, M-5 and M-7, which contained 70% of sample 21 and 30% of samples 22, 26, 31 and 34, respectively. Finally, fractions from samples 22 to 34 were also divided to < 1.12/tm for mineralogical study. Techniques Each sample and its fractions were analysed by the Jakob method (1944), except for alkali metals, which were determined by emission spectrophotometry on the extracts resulting from attacking the samples by the conventional three-acid method according to Bennet et al. ( 1962 ). The extracts were also occasionally used to determine iron, aluminium, titanium, magnesium and calcium by atomic absorption spectrometry. The DTA recordings of the above fractions were obtained on an Aminco 24442 instrument at a heating rate of 16 ° C/min. Thermal treatments were carded out in 50-mm wide, 5-mm thick cylinders prepared from the <0.12 m m materials and cast at a pressure of 47.5 MPa. The cylinders were heated in independent series in a muffle furnace under a static air atmosphere from room temperature to 950, 1000 and 1100°C, after which they were kept at the final temperature for I h. One cylinder per sample or mixture per temperature was put aside for subsequent XRD analysis and

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study of physical properties. The heating rate was 10°C/min; cooling was effected at 10 oC / m i n from the highest temperatures (900, 1000 or I 100 ° C ) to 525°C, and then to room temperature in an uncontrolled, free fashion in the closed muffle furnace. The different cylinders were subjected to powder X R D analysis by using CuK~ radiation at 35 kV and 20 mA, once cold. For the fractions of < 1.12 /tm, patterns were also obtained for oriented aggregates, samples solvated with glycerol and samples heated at 550°C. The compressive strength, expressed in MPa, was measured on 14 X 1 × 1 cm 3 prismatic bars prepared from the materials of < 0.12 m m by firing at 950 and 1100 oC, which were positioned between supports lying 10 cm apart. The linear firing shrinkage was obtained by directly measuring the diameter of the above-mentioned cylinders fired at 950 and 1100°C, and was expressed as a percentage of the original diameter. The porosity of the fired pieces was measured as the water-absorption capacity on the same cylinders by immersing and boiling them in distilled water, and was expressed as a percentage of the weight (mass/mass) of the dry fired cylinder. Finally, a dilatometric study was carried out on 76.3-mm long, 12.7-mm wide cylinders prepared from the materials of < 0.12 m m by using a horizontal-expansion dilatometer between 20 and 1000°C at a heating rate of 6 ° C / m i n . RESULTS AND DISCUSSION

Composition of the samples and clay mixtures Table 1 lists the chemical compositions of the fractions of < 0.12 m m for samples 22 to 34, analysed here, in addition to that of sample 21, taken from Gonz~ilez-Garcia et al. ( 1988 ), and those of mixtures M-1 to M-7, calculated from the previous ones. The high silica contents of samples 22 to 34 prompt the occurrence of free silica. This finding was confirmed by X R D data. The alumina content was somewhat low in samples 22 and 34, and higher in samples 26 and 31. The X R D patterns of the first two showed both the quartz diffractions and those of illite (strong) and kaolinite, plagioclases and hematite (weak), but none of calcite. On the other hand, samples 26 and 31 yielded strong diffractions of kaolinite, accompanied by others of m e d i u m intensity of illite and even some weak ones of smectite in sample 26. Neither yielded calcite diffractions. In summary samples 22 and 34 were basically illitic, while samples 26 and 31 were rather kaolinitic, though none contained CaCO3. These conclusions are consistent with the DTA recordings (Fig. 1 ). The fractions of < 1.12/tm of samples 22 to 34 from which organic matter had been previously removed showed endothermal effects at 150 and 550 °C which can be attributed to the presence of altered illites. The sharper, stronger effect of sample 26 at 550°C and its exothermal effect at 925°C confirm the

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TABLE I

Chemical composition of the < 0.12-mm materials from clay samples 21 to 34 and calculated chemical compositions of mixtures M-l, M-3, M-5 and M-7 Substance

Samples: 21

LS (%) SiO2 (%) AI203 (%) Fe203 (%) TiO2 (%) CaO (%) MgO (%) Na20 (%) K20 (%)

Total Carbonates (as % CaCO3 ) SIO2/A1203 (mol) CaO/SiOz (tool) CaO/AI203 (mol) CaO/(SiO2+AI:O3) (mol)

11.00 48.50 13.30 4.34 0.74 13.92 2.93 0.60 2.22 100.55 25.20 3.23 0.308 1.904 0.269

22

26

31

34

3.45 74.27 12.94 3.76 1.09 0.83 0.72 0.60 2.28 99.94

8.05 55.39 23.70 7.05 0.65 1.26 1.26 0.46 2.11 99.93

6.30 51.76 25.14 8.34 0.82 1.67 1.95 1.19 2.75 99.92

M-1

M-3

M-5

M-7

5.32 10.78 65.45 56.03 15.22 13.13 6.59 4.14 1.01 0.84 1.56 9.94 0.59 2.26 0.32 0.60 3.90 2.24 99.96 99.96

12.16 50.36 16.36 5.13 0.71 10.07 2.43 0.56 2.18 99.96

11.63 49.28 16.79 5.52 0.76 10.19 2.63 0.78 2.38 99.96

11.34 53.38 13.82 4.95 0.82 10.15 2.22 0.52 2.72 99.96

17.64

17.64

17.64

17.64

7.28 0.189 1.375 0.162

5.23 0.214 1.119 0.180

5.00 6.57 0.221 0.203 1..103 1.385 0.184 0.172

LS = ignition loss.

chiefly kaolinitic nature of this material. The recording of sample 31 is consistent with an illitic material, with the exception of the exothermal effect starting at 925°C, which is typical of kaolinite. As stated above, the X R D data are clearly indicative of the occurrence of kaolinite in this material. The recordings of the < 0.12-mm fractions from the same samples were more complex and prevented the obtainment of a good DTA recording as a result of their coarseness and heterogeneity; nonetheless, sample 26 was clearly kaolinitic in nature according to its DTA recording. Both types of fraction showed small endothermal effects at about 340°C, which correspond to hydrated iron oxides, and a few other weak endothermal effects at about 670°C that could be ascribed to smectites - however, these were only detected in sample 26 by XRD. From the X-ray diffraction intensity of each mineral the mineralogical composition of each sample fraction was estimated by using reported intensity factors (Schultz, 1964; Martin et al., 1968; Gahin et al., 1982 ). The < 1.12-gin fractions of samples 22 and 34 contained roughly 75-80% iUite, 5-7% kaolinite, 9% and 6%, respectively, of Fe203 and 5% quartz. Those of samples 26 and 31 contained 24 and 29% ofillite, 57 and 50% of kaolinite,

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Approximate mineralogical composition (%) of the < 0.12-mm materials from the clay samples and mixtures studied Samples

I (%)

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F%O3 (%)

C (%)

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10 7 45 45 5 8 20 20 8

8 8 5 8 5 5

15 27 12 9 15 18 14 13 15

5 7 6 6 9 5 6 5 6

4 4 7 8 6 5 5 6 5

25 17.6 17.6 17.6 17.6

I = ill ire: K = kaolinite; S = smectite; Q = quartz; FP = feldspar plagioclases; C = calcite.

and 8 and 14% of Fe203, respectively, plus 10% smectite (sample 26 ) or 7% quartz (sample 31 ). Table II lists the mineralogical composition of the < 0.12-mm materials of greatest interest to this work on account of their similarity to those used by the ornamental tile industry and their use in studying the firing transformations and physical properties of the fired ware. The table also lists the mineralogical composition of the analogue fraction of sample 21 obtained from the

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XRD data reported by Gonz~ilez-Garcia et al. (1988), as well as those of mixtures M-1 to M-7, which were determined from those of their constituent clays. We should emphasize the high proportion ofillite in samples 21,22 and 34, the high kaolinite content of 26 and 31, the significant proportion of free quartz in all the fractions, the scarce occurrence of smectites and the appreciable proportion of feldspars and Fe203 in all of them, and the high CaCO3 content of sample 21. This last compound also occurred in samples M-1 to M-7, in which sample 21 was the major component. As far as mixtures M-I to M-7 are concerned, it is worth noting their high illite contents, particularly those of M-1 and M-7, and the significant proportion of kaolinite in M-3 and M-5, the high free-quartz contents of all four and their calcareous nature, a major feature of their main component (sample 21).

Firing transformations The chief aim of this work was to gather knowledge about the transformations undergone by mixtures of clays (M-1 to M-7 ) customarily used by the ornamental tile industry on firing. We shall first consider the transformations experienced by the constituent clays of the mixtures and then those undergone by these last. As ceramic pastes employed in making ornamental tiles are typically fired at temperatures above 950°C, the clay mixtures were heated up to final temperatures between 950 and 1100°C at a rate of 10°C/min. Heating samples 22, 34, 26 and 31 (th~ former and latter two being of a marked illitic and kaolinitic nature, respectively, and containing no CaCO3 in any case) resulted in neither calcium silicate, nor aluminosilicate, wollastonite or gehlenite formation. The cylinders of the four samples yielded spinel diffractions at 2.86, 2.43 (the latter somewhat more intense than the former) 2.02, 164 A, etc., above 1020 ° C. At this temperature also mullite diffractions were observed at 5.38, 5.40, 3.42, 3.38 A, etc., in all cases, the intensity of all these diffractions increased at 1100 ° C. While these findings do not refute the hypothesis that the formation of mullite involves that of spinel in an intermediate step, they do not support it completely as both phases appear to be formed simultaneously, at least up to 1100°C. As stated in the Introduction, Gonz~ilez-Garcia et al. (1988) found that heating cylinders of sample 21 (illitic-kaolinitic and CaCO3-containing) at 950°C resulted in the formation of wollastonite, calcium plagioclases and gehlenite, the proportion of the first two increasing and that of the last decreasing at 1020 and 1100 ° C. Increasing temperatures also resulted in decreasing proportions of quartz and slightly increasing hematite contents. Heating cylinders prepared from <0.6-mm material of these samples

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(analogous to that used industrially) in an industrial kiln in a 40-h cycle including a 2-h treatment at 980 ° C gave rise to the formation of large amounts of wollastonite and calcium plagioclases and sparse gehlenite, iarnite and hematite. No mullite was formed in this assay or in the experiments referred to in the previous paragraph. Another assay carried out on an analogous < 0 . 6 - m m material but from sample 22 (illitic-kaolinitic and containing no CaCO3) revealed the formation of significant amounts of spinel phase together with smaller amounts of mullite hematite and some plagioclases - the latter two were already present, in a lower proportion, in the starting sample. These findings reflect the influence of the heating conditions and time and, specially, the presence or absence of CaCO3 on the transformations on thermal treatment of clays, particularly as regards the nature of the crystalline phases obtained. Figs. 2 and 3 show the X R D patterns of the cylinders prepared from the < 0.12-mm materials from M- 1, M-2, M-3 and M-7, the subject matter of this work, obtained after heating them at 950 and 1100°C and subsequently al-

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lowing them to cool to room temperature as described above. All the diffractions of calcite and clay minerals were absent at 950 ° C, and only some, very weak ones of illite were observed; no diffractions of calcium oxide were found; yet, the patterns included those of gehlenite and woUastonite (of medium to weak intensity), those of hematite (also weak) and, particularly, those of anorthitic plagioclases (of medium to strong intensity). Strong diffractions of quartz were also present. Heating at 1100 °C decreased the intensity of the quartz and gehlenite diffractions in all four mixtures and increased those of wollastonite moderately, those of calcium plagioclases markedly and those of hematite very slightly. A semi-quantitative estimation of these crystalline phases based on the diffraction intensities of each mineral and the intensity factors reported in the literature is shown in Fig. 4, together with that of sample 2 l, which was obtained from Gonz~ilez-Garcia et al. ( 1988 ). As can be seen, the transformations undergone by these mixtures on heating was strongly influenced by the nature of their major component, viz. sample 21. However, there were some differences such as the formation of lesser

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FIRING TRANSFORMA1TONS OF MIXTURES OF CLAYS

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view of the advances in this field and the difficulties involved in studying this type of system was reported by Fraser ( 1977 ). Nevertheless, the problem can be approached by studying the thermodynamic feasibility o f a series o f reactions potentially involved in the firing o f the materials through the variation o f the Gibbs standard free energy by assuming all the substances to have unity activities and solving their respective equations, A G ° = f ( T ) , and plotting their respective diagrams (Hougen et al., 1964). The equations used here were obtained from the data reported by Robie and W a l d b a u m ( 1968 ) for the reactions listed in Table III. The corresponding plots are shown in Fig. 5. Curves l to 4 in Fig. 5a correspond to the formation of larnite and wollastonite on reaction between silica and CaO or CaCO3; they were obtained by assuming the uptake o f 2 mol o f the latter two to make t h e m comparable. According to their position in the diagram, wollastonite was the most stable o f all u n d e r the conditions and at temperatures where the diagrams were obtained. On the other hand, curve 5 shows that, in the presence of enough silica, larnite is transformed into wollastonite, with a decrease in the free energy. TABLEIII Likelychemicalreactionsinvolvedin the formationof the crystallinephasesoccurringin the firingof claysor mixturesof clayscontainingCaCOa ( 1) 2CaO+ SiO2 = C a2SiO4 larnlte

(2) 2CaO+2SiO2= 2CaSiO3 wollastomte (3) 2CACO3+ SiO2= C.a2SiO4+ 2CO2 larnlte (4) 2CaC03+3Si02= 2CaSi03 +2C02 wollastomte (5) Ca2SiO4+SiO2= 2.CaSiO3 wollastomte (6) A1203+ SiO2+ 2CaO= Ca~A12SiO7 ge-hlenite ( 7 ) 2A1203+ 4SIO2+ 2CaO= 2CaSi2A1EOs anorthite (8) A12Oa+ SiO2+ 2CACO3= Ca2Al2SiO7+ 2CO2 ge-hlenite (9) 2A1203+ 4SiO2+ 2CaCO3= 2CaAl~Si2Os+ 2CO2 anorihite (10) 2Ai203+4/3 SIO2=2/3 Si~A16013 mu-llite ( 11 ) Si3AI30~1012H)2K+ 2CACO3+ Si02--C~e~l~l¢2nSiit?7+ KK~i~Oasr+ 2CO2+ H20 ( 12) 2Si3AI3Q~(OH) 2K + 2CACO3+ 4SIO2= 2CaSi2Al~Os+ 2KSi3A1Os+ 2CO2+ H20 flate anorthiie K fel~lspar ( 13) 2Si3A13Qj~(OH ) 2K+ SiO2= Si2A]6.O13+ 2KSi~A1Os+ 2H20 tlllte mull~te K feKispar (14) CazA12S.iO7+ A1203+ 3SIO2= 2CaSi2A.120s gehlenlte anorthfle

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FI RING TRANSFORMAITONS OF MIXTURES OF CLAYS

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gehlenite and anorthite, respectively, from illite (represented by moscovite here as no thermodynamic data for illite were available) and silica and CaCO3, with consumption of 2 mol of the last. Again, the free energy decrease was more marked in the formation of anorthite. On the other hand, curve and reaction 14 indicate that, under the theoretical conditions used and in the presence of enough silica and alumina, gehlenite could be transformed into anorthite, with a decrease in the free energy, which may contribute to the decrease in the proportion of gehlenite found on increasing the temperature of the real systems studied. The diagram includes curve 13, corresponding to the formation of mullite on reaction between 2 mol of illite and the required a m o u n t of silica; again, under the conditions used and in the presence of CaCO3, anorthite was much more stable than mullite, which in turn would be even less stable than gehlenite below about 1055 ° C.

Physical properties of the fired ware As far as the physical properties of the ware are concerned, the linear shrinkage of mixtures M-1 to M-7 on firing ranged between 12 and 13% (length per length) at 950°C and between 12 and 14% at 1100°C, while porosity, measured as the water-absorption capacity, varied in the ranges 8.610.6% and 8-11.5% ( m a s s / m a s s ) at 950 and 1100°C, respectively. All these values lie within the traditionally acceptable range for adequate quality and

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F. GONZALEZ-GARC|A ET AL.

porosity allowing proper enamelling. The compressive strength of the mixtures was 28.8 MPa at 950°C and 32.8 MPa at 1100°C, i.e. much higher than that of the cylinders prepared from the individual unmixed samples and thus ensuring a high quality in the fired ware. To our minds, such high compressive strengths are probably related to the high proportion of crystalline phases formed in the firing process and to their nature. Finally, while the cylinders of sample 22, pre-fired at 1000 ° C, showed marked expansion variations with temperature, and those of sample 21 (the major component of the samples studied) underwent an abrupt expansion change between 550 and 575 °C as a result of the transformation of t~ into fl quartz, the aforesaid mixtures, with the exception of M-l, which featured a high quartz content, yielded virtually linear expansion-temperature curves (Fig. 6 ). This means that the linear expansion coefficient must be virtually constant between 50 and 1100 ° C, which is of great interest to the enamelling of the pieces and the stability of the enamel-support system. CONCLUSIONS

Heating industrial illitic-kaolinitic clays containing various impurities such as iron oxides and quartz but n o C a C O 3 , results in the formation of mullite from 950 or 1100°C onwards depending on the heating period and cycle employed. Analogous clays including significant proportions of CaCO3 do not form mullite, but gehlenite, larnite and, especially, wollastonite and anorthitic plagioclases on heating above 950 ° C. The proportion of the last two crystalline phases increases and that of gehlenite decreases with increasing firing temperature. Mixtures of illitic-kaolinitic clays containing CaCO3 and other non-calcareous kaolinitic or illitic clays also form gehlenite, wollastonite and anorthitic plagioclases from 950°C onwards. The proportion of wollastonite in these mixtures increases with the temperature (only moderately in wollastonite and more substantially in the plagioclases), and it decreases in gehlenite. The proportion of anorthitic plagioclases is higher in the mixtures containing kaolinitic clays. The nature of the crystalline phases formed and their relationship with the chemical composition of the mixtures and the firing temperature can be accounted thermodynamically. Such mixtures yield fired ware with excellent physical properties for the ornamental tile industry.

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