Microstructural Evaluation and Properties of a Ceramic Body

ISSN 1517-7076 

Revista Matéria, v. 10, n. 4, pp. 526 – 536, 2005 

http://www.materia.coppe.ufrj.br/sarra/artigos/artigo10689 

Microstructural Evaluation and Properties of a Ceramic Body for 

Extruded Floor Tile 

C. M. F. Vieira 1 , P. R. N. da Silva 2 , F. T. da Silva 3 , J. L. Capitaneo 3 , S. N. Monteiro 1 

1 

State University of the North Fluminense - UENF Advanced Materials Laboratory – LAMAV 

e-mail: vieira@uenf.br, sergio.neves@ig.com.br 

2 

Chemical Science Laboratory - LCQUI Av. Alberto Lamego, 2000, Campos dos Goytacazes, RJ, 28013- 

602, Brazil 

e-mail: nagipe@uenf.br 

3 

Federal University of Rio de Janeiro / Program of Metallurgical and Materials Engineering - 

PEMM/COPPE/UFRJ 

e-mail: flatesi@metalmat.ufrj.br, jeff@metalmat.ufrj.br 

ABSTRACT 

Campos dos Goytacazes in the northern of the State of Rio de Janeiro, Brazil, has became a larger 

producer of red ceramic wall bricks. The industries are now interested in the fabrication of products with 

greater intrinsic value such as extruded floor tiles. However, the refractory behavior of the local clays 

presents some serious difficulties. Therefore, the objective of this paper was to evaluate the firing behavior, 

technological properties and the microstructure of a typical industrial extruded floor tile body from Campos 

dos Goytacazes, Brazil. The firing behavior was studied by thermal analysis (DTA/TG) while the 

microstructural characteristics were evaluated by mercury porosimetry, nitrogen adsorption/desorption, 

scanning electron microscopy and X-ray diffraction. Technological properties such as water absorption, 

linear shrinkage and flexural strength of specimens fired from 900 to 1200 o C were also evaluated. The results 

showed that the industrial ceramic body presents a high porosity, which does not attend floor tile 

specifications even at a firing temperature of 1200 o C. This is due to its kaolinite predominance and relatively 

high weight loss during firing, associated with a significant amount of clay mineral and aluminum hydroxide 

decomposition. 

Keywords: Clays, microstructure, properties, traditional ceramics. 

1 INTRODUCTION 

The required technical performance of conventional ceramic tiles is normally associated with their 

dimensional parameters, mechanical strength, water absorption, chemical stability and wear resistance. These 

are largely dependent on microstructural characteristics such as porosity, defects and second phases [1-4]. 

The final microstructural features of the ceramic are developed during processing and consolidated in the 

firing stage, in which physical and chemical reactions occur among the different constituents [5-11]. 

Ceramic tile bodies are normally composed of two groups of materials: plastic and non-plastic. The 

plastic material is predominantly based on clays and sometimes kaolin, which are essential to the 

development of plasticity as well as to reach satisfactory green and dry mechanical strength. The non-plastic 

part is associated with inert, fluxes and flux-inducing materials [12-15]. The correct mixture of these 

materials makes the products able to present the desired technological properties and allows them to be easily 

processed with the lowest possible cost. 

It is well known in the literature that kaolinitic clayey bodies are associated with refractory behavior 

during the firing stages [15-17]. This is due to their high amount of alumina and low percentage of alkaline 

oxides, which are responsible for sintering consolidation through liquid phase formation. As a consequence, 

kaolinitic-based clays cannot be used to produce ceramic tiles in a short time, usually less than one hour, 

firing cycle. Under this process condition ceramic tiles are normally formulated with illitic clays with elevate 

amount of flux materials to promote a fast and efficient sintering process by viscous-flow. On the other hand, 

extruded products processed by conventional firing cycles, i.e., from 20 to 72 hours, can accept a more 

refractory formulation comprising kaolinitic clays. In this case, both solid state sintering and vitrification 

Autor Responsável: Carlos Maurício F. Vieira Data de envio: 27/09/05 Data de aceite: 11/12/05

VIEIRA, C.M.F., da SILVA, P.R.N., da SILVA, F.T., CAPITANEO, J.L., MONTEIRO, S.N., 

Revista Matéria, v. 10, n. 4, pp. 526 – 536, 2005. 

mechanisms may be sufficient to attend the required quality. Based on this idea, one would expect that 

kaolinitic clays might be used for tile fabrication in conventional furnaces in which a large firing cycle would 

compensate for the absence of flux. 

In the county of Campos dos Goytacazes, northern of the Rio de Janeiro State, there is a small 

production of extruded floor tile obtained by conventional firing cycles of 72 hours at 900 o C in dome type 

furnaces. This product is considered of low quality and does not attend specifications. In principle, through 

the knowledge of the microstructural aspect and characteristics of the ceramic body, it was imagined that an 

eventual reformulation of the clayey body and changes in process conditions could improve the properties to 

the point of reaching floor tile specifications. 

In this context, the objective of the present work was to characterize an industrial extruded floor 

tiles body from Campos dos Goytacazes and to evaluate its firing behavior and microstructure, in terms of 

porosity and crystalline phases. The main purpose was to verify if changes in process conditions could 

improve the fired properties, such as mechanical strength, water absorption and linear shrinkage. 

2 MATERIALS AND METHODS 

The material employed in this work was a clayey body typically used in the fabrication of extruded 

floor tiles by industries located in the region of Campos dos Goytacazes. The formulation of this industrial 

body corresponds to 90 wt.% of three different types of kaolinitic clays and 10 wt.% of a quartzitic sand. 

The XRD of randomly oriented powder was carried out in a Seifert model URD 65, diffractometer, 

equipped with a graphite monochromator, operating with Cu-Kα radiation for a 2θ range from 5º to 40º. The 

chemical composition was obtained by fluorescence spectrometry on a Philips equipment, model PW 2400. 

The particle size distribution of the raw materials was performed by sieving using 20, 40, 60, 100 and 200 

mesh, and by sedimentation method according to the Brazilian standard NBR 7181 [18]. The workability of 

the industrial body was evaluated by determining its plasticity by the Atterberg limits [19, 20]. 

Rectangular 100 x 30 x 10 mm samples were fabricated using a non-vacuum laboratory extrusion 

apparatus. These samples were partially dried at room temperatures for five days, and then at 110 o C for 24 

hours in a stove. The dried samples were subsequently fired at temperatures varying from 900 to1200 o C in an 

electric muffle furnace with a heating rate of 10 o C/min and three hours soaking at the maximum temperature. 

Cooling occurred by natural convection after turning off the furnace. The fired technological properties 

evaluated were: water absorption, linear shrinkage and flexural strength. Three points bending tests were 

performed in an universal Instron 5582 machine according to standard procedure [21]. Thermo analytical 

techniques such as thermogravimetry (TGA) and thermodifferential (DTA), simultaneously conducted in a 

TA instrument model SDT 2960, were conduced in a clay powder sample (25 mg), screened at 40 mesh, 

operating under a 100 ml.min -1 flow of air and heating rate of 10 o C/min. 

Microstructural analysis of specimens fired at 1050 o C was performed by X-ray diffraction (XRD), 

mercury porosimetry, nitrogen adsorption and scanning electron microscopy (SEM). This temperature 

corresponds to the maximum possible to be reached by the major industrial furnace used by the local ceramic 

industries applying conventional firing cycles. The XRD of fired powder samples was carried out by the 

same procedure described previously. 

The pore-size distribution and surface area were determined by mercury intrusion porosimetry, 

using an Autoscan Quantachrome Porosimeter. To complement this porosity analysis, nitrogen 

adsorption/desorption isotherms were determined at 427 o C using an Autosorb apparatus. A comparison was 

made between surface areas measured by Hg porosimetry and N2 adsorption/desorption. The mesopore 

diameter distribution was calculated from the desorption branch of the N2 adsorption/desorption distribution 

curve using the BJH formula [22, 23]. 

SEM observation of gold platted specimens of the fired ceramic fracture surface was carried out in a 

model DSM 962 ZEISS equipment with secondary and backscattered (BSE) electron imaging. 

3 RESULTS AND DISCUSSION 

3.1 Characteristics of the Ceramic Body 

According to the XRD pattern in Figure 1, it is possible to identify the kaolinitic predominance of 

the industrial ceramic body. Others secondary peaks minerals are associated with quartz, muscovite mica and 

gibbsite. These results are in accordance with mineralogical composition of clayey materials from Campos 

dos Goytacazes [24]. The presence of gibbsite is considered undesirable for low porosity products, since the 

aluminum hydroxide contributes to increase both, the weight loss and the refractoriness by aluminum oxide 

formation during firing [25]. 

527

Intensity (Counts) 

600 

500 

400 

300 

200 

100 



M 

K 

Gi 

K Q 

K 

K 

KQK 

K 

0 

5 10 15 20 25 30 35 40 

Figure 1: X-ray diffraction pattern of the industrial body. Gi = gibbsite; K = kaolinite; M = muscovite mica; 

Q = quartz. 

Table 1 shows the chemical composition of the industrial body used in this work as well as another 

one from Spain. In comparison with the extruded floor tile body from Spain [16], the industrial body from 

Campos has a relatively low amount of SiO2, high amount of Al2O3 and low amount of alkaline oxides. 

These are typical characteristics of kaolinitic clays. The high loss on ignition (LoI) indicates that the clay 

fraction, or that of clay mineral with particle size < 2 μm, is very significant. 

Table 1: Chemical composition of the ceramic body (wt.%) 

Industrial Body SiO2 Al2O3 Fe2O3 TiO2 K2O Na2O CaO MgO LoI 

Campos 

(Brazil ) 

Estercuel-Crivillén 

(Spain) 

2θ 

K 

Q 

53.14 28.32 3.10 1.06 1.29 0.11 0.33 0.56 12.39 

58.80 23.30 3.60 0.95 3.19 0.11 0.95 - 7.9 

Figure 2 shows the particle size distribution of the industrial body. As seen in this figure, the 

ceramic body presents 48.1 wt.% of clay minerals, associated with particle size < 2 μm, which are essential 

for the development of plasticity. The silt fraction is related to particles with equivalent spherical diameter 

(φ) in the range of 2 and 60 μm. The sand fraction are associated with particle of φ>60 μm. Silt and sand 

particles, considered non-plastic materials, are present in the ceramic body in the amounts of 33.6 and 18.3%, 

respectively. 

528

Cumulative mass percent finer 

100 

90 

80 

70 

60 

50 

40 

Clay mineral fraction 

3.2 Extrusion Behavior 

1 



10 

Silt 

100 

Diameter (μm) 

Sand 

1000 

Figure 2: Particle size distribution of the industrial body. 

The forming by extrusion of a clayey body is a critical stage in the ceramic process. The body 

composition must present an adequate mixture of plastic and non-plastic materials to permit an elevate 

productivity associated with the minimum possible mechanical strength of the green pieces and a satisfactory 

drying stage [26]. All these are related to the plastic diagram shown in Figure 3, which represents an 

extrusion prognostic related to Atterberg limits [27]. In this figure it is observed that the industrial body 

investigated in this work is located in a region considered acceptable for extrusion. This result, corresponding 

to a relatively high plasticity index, also indicates that the industrial body presents an excessive amount of 

clay mineral. Therefore, an optimization of the extrusion stage could be attended by merely increasing the 

non-plastic materials. 

Figure 3: Extrusion prognostic through Atterberg limits [27]. (•) Location of the industrial body. 

3.3 Thermal Behavior 

Figure 4 shows the thermal analysis of the industrial body. The first endothermic peak 53.1 o C, in the 

DTA curve is related to the loss of hygroscopic water. The second endothermic peak, 268.8 o C, is associate 

with a decomposition of hydroxides, essentially gibbsite - Al(OH)3 and, eventually, amorphous iron 

hydroxide. Its corresponding weight loss was 1.96%. The third and deepest endothermic peak, 496.8 o C, is 

529



due to the dehydroxylation of the kaolinite [28]. Its weight loss of 8.52% confirms the elevated percentage of 

clay minerals. The last endothermic peak, at 570.1 o C, is associated with the allotropic transformation of 

quartz. The only observed exothermic peak at 973.9 o C is typical of the metakaolinite decomposition, 

forming Al-Si spinel, amorphous silica and mullite phases [29, 30]. The ATD/TG curves of the industrial 

body confirm the high weight loss, which is mainly due to the metakaolinite dehydroxylation. The water 

vapor released during the firing stage increases the porosity of the piece and contributes to decrease the 

mechanical strength as well as to elevate the shrinkage. 

3.4 Fired Properties 

Figure 4: Thermoanalysis curves (DTA/TG). 

The evolution of the linear shrinkage and water absorption as a function of the firing temperature is 

given in the gresification diagram shown in Figure 5. The corresponding curves exhibit a slight decrease in 

water absorption and an increase in linear shrinkage up to 1000 o C. It can be observed that above 1000 o C a 

sharp change occurs in the trend of the curves, indicating a significant decrease in porosity. This is due to 

sintering mechanism, essentially of viscous-flow type, that promotes the densification of the samples [31]. 

The sintering of a clay involves complex reactions governed by non-equilibrium conditions and characterized 

by the presence of different reacting sub-system [9]. The gresification diagram behavior of the industrial 

ceramic body is typical of kaolinitic material. Although the industrial body shows a sharp decrease in the 

porosity above 1000 o C, its vitrification temperature, i. e. when the water absorption reaches the zero value, 

was not obtained [15]. According to the Brazilian standard NBR 13818 for floor ceramic tiles [32], the 

maximum value of water absorption is 10%. It is important to notice that this recommended value is not 

reached at any of the investigated temperatures. 

530

Water absorption-WA (%) 

22 

20 

18 

16 

14 

12 

10 



WA 

LS 

900 950 1000 1050 1100 1150 1200 

Temperature ( o C) 

Figure 5: Gresification diagram of the industrial body. 

Figure 6 shows that the flexural strength of the ceramic body increases with the firing temperature. 

This behavior can be explained by the decreasing in porosity of the samples. The minimum value 

recommended by norm for application of ceramic tiles on floors is 18 MPa [32]. It can be seen that this 

minimum value was not reached at any of the investigated firing temperatures. Intrinsic flaw size is perhaps 

the predominant factor affecting the mechanical strength of a clayey ceramic [30]. In the case of a kaolinitic 

material fired at low temperature, the intrinsic flaws are basically the pores. The other strength-controlling 

factors in multiphase polycrystalline ceramics [30], such as thermal expansion coefficients of the phases, 

particle size, phase transformation, volume fraction and elastic properties of the phases, have secondary 

influence on the mechanical strength of the investigated industrial body due to its high porosity. 

Flexural strength (MPa) 

16 

15 

14 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

900 950 1000 1050 1100 1150 1200 

Temperature ( o C) 

Figure 6: Flexural strength of the industrial body as a function the firing temperature. 

3.5 Pore Size Distribution 

Figure 7 shows the pore size distribution, measured by N2 adsorption/desorption, of the industrial 

body fired at 1050 o C, in which a low amount of mesopores centered at 17x10 -4 μm is observed. The very 

8 

6 

4 

2 

0 

Linear shrinkage-LS (%) 

531



small calculated total pore volume of 1.26x10 -2 cm 3 /g reflects the considerably small BET specific surface 

area of 7.0 m 2 /g. This extremely low volume indicates that the vitrification process at 1050 o C was almost 

sufficient to completely fill the smaller pores of the ceramic body. 

Pore volume (cm 3 /g)x10 4 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

5 10 15 20 25 30 35 40 45 

Pore diameter x10 -4 (μm) 

Figure 7: Pore size distribution, by nitrogen adsorption, of the industrial body fired at 1050 o C. 

In Figure 8 it is shown the pore size distribution obtained by mercury porosimetry. In this technique, 

the pore sizes that can be evaluated by the equipment comprise values higher than those obtained with the N2 

adsorption/desorption technique. A bimodal distribution with most pore sizes between 0.13 and 2.70 μm is 

observed. 

intruded mercury volume x 10 3 (cm 3 /g) 

12 

10 

8 

6 

4 

2 

mesopores 

0 

0.001 

0.01 

0.1 1 

Pore diameter (μm) 

Figure 8: Pore size distribution, by mercury porosimetry, of the industrial body fired at 1050 o C. 

It is worth mentioning that the distribution curve in Figure 7 is in a much-expanded scale than that 

in Figure 8. If properly located in Figure 8, the curve from Figure 7 would correspond to a very small bump 

with 0.16 units of height, extending from the beginning of the pore diameter scale (0.001 μm) to the marked 

dashed line (0.005 μm). Therefore, according to the present porosity analysis, the amount of mesopores is 

10 

100 

532



insignificant as compared to the larger pores determined by Hg intrusion. This is probably a consequence of 

both the vitrification process that preferentially closes the small pores [2, 4] and the packing degree of the 

body. A comparison was made between surface areas measured by both techniques. In the Hg porosimetry, 

the range of calculated surface area was 1–7 m 2 /g. The calculated upper limit of 7 m 2 /g coincides with the 

BET specific surface area obtained for the mesopores in Figure 7. This is an indication that the two methods 

overlap in terms of detected pores. Consequently no additional porosity might be expected to exist in the 

fired ceramic. Even though the volume of mesopores is negligible when compared to that of the larger pores 

in Figure 8, their surface area predominates in the fired ceramic. However, from a technical point of view, the 

maximum value of 7 m 2 /g is still too small to have any significant influence in the behavior of the material. 

3.6 Crystalline Phases 

Figures 9 shows the XRD pattern of the ceramic body fired at 1050 o C. It can be observed that the 

crystalline phases identified by corresponding peaks are quartz (predominant), plagioclase feldspar, hematite 

and mullite. Quartz is a residual phase, i.e. a component of the natural clay that did not suffer chemical 

transformations during the firing stage. On the contrary, plagioclase feldspar, hematite and mullite were 

formed during the firing. Hematite was probably formed after the loss of hydroxyl groups (OH - ) from iron 

hydroxides at temperatures above 268 o C. Mullite was formed from the metakaolinite. These results are in 

agreement with other works reported in the literature evaluating the phase evolution with firing temperature 

of the kaolinitic material from Campos dos Goytacazes [33, 34]. 

3.7 Microstructure 

Intensity (a.u.) 

Q 

Pl 

Q 

Pl 

Mu 

H 

Mu 

Mu Q 

Q 

Q 

H 

Mu 

10 20 30 40 50 60 

Figure 9: XDR patterns of the industrial body fired at 1050 o C. 

Figure 10 shows the microstructure of the industrial body fired at 1050 o C with different 

magnifications. The BSE image, Figure 10(a), demonstrates the presence of large cracks and a significantly 

rough texture. These large cracks are probably due to the stress generated during the extrusion process. It is 

also observed that the fracture appearance is typically intergranular. Figure 10(b) shows a poorly compacted 

microstructure, with the presence of individual plates, indicating the existence of region with connected 

porosity. These are characteristics associated with weak consolidation due to a low development of liquid 

phase, which is typical of kaolinitic clays. 

2θ 

Q 

Q 

Q 

Q 

Q 

533



Figure 10: SEM micrographs of the fracture surface of the industrial body fired at 1050 o C. 

(a) 100x (b) 2000x. 

3.8 Final Remarks 

The results obtained for the characteristics and properties of an industrial ceramic body that is 

currently being used to fabricate extruded floor tiles in Campos dos Goytacazes, showed no possibility of 

attending the technical specifications, even at temperatures as high as 1200 o C. The reason is the intrinsic 

refractory nature of the kaolinitic clays, which has a typical occurrence in that region. In other words, 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 time 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 in 

Campos dos Goytacazes. 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. This is a novelty since the general 

thought was that kaolinitic clays could be consolidated to attend the floor tile specifications at temperatures 

no greater than 1200 o C. The high porosity of the industrial ceramic body composed mainly of kaolinitic clays 

and its consequent elevated water absorption and low mechanical strength are, therefore, problems difficult to 

overcome in terms of the floor tile specifications. The porosity can be decreased, however, without detriment 

to the plasticity/workability of the body, by increasing the flux content. A substantial reformulation of the 

body composition that could include larger amounts of alkaline oxides and lower loss on ignition should be a 

proper solution. 

4 CONCLUSIONS 

An investigation on the characteristics, firing behavior, technological properties and microstructure 

aspects of a ceramic body from Campos dos Goytacazes, Brazil, used in the fabrication of extruded floor 

tiles, displayed relevant results that can help to improve the ceramic processing and the quality of the 

product. 

The high amount of clay minerals existing in the local kaolinitic clays was responsible for the 

industrial extruded ceramic body to stay outside the optimal extrusion condition. The predominance of 

kaolinite and the presence of hydroxides were the main characteristics responsible for the refractory firing 

behavior and elevated loss on ignition. Consequently, a high porosity still exists even at temperatures as high 

as 1200oC. The microstructure of the ceramic body fired at 1050oC, which is the maximum temperature that 

could be used by the local industries due to limitations in the quality of their furnaces’ refractory bricks, 

showed a typical intergranular fracture, associated with the presence of large cracks. Therefore it is not 

possible to attain the specifications required for floor tiles by simply using local clays. To reach the required 

values for the technological properties, one should significantly decrease the refractoriness of the ceramic 

body by flux addition and/or reduce the loss on ignition. 

534

5 ACKNOWLEDGMENT 



The authors would like to thank the financial support to this investigation provided by the following 

Brazilian agencies: CNPq, CAPES, FENORTE/TECNORTE and FAPERJ. 

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