Incorporation of Granite Waste into Vitrified Ceramic Tiles

Incorporation of Granite Waste into Vitrified Ceramic TilesC. M. F. Vieira, S. N. Monteiro
State University of the North Fluminense Darcy Ribeiro-UENF Advanced Materials Laboratory-LAMAV
Av. Alberto Lamego 2000, 28013-602, Campos dos Goytacazes-RJ, Brazil *[email protected]
Keywords: Firing; Porosity; Granite waste; Traditional Ceramics. Abstract: The incorporation of a granite waste into a vitrified ceramic tile body was investigated. The granite waste came from an industry of stone sawing operations located in the municipal area of Santo Antônio de Pádua, State of Rio de Janeiro, Brazil. The investigated compositions were elaborated substituting partially and totally the feldspar by the granite waste. Cylindrical specimen were prepared by uniaxial pressure at 30 MPa and then fired at temperatures varying from 1050 to 1200oC. Fired properties such as bulk density, linear shrinkage, water absorption and diametrical compression were determined. The microstructure of fired samples was evaluated by scanning electron microscopy. The results showed that the replacement of feldspar by granite waste promoted a better vitrification, decreasing the open porosity and increasing the mechanical strength.
Introduction
Raw materials used in the composition of vitrified ceramic tiles comprise materials considered as plastics, inerts and fluxes. The last one are materials with a high amount of alkaline oxides, mainly K2O and Na2O, which, in reaction with silica and alumina, promote liquid phase formation that facilitates the densification. The liquid phase surrounds the solid particles and, by surface tension, enables particles to approach and then close the porosity [1- 2]. The most common flux employed in vitrified ceramic tiles is the feldspar [3,4], however, other materials could serve for this purpose. The powder waste from sawing operations of ornamental rocks, such as granite, is a potential flux source that presents some additional favourable circumstances. Its relatively low or practically no cost can be an economic advantage to the ceramic sector. The incorporation of wastes from several industrial activities in clayey ceramic products is one technological alternative to reduce the environmental impact caused by indiscriminate disposal. Furthermore, the use of wastes in ceramic processes allows for the saving of raw materials.
Granite is considered a flux material due to its large amount of alkaline oxides. These oxides derive from feldspars and micaceous minerals that are common constituents of granitic rocks. Previous works have utilized granite waste from sawing operations in the production of ceramic tiles [5-8]. The major observations were that the granite waste shows physical and mineralogical characteristics similar to those of raw materials used in the body composition and that the technological properties of some waste/clay mixtures fulfil the required tile properties. Moreover, it was found that the incorporation of granite waste in ceramic tile body could improve the sintering process and reduce the water absorption.
The objective of the present work was then to investigate the substitution of feldspar, in a vitrified ceramic tile body, for a granite waste from sawing operations. The idea was to have the granite waste acting as a flux material to improve the characteristics of vitrified ceramic tiles. Experimental Procedure
The granite waste came from an industrial facility of stone sawing operations located in the municipal area of Santo Antônio de Pádua, State of Rio de Janeiro, Brazil. This waste was previously characterized [9] and found to present biotite, albite, anortite, hornblend and quartz as main mineral phases. Its particles size varies approximately from 1 to 300 µm, with a frequency maximum corresponding to 8-10 µm. The amount of alkaline oxides, K2O + Na2O was found to be 8.11 wt.%.
The clay used in the present investigation was collected from the municipal areas of Silva Jardim also located in the State of Rio de Janeiro. The other raw materials, such as kaolin, quartz, potash feldspar and talc were obtained from mining facilities outside the State of Rio de Janeiro. Table 1 shows the chemical composition of the raw materials used in this work. In comparison with the feldspar, the granite waste presents practically the same amount of SiO2 but comparatively lower amounts of Al2O3 and K2O + Na2O. A negative chemical aspect of the granite waste is its relatively higher amount of Fe2O3, 4.40 wt.%, that will darken the fired body. In many cases, whiteware ceramics are required to have light colors due to market interests. Table 1. Chemical composition of the raw materials (wt. %).
Clay Kaolin Quartz Feldspar Talc Granite waste SiO2 52.10 44.2 99.66 66.8 62.0 67.14 Al2O3 30.30 40.2 0.15 18.3 0.01 14.92 Fe2O3 1.76 0.34 0.04 0.1 0.07 4.40 TiO2 1.25 0.5 0.01 0.01 0.01 0.73 MgO 0.30 0.09 - 0.1 32.3 0.73 CaO 0.13 - - - 0.01 1.91 K2O 3.34 0.41 - 8.13 0.01 5.18 Na2O 0.30 0.38 - 3.7 0.01 2.93 LoI 10.20 15.0 0.25 0.01 5.0 0.50
Three experimental body compositions A, B and C were prepared by substituting the feldspar for granite waste while maintaining constant the amount of the other materials. These raw materials were thoroughly mixed according to the proportions shown in Table 2. It should be noticed in this table that the feldspar was partially or totally substituted for the granite waste. Table 2. Prepared compositions (wt. %).
Raw materials Compositions Clay kaolin quartz Talc Feldspar Granite waste
A 30 15 5 5 45 - B 30 15 5 5 22.5 22.5 C 30 15 5 5 - 45
The prepared experimental bodies were initially wet-milled in porcelain pots during two hours. The resulting slurries were then sieved through 325 mesh (44 µm) and allowed to dry at 110oC before a final sieving through 20 mesh (820 µm). Cilyndrical specimens with a diameter of 20 mm were molded under uniaxial pressure at 30 MPa. Specimens initially containing 8% of moisture were dried at 110oC until a constant weight was achieved and then fired at temperatures in the range of 1050 to 1200oC. A controlled heating rate of 10oC/min was maintained up to the desired temperature, at which each specimen was left for 6 minutes. Cooling occurred by natural convection after turning the furnace off and leaving the specimen inside. The evaluated technological properties were: bulk density, water absorption, firing linear shrinkage and diametrical compression. The bulk density of both dried and fired specimen was determined by the dimensional method dividing the mass by the external volume. The water absorption was determined according to standard procedure [10]. The linear shrinkage was obtained by by mesuring the diameter of the samples, before and after the firing stages, using a caliper with a precision of ± 0.01 mm. The diametrical compression was determined in a universal Instron 5582 machine, following the norm [11]. The microstructure of the fracture surface of selected fired samples was studied by scanning electron microscopy, SEM, using a Zeiss model DSM 962 equipment.
Results and Discussion Table 3 presents the dry bulk density of the elaborated compositions. It is observed that the three compositions have similar porosity. This fact guarantees that the microstructure evolution of the fired compositions was not influenced by differences in the packing degree of the compositions. Table 3. Dry bulk density of the compositions (g/cm3).
A B C 1.91 ± 0.02 1.93 ± 0.03 1.92 ± 0.01
The fired technological properties determined for the investigated compositions are
presented in Figs. 1 and 2. One should note that both the partial and the total substitution of feldspar for granite waste promote a better flux action at lower temperatures. This can be attributed to the lower amount of Al2O3, in the waste, which reduces the refractoriness of the standard composition A.
According to Fig. 1(a), the water absorption decreases with the firing temperature with a more pronounced fall from 1100 to 1150oC. This is probably due to the melting of the feldspar phases. It is also observed that the compositions with partial or total replacement of the feldspar by granite waste, B and C, display lower water absorption in comparison with the standard composition, A. At 1200oC all compositions have zero water absorption, indicating total vitrification.
Figure 1(b) presents the linear shrinkage of the compositions with the firing temperature. It is observed that the linear shrinkage of the three compositions increases from 1050 to 1150oC. The relatively higher linear shrinkage values of compositions B and C at 1100oC in comparison with that of the standard composition A, indicates that the granite waste has possibly a more efficient vitrification action than that of the feldspar. On the other hand, at 1200oC, the compositions with granite waste, B and C, present a decrease in the
linear shrinkage. This behavior can be attributed to the gas release, O2, from the Fe2O3 reduction.
1050 1100 1150 1200
2
4
6
8
A B C
Figure 1. Water absorption (a) and linear shrinkage (b) of the compositions as a function the firing temperature. Figure 2 presents the variation of the diametrical compression of the compositions with the firing temperature. It is observed an increase in the mechanical strength of the three compositions from 1050 to 1150oC. This is due to the well-known vitrification process [1] that eliminates the porosity and promotes microstructural consolidation among the particles. At lower temperatures the compositions with granite waste displays comparatively higher values of mechanical strength. At 1200oC, the reduction of the Fe2O3 probably caused an increase in close pores that will act as defects, decreasing the mechanical strength of the ceramics.
1050 1100 1150 1200
A B C
Figure 2. Diametrical compression of the compositions as a function the firing temperature. Figures 3 to 5 presents micrographs corresponding to the fractured surface of the compositions fired at 1150oC. All compositions display a relatively finer texture associated with some isolated defects such as pores and cracks. It is worth noticing in these figures that
the compositions with granite waste, B and C, present a fractured region, apparently more vitrified, which is in accordance with the evaluated properties.
Figure 3. SEM photomicrographs of the fractured region of the composition A fired at 1150oC.
Figure 4. SEM photomicrographs of the fractured region of the composition B fired at 1150oC.
Figure 5. SEM photomicrographs of the fractured region of the composition C fired at 1150oC.
Conclusions
In this work, feldspar was partially or totally replaced by a granite waste, obtained from sawing operations, in the production of vitrified ceramic tile bodies. It was observed that the granite waste presented a better sintering behavior, as compared with the feldspar, by reducing the vitrification temperature. The partial replacement of feldspar by granite waste promoted a decrease in the water absorption and an increase in the mechanical strength from 1050 to 1150oC. In the industrial production of ceramic tiles, in which the whitener of the fired product is not a required parameter, the substitution of feldspar for granite waste can be advantageous. Due to the relatively higher cost of feldspars, the ceramic tiles industries that are located near the regions of ornamental rocks activities can profit from this procedure. The incorporation of granite waste into ceramic products also represents an environmentally correct solution for the disposal to this type of waste.
Acknowledgements
The authors would like to express their thanks for the financial support provided by the FAPERJ, CNPq, Capes and Tecnorte/Fenorte.
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