THE EFFECT OF MAGNESITE ADDITION ON THE PROPERTIES AND PRODUCTION PROCESS OF LOW POROSITY PORCELAIN TYPE CERAMICS

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1 Presentation 13/01/2003 THE EFFECT OF MAGNESITE ADDITION ON THE PROPERTIES AND PRODUCTION PROCESS OF LOW POROSITY PORCELAIN TYPE CERAMICS C.A.Sikalidis (1), Th. Zampetakis (2), C. Dagilas (2) (1) Department of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece (2) Grecian Magnesite Research Center, Vassilica, Thessaloniki, Greece ABSTRACT The present paper focuses on the effect of magnesite addition on the properties of low porosity porcelain type ceramics. The addition of magnesite on a percentage of 2 to 4wt% reduces the peak temperature and also improves the properties of the final products by decreasing the porosity (water absorption) and increasing the mechanical strength. INTRODUCTION In order liquid phase sintering to be achieved in ceramic products many minerals are used as fluxes in a ceramic batch formula. Since a long time limestone was used as a cheap flux in the production of ceramics. Problems were arise when the thermodynamic and the kinetic of the thermal treatment of the ceramic products were such that didn't allow the complete removal of the CO2 produced by decomposition of the limestone. This problem was almost unsolved when limestone was used in the production of low porosity fast fired porcelain type ceramic tiles. The fact that CaCO3 decomposes above the temperature of 850o C and at the same time the thermal treatment that is usually followed for fast fired porcelain type ceramic tiles is around 30 minutes cycle with very short time at the peak temperature of 1180o C, result to the non reachable demand of low porosity (less than 0,5 wt% water absorption) of the product. Magnesia (MgO) that is created due to the thermal decomposition of magnesite rock (MgCO3) although is refractory by itself, reacts with SiO2 and other substances, as the K2O and Na2O, to give phases having low softening temperature. Consequently can act as a flux in ceramic masses and in ceramic glazes. Generally can react with clays, silica and feldspars to give a "glassy" phase bond, assisting and improving the liquid phase sintering. As a result the ceramics produced will gain decreased porosity and improved mechanical properties. MgCO3 decomposes at the temperature of 650o C, that is almost 200o C below the one of CaCO3, suggesting the use of magnesite rock in the production of low porosity fast fired porcelain type ceramic tiles. In the present study, the magnesite rock is directly introduced in the ceramic batch formula and its decomposition to MgO is a result of the thermal treatment of the ceramic product. The aim of this study is to qualify and quantify the technical parameters and presuppositions that are needed in order to be used the magnesite rock as raw material (flux) for the production of ceramic products with low porosity (water absorption 0,5 wt%) and improved mechanical strength, at lower firing temperature.

2 MATERIALS AND METHODS For the investigation of the effect of magnesite addition on the properties and production process of granite type ceramics the following steps were followed. 1. Selection of the batch formulas for the preparation of low porosity ceramic specimens, shaped by powder pressing. 2. Characterization and selection of the raw materials used in the batch formulas and of the magnesite, which was added into the batches in addition to the other raw materials. 3. Experimental verification of the mixtures by preparation of ceramic specimens without and with addition of magnesite following thermal treatments with different peak temperatures. 4. Testing of the fired specimens for the investigation and determination of the effect of the addition of magnesite on the properties of the final products. For the preparation of the specimens one basic formula was used (Table 1). The plastic materials were 45wt% into the mixture, which consider to be typical for granite type tiles. Table 1 Batch formula used for the preparation of the ceramic specimens Materials (wt%) Mixture GRPI * China Clays 25 Plastic clays 20 Feldspar 40 Quartz 15 * Raw magnesite was added in percentages 0, 1, 2, 4 and 6wt% in addition to the total of the other materials. The china clays used were a kaolinitic clay coded LCl, from Lefkogia (N.E.Greece), together with a kaolinitic clay coded ECl. The plastic clay used was from Ukraine coded UCl. Their determined characteristics are presented in (Table 2). Using a Siemens type D5000 automatic X-Ray Diffractometer and a Setaram 9216 Thermo Gravimetric and Differential Thermal Analyser, mineralogical analyses were performed. The BET specific surface area was measured using Micromeritics instrument while the particle size distribution a Malvern one. Table 2 Characteristics of china clays and plastic clays used X-RD TGA SSA Sample SiO2 Kaolinite (12.2o) Moscovite (9.9o) Kaolinite.. RA FWHM RA FWHM RA FWHM % m2/g LC EC UC XRD: X-Ray Diffraction data, TGA: Thermo-Gravimetric Analysis data SSA: Specific Surface Area, FWHM, RA: Full Width at the Half Maximum and Raw Area of XRD for SiO2, kaolinite and muscovite main peaks.

3 Feldspar was of Greek origin, containing mainly albite, with Na2O > 9.5 wt%, K2O ~1.5 wt% and Fe2O3 < 0.1 wt%, supplied by Mevior S.A. Quartz was also of Greek origin, with SiO2 > 99.5 wt% and Fe2O3 < 0.1 wt%, supplied by Mevior S.A. The raw magnesite used was the type KERMA with low iron, supplied by Grecian Magnesite having the characteristics presented in Table 3. Table 3 Characteristics of raw magnesite Chemical analysis (wt%) S.S.A (m2/g) Grain size Whiteness (%) MgCO3 94,8 SiO2 3,20 50% < 5μm CaO 0,74 15,3 96 Fe2O3 0,05 90% < 35μm Al2O3 0,08 The specimens prepared were coded GRPI-0, GRPI-1, GRPI-2, GRPI-4 and GRPI-6, respectively to the addition of magnesite 0,1,2,4 and 6 wt%. The slip was dried and the cake taken was milled at a Retsch ZM100 mill. The dry powder was wetted, with 4 wt% water, passed through a 1000 μm sieve and pressed at 550 kg/cm2 in a hardened steel mold (120x50mm) using a Walter Bai laboratory hydraulic press. The pressed specimens were dried at 110 oc for two hours; the drying shrinkage was not significantly with the addition of magnesite. The dried specimens were subjected to thermal treatment in a Thermawat box furnace. Weight loss due to firing, firing shrinkage, water absorption, modulus of rupture, Mosh hardness and abrasion resistance using a Gabrielli CAP12 instrument, were determined according to European standards, EN. The development of crystalline phases was studied by X-ray diffractometry and the texture and microstructure by using a Jeol type scanning electron microscope, an Olympus BH2 optical microscope and an Olympus SZH10 stereoscope. RESULTS AND DISCUSSION Addition of magnesite in the GRPI formula. The results of the effect of the addition of magnesite on the properties of specimens prepared from mixture GRPI and magnesite addition, fired at the peak temperatures of 1100, 1120 and 1130 oc are given in Table 4. Table 2 Weight loss in wt% (WL), firing shrinkage in % (FS), water absorption in wt% (WA) and modulus of rupture (MOR) in N/mm2 of specimens GRPI-0, GRPI-1, GRPI-2, GRPI-4 and GRPI-6 fired at different peak temperatures.. Thermal treatment30 mins at 1100 oc Tmax Thermal treatment 30 mins at 1120 oc Tmax Thermal treatment 30mins at 1130 oc Tmax WL FS WA MOR WL FS WA MOR WL FS WA MOR GRPI

4 GRPI GRPI GRPI GRPI From the above results was concluded that the addition of magnesite, improves both water absorption and mechanical strength of the ceramic tiles. For the same thermal treatment the improvement increases with the amount of magnesite, in the various formulation. Consequently the required firing temperature to obtain the desired water absorption and mechanical strength decreases with the addition of magnesite (fig.1 and fig.2). The increase of water absorption from the addition of 4 to 6wt% on specimens fired at 1130oC, is related to the increase of internal porosity presented as blistering and blowing due to over firing (1). Fig.1 Effect of the magnesite addition on water absorption in accordance with the firing temperature on GRPI specimens. Fig.2 Effect of the magnesite addition on modulus of rapture in accordance with the firing temperature on GRPI specimens. The results of table 5 show that both superficial hardness (Mohs hardness) and body hardness (abrasion resistance) increase with magnesite addition and firing temperature.

5 Table 5 Mohs hardness and abrasion resistance values determined for the specimens GRPI-0, GRPI-1, GRPI-2, GRPI-4 and GRPI-6. Peak temperatures of thermal treatment Specimens Mohs hardness Abrasion resistance * C 1120 C 1130 C 1100 C 1120 C 1130 C GRPI-0 < GRPI , GRPI , , GRPI ,5 8-8,5 27, GRPI , ,5 28, * Lower value corresponds to higher abrasion resistance. The results showed that the hardness of the specimens increases with the addition of magnesite. The same effect was observed with the increase of the peak temperature of the thermal treatment as it was expected. These results suggest that the addition of magnesite might lead to ceramics having increased hardness even when fired at relatively lower temperatures. As it is observed the abrasion resistance of the specimens was increased up to the addition of an optimum percentage of magnesite. As it was also expected the increase of the firing temperature has also a positive result. The optimum addition of magnesite is related to the peak temperature of the thermal treatment. The microstructure of the specimens GRPI-0, GRPI-2, GRPI-6 fired at the peak temperatures of 1100, 1120 and 1130 oc, are shown in Figure 3. Magnesite addition has a positive effect in microstructure decreasing the porosity. The pores become smaller and rounded. The addition of 6% wt of magnesite and the peak temperature of 1120oC resulted to the best microstructure, while the peak temperature of 1130oC (for the same magnesia addition), caused an increase on the bodies' porosity (fig.3), which is related to over firing. The above good results from the use of magnesite have not been proved in the study of Jose Angelo and collaborators (2) for porous bodies. But they worked with too high MgCO3 amounts (5-20% wt/wt), despite that the role of MgO as fluxing agent is well known to the ceramists (3). Fig.3 Microstructure of the burning specimens GRPRI

6 Industrial firing trials. Ceramic specimens prepared from the same raw materials under the same conditions, were fired on an industrial roller kiln, in order to verify the above good laboratory results. The thermal treatment, around 45 minutes cycle with 10 to 15 minutes at the peak temperature of 1190 C, was the one followed for fast fired porcelain type ceramic tiles. The results of the effect of magnesite on the properties of specimens fired at industrial conditions are given in table 6. Table 6 Weight loss in %wt (WL), firing shrinkage in % (FS), water absorption in %wt (WA), modulus of rupture (MOR) in N/mm2 and abrasion resistance in mm of length of chord. Specimens WL FS WA MOR AR * GRPI GRPI GRPI GRPI * Lower value corresponds to higher abrasion resistance. The above results confirm the conclusions from the laboratory trials. The addition of magnesite improves both water absorption and mechanical strength of the ceramic tiles (fig.4,5,6).

7 Fig.4 Effect of the magnesite addition on water absorption on industrial fired specimens Fig.5 Effect of the magnesite addition on modulus of rapture on industrial fired specimens Fig.6 Effect of the magnesite addition on abrasion resistance on industrial fired specimens CONCLUSIONS The integration of trials for pressed specimens prepared using formula GRPI showed that the addition of magnesite increases significantly the mechanical strength, decreases the water absorption, increases the hardness and the abrasion resistance and improves the microstructure. The above results are achieved at lower firing temperatures compared to the standard formulation without MgCO3. In general an addition of 2-4% MgCO3 according to the used formulation could decrease the firing temperature by C associated with improved properties.

8 If we consider that MgCO3 plays the role of MgO source it has numerous advantages compared to other sources such as dolomite and talc. The lower decomposition temperature (fig.7) is the most important. Magnesium carbonate decomposes at the same range of temperature at which constitution water is released ( C). CaMg(CO3)2 and talc decompose at much higher temperatures and the released CO2 and other gases could create residual porosity and body distortion. Also MgCO3 is the richest MgO source compared to all the others. Fig.7 REFERENCES 1. Kingery - Bowmen - Uhlmann, "Introduction to Ceramics", John Willey & Son 2nd Edition, page Jose Angelo and collaborators, ".Study of Magnesite Behavior in Porous Single Fire White

9 Bodies". 3. Salmag - Scholtze (1983), "Keramic Teilz" page 19, springle verlag Berlin 1983.