The effects of sintering temperature on phase and pore evolution in porcelain tiles
Introduction
The Acimac (The Association of Italian Manufactures of Machinery and Equipment for Ceramics) Statistics Department reported that world tile production reached 11.9 million m2 in 2013; a 6.4% rise compared to 2012. World consumption also maintained strong growth reaching 11.5 million m2; a 5.9% increment compared to 2012 [1]. These trends show that demand for tile has increased year by year. In addition, demand for porcelain stoneware tile has shown a higher rate than other types of tiles.
Porcelain tile is composed of a vitreous phase (generally consisting of feldspathic ingredients) and crystalline phases of quartz and feldspar as residual phases as well as mullite that is formed due to reaction between plastic raw materials and feldspar. The amount of vitreous phase lies between 45–80 wt%, with the quartz phase usually higher than 30 wt%, while mullite is limited to 8 wt% as result of fast sintering processes and lower sintering temperatures [2], [3], [4]. Martin-Marquez et al. studied the composition of fired porcelain tile and showed that the tile tested was composed of 61% amorphous phase at 1230 °C [5]. Zanelli et al. investigated ninety three tile compositions and showed that the amount of the amorphous phase changed between 40–75 wt% and identified it as the major phase after 1075 °C [6].
According to Salem et al., the densification of tile takes place in three stages. Initially, the formation of the first liquid phase takes place at around 980 °C, flowing and wetting the particles which increase with increasing temperature. After this, capillary pressure is produced along the particles, with the liquid phase filling the open pores. The interconnected structure of the pores then starts to break down, showing the start of the final stage. In this final stage, the liquid phase flow is terminated by the isolated gas and closed pores remain in the body [7]. Martin-Marquez et al. show that the amount of closed pores is not dependent on sintering temperature, while the amount of open pores decreased from 9% to 5% between 1200 and 1250 °C [8].
Mercury intrusion porosimetry (MIP) accepts all the pores of cylindrical geometry, but the real pore shape is irregular in the material and, therefore, the results obtained from this technique should be interpreted together by microscopic analysis. Using an amount of intruded mercury, it is possible to follow the breakdown of pore interconnectivity to the sintering stages.
In this study, the relationship between porosity and phase development of porcelain tiles against sintering temperature is discussed. To obtain different pore and phase characteristics, standard porcelain tile granules are sintered at different temperatures with their development being investigated.
Section snippets
Experimental
Standard porcelain tile granules were pressed in a 50 mm × 100 mm rectangular die at 450 kg/cm2, and dried at 110 °C. Sintering was carried out in a fast-firing laboratory roller kiln (Nannetti ER-30) at temperatures of 1150–1230 °C with an industrial fast-firing cycle (total 45 min, including cooling).
An X-ray fluorescence (XRF, PANalytical Axios) study was performed to obtain granule composition. The composition of the granules used for this study is given in Table 1 and the amounts of oxides are
Results and discussions
As a general observation upon heating, the bulk density and firing shrinkage of all the bodies continued to increase, reaching a maximum and then decreasing due to over firing. This relationship is typical of ordinary porcelain. Water absorption values reach a minimum level with an increase in sintering temperature. The ISO standard prescribes a maximum water absorption value of 0.5% for the porcelainized tiles. For 1220 and 1230 °C sintering temperatures, this water absorption value was
Conclusions
The sintering behavior of porcelain tiling is investigated by observing phase and pore changes at different sintering temperatures. XRD analyses show that albite and quartz are dissolved; the amorphous phase and mullite are formed with increased temperature. Using the amount of pore area and pore size, the sintering steps of the tiling is determined. The second step occurs below 1150 °C, since the pore area is calculated as 24.5%. The third step occurs after 1200 °C when the pore area change
Acknowledgement
Financial support from the Anadolu University Scientific Research Project, Project No. 080210, is gratefully acknowledged.
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