Abstract
This study is focused on the behavior of the cream-firing clays from Opoczno region (Poland). The ceramic masses on which tests were carried out consisted of cream-firingBorkowice clay and dolomite in two different grain sizes as an additive that changes the color of ceramic materials. Test samples were prepared by plastic method and fired at range of 1100–1240 °C. Phase composition of theinvestigated materials was characterized by XRD method with quantitive analysis of the amorphous phase determined by the Retvield technique. Color properties of the surface of the obtained ceramic materials were determined in CIE-Lab color space. The phase composition of the obtained ceramics depends on the firing temperature. The color of the surface of the ceramic materials also depends on the firing temperature. There was a tendency to decrease the brightness, decrease the blue shade, and increase the yellow shade of the surface of materials with increasing the temperature. The conducted tests allowed to conclude that the color of ceramic materials depends on their phase composition. The most important role in the formation of color correspond to the amorphous phase, formed during the process. The lower content of the amorphous phase in the material allows to obtain brighter products with a lower proportion of yellow, and therefore closer to white. Moreover, following tests were carried out: total water absorption, total open porosity, linear shrinkage, and flexural strength. With increasing the temperature, total water absorption and total open porosity decrease, and total linear shrinkage increases due to the progressive sintering process. Flexural strength increases with the increase of the firing temperature for materials consisting of Borkowice clay. The addition of dolomite introduced new pores into the material, which resulted in an increase in flexural strength at lower firing temperatures and a decrease in flexural strength at higher firing temperatures.
Highlights
The exposure of phyllosilicates and other accompanying minerals such as quartz, feldspar, hematite, dolomite, and calcite to high temperature causes a series of phase transformations that affect the properties of the final ceramic material [1]
The highest value of shrinkage for the reference material 0 was recorded at 1200 ◦C, and the lowest at the lowest temperature of 1100 ◦C. These results show that the reference material is characterized by a higher degree of sintering than plastics with the addition of dolomite (D15, D35), regardless of the firing temperature
The addition of dolomite to the ceramic mass causes a change in the phase composition of fired sintered materials
Summary
The exposure of phyllosilicates and other accompanying minerals such as quartz, feldspar, hematite, dolomite, and calcite to high temperature causes a series of phase transformations that affect the properties of the final ceramic material [1]. The minerals contained in clays are structurally and chemically modified that significantly transformthe original clay materials. These transformations are due to the high temperature and low pressure which characterize the firing process and are dependent on the chemical and mineralogical compositions of the original clay, its grain-size distribution, the maximum heating temperature, heating rate, duration of firing and kiln atmosphere [2,3]. As described above, during firing, thermal decomposition of the majority of the original crystalline phases occurs (excluding quartz) while new crystalline phases (especially mullite) and chemically differentiated glassy phase are formed [4]. Gehlenite and anorthite phases are formed during the firing of the ceramic masses, which contain illite, kaolinite, quartz, and calcite [6]
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