This study investigates the effects of adding clay and fireclay on the physical and mechanical properties of magnesia-based refractories such as contraction, bending strength, bulk density, and apparent porosity. Domestic raw materials were used for the preparation of samples fired at 1350, 1450, and 1550 °C for 2 h. Adding clay exhibited no significant effect on the density and porosity, whereas adding fireclay had a remarkable influence on the shrinkage. Nevertheless, the effects of clay and fireclay on the strength of magnesia were unnoticeable. X-ray diffraction results showed that, after firing, the main phase compositions of the samples with clay addition were periclase and forsterite. Adding fireclay led to the synthesis of magnesite spinel, which can be attributed to the high alumina content. Based on scanning electron microscopy, no liquid phase was formed indicating that the sintering was a solid-state evolution with the synthesis of forsterite.
During the liquid phase sintering process a liquid is formed, penetrates into the pores, causes rearrangement of particles and shrinkage and forms an amorphous and/or crystalline intergranular phase after firing which re-melts upon heating to the same sintering temperature. In this research a method is proposed to alter the intergranular phase to a phase with a melting point higher than the sintering temperature. The hypothesis is based on the shape of the phase diagram and precise control of particle sizes of liquid forming species. Results show that in the CaO–Al2O3 system a liquid is formed at 1 500 °C and it will then change to CaO. Al2O3 with a melting point of more than 1 600 °C. Differential scanning calorimetry analysis shows that the processes of formation of liquid and crystallization of concomitant crystalline phases occur obviously separately in this system. The system CaO–TiO2 also confirms the hypothesis, although more analytical evidence is needed for the latter.
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