Abstract:The effects of Fe₂O₃ content on the microstructure and mechanical properties of the CaO-Al₂O₃-SiO₂ system were investigated by differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron spin resonance (ESR), and M？ssbauer spectroscopy. The results show that the addition of Fe₂O₃ does not affect the main crystalline phase in the prepared glasses, but it reduces the crystallisation peak temperature, increases the crystallisation activation energy, and reduces the crystal granularity. The ESR results indicate that Fe₂O₃ can promote crystallization, as it leads to the phase separation of the CaO-Al₂O₃-SiO₂ system due to axial distortion. Moreover, Fe₂O₃ alters the network structure of the CaO-Al₂O₃-SiO₂ system, allowing Fe³⁺ to enter octahedral sites that exhibit higher symmetry than tetrahedral sites. All of these factors are favourable to increasing the bending strength. The M？ssbauer results reveal that there are two types of coordination for both Fe³⁺ and Fe²⁺ and the bending strength of the CaO-Al₂O₃-SiO₂ system increases with the amount of six-coordinate Fe³⁺. The increasing interaction between Fe³⁺ and Fe²⁺ can also enhance the bending strength of the CaO-Al₂O₃-SiO₂ system. The microhardness of the CaO-Al₂O₃- SiO₂ system was determined to be HV 896.9 and the bending strength to be 217 MPa under the heat treatment conditions of nucleation temperature of 700 °C and nucleation time of 2 h, crystallization temperature of 910 °C and crystallization time of 3 h.

Key words: glass-ceramics; CaO-Al₂O₃-SiO₂ system; Fe₂O₃; electron paramagnetic resonance; M？ssbauer spectroscopy; mechanical properties