Effects of Fe2O3 content on microstructure and mechanical properties of CaO-Al2O3-SiO2 system

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 Mössbauer 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 Mössbauer 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.