The Studies of the effect of MgO and TiO2 on the crystallization process of fly ash derived glass

Nowadays there is a tendency toward the research of a way to reuse coal fly ashes produced by the coal-burning power plant in order to protect the environment. In the present work the nucleation and crystal growth processes of fly ash derivated glass-ceramic, induced by MgO and TiO2, were studied. The high viscosity of melted glass obtained from fly ash [1] was reduced adding MgO whereas TiO2 was used to enhance the nucleation mechanism. The analysis was carried out by differential thermal analysis (DTA), thermal dilatometry (TD), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The glasses were synthesized mixing fly ash with MgO and TiO2. After the melting at 1500 °C for 4 h, glasses were quenched and annealed at 100 °C for 30 min. DTA thermograms were recorded in air at 10°C/min using powdered Al2O3 as reference material and to improve the heat exchange. A finely ground sample was used for XRD analysis. SEM samples were etched in 1% HF and coated with a thin Au film. DTA curve of as-quenched samples, (a) Fig.1, shows a glass transition with a Tg=730 °C and two exothermic peaks at 820°C and 975°C respectively. Samples, subjected to a heat treatment at 730 °C for 2 h (b), 4 h (c), 8 h (d) of Fig. 1, show a decrease of the first exo-peak whereas the second one was not affected by heat. Curve (d) also shows a glass transition at the temperature of 940°C that indicates the presence of the phase separation. This is confirmed in the dilatometry analysis (Fig 2.) in which curve (b) shows a glass transition at 900°C. XRD analysis (Fig.3) of the as-quenched sample (a) and of that heated 8 h at 730 °C (b F) shows a still amorphous structure. XRD Pattern of the glass heated to 820°C (temperature of the first DTA exo-peak in (c) of Fig.1) does not exhibit a crystalline structure with only a small reflection peak. The sample heated to 975 °C (temperature of the second DTA exo-peak in (d) of Fig.1) is more crystallized. According to these results, two heat programs were proposed to turn glass into ceramic-glass material: Program 1: 730°C (8 h)  900 °C (8 h)  1020 °C (20 h) Program 2: 850°C (4 h)  950 °C (8 h)  1020 °C (20 h) In both programs, the crystalline phase is produced in the second step and the formation of anorthite and cordierite is observed in the third step, but they have a different glass-ceramic morphology. SEM of program 1 treated sample and of program 2 treated sample (Fig. 4) show an amorphous high viscosity phase and a crystallized low viscosity phase. Finely, the thermal expansion coefficients of two heat-programs glass-ceramic materials are lower than that of as-quenched and the heated 8 h at 730°C glass. Bibliography [1] Cioffi, R., Pernice, P., Aronne, A., Marotta, A. & Quattroni, G., Nucleation and crystal growth in a fly ash derived glass. J. Mater. Sci., 1993, 28, 6591-4.