Nucleation and Crystal Growth of a MgO–CaO–Al2O3–SiO2Glass with Added Steel Fly Ash

The nucleation and growth of diopside Ca(Mg,Al)(Si,Al)2O6crystals on the free surface of a 24 wt% MgO, 14 wt% CaO, 9 wt% Al2O3, and 53 wt% SiO2glass, with a 2 wt% addition of steel fly ash, were investigated through DTA, XRD, SEM, and optical microscopy. Crystallization was complete at ∼920°C with an activation energy of 589 kJ/mol. Samples with polished free surfaces were nucleated at selected temperatures in the range of 730° to 820°C, and then heat-treated at 870°C for 15 min for crystal growth. Nucleation was predominantly observed at the surface, and the number of diopside crystals per unit of area and the mechanism of crystallization were determined. It was concluded that nucleation reaches a maximum at 750°C, corresponding to an average density of diopside crystals of 8.4 × 106 nuclei/cm2, and that between 900° and 1100°C, a uniformly crystallized layer is formed at an exponential rate. The crystallized volume fraction increased significantly in the 880°–890°C growth range, and remained almost constant at higher temperatures. In the 860°–910°C range, the size of the diopside crystals formed in the samples nucleated at the temperature of the maximum nucleation rate, and linearly increased, reaching values between 1.0 and 3.0 μm at 870° and at 910°C, respectively.