Alloy name: Magnesia
Diagram No.: 947
Type of diagram: TTT
Chemical composition in weight %: MgCO3
Alloy group: Oxides
Note: Time-temperature-transformation diagram of the magnesia
X-ray diffraction was used to study the transformation of magnesite (MgCO3) to periclase (MgO), and also obtain the CS of periclase. The TTT diagram of caustic calcined magnesia was established based on the XRD patterns, by determining when at a given temperature, magnesite peaks disappear and periclase prominent peaks emerge (Figure). In this diagram, Curve A represents the initiation of reaction, where periclase appears in the X-ray patter, and Curve B shows the time and temperature at which the transformation of magnesite to periclase is complete. Based on the XRD analysis, magnesite is absent beyond Curve B, where the dominant phase is periclase. Compared to dead–burned magnesia with virtually zero LOI, this product has an LOI up to 10%. The area between these two curves represents the transitional stage where both magnesite and periclase co–exist. As it can be seen from Figure, at the lower calcination temperatures the phase transformation is taking place at a slower rate. For example, at 600°C the transformation starts in less than 15 min and is completed within 2 h. At 800°C, on the other hand, the decomposition is completed in less than 30 min, while at 500 °C calcination longer than 8 h is required (Fig).
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