Blast Furnace Slag

Name: Blast Furnace Slag
Diagram No.: 971
Type of diagram: TTT
Chemical composition in weight %: 42.99% CaO, 33.52% SiO2, 13.77% Al2O3, 6.28% MgO, <0.37% Fe2O3, 0.27% MnO
Group: Slags
Note: Top diagram: TTT diagram of the beginning of crystallization obtained by in situ observation of BF slag (tentative diagram).
Figure shows the tentative TTT diagram of the BF slag obtained. The horizontal axis is the time in a natural logarithm and the vertical one is the temperature that a crystal precipitated. The nose position of BF slag was about 9 s at 1 250°C and was 4 s at 1050°C, which meant the double nose TTT diagram. In addition, the melting point of BF slag was about 1400°C. Generally, the double nose TTT diagram will have more than two kinds of crystals. In the case of BF slag, there is no information about the kind of crystal phase, only TTT diagram having a close chemical composition of BF slag was published by P. Rocabois et al.) in which several crystal phases were reported such as pseudowollastonite (CS: CaO ·SiO2), anorthite (CAS2: CaO·Al2O3 ·2SiO2), Gehlenite (C2AS: 2CaO· Al2O3 ·SiO2) and Akermanite (C2MS2: 2CaO ·MgO·2SiO2), but there is a little information about optical characteristics during precipitation, which is related to the result of in situ observation. As shown in Fig., there are relatively large scattering of the data.
Bottom diagram: TTT diagrams of the beginning of crystallization obtained by in situ and ex situ observations by optical microscope and XRD analysis.
Finally, the TTT diagram of BF slag was determined as shown in Fig. The upper nose in the tentative TTT diagram obtained by the in situ observation would be the nose of Gehlenite which located inside the TTT diagram of Merwinite. The crystal of Merwinite precipitated in high temperature was a transparent or a translucent which was difficult to see in high temperature. The nose position of Merwinite located around 4 s at 1090°C. The reason why the Merwinite precipitates faster than the Gehlenite will be resulted from the lower free energy (Fig. A1), however, the behavior of crystallization until a stable state (100% of crystallization degree) will be affected by the difference of the diffusibility of cations that construct the Merwinite and the Gehlenite. Especially, as Ca and Si are common elements in the both compounds, the difference of the diffusibility of Mg and Al will be important to clarify the crystallization behavior around 1300°C.
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