CaO-Al2O3-MgO Slag

Name: CaO-Al2O30-MgO
Diagram No.: 1153
Type of diagram: CCT, TTT
Chemical composition in weight %: No data
Group: Slags
Note: The most obvious environmental effect is that of water vapor in the atmosphere. In general humidity increases the crystal growth rate and makes the slags solidify at a higher cooling rate making the slags harder to form as a glass.

In order to study the effect of water vapor on crystallization, experiments were conducted in two different kind of atmospheres: an Ar atmosphere and an Ar +water vapor atmosphere. The Ar -water vapor mixture was prepared by passing Ar gas at 150 ml/min through a water column, whose temperature was fixed and controlled by the condenser. The experimental partial pressure of water vapor were 0.031, 0.042 and 0.054 atm, which are equivalent to full humidity at 25, 30 and 35 C, respectively. From the work by Schwerdtfeger and Schubert , the solubility of water vapor in CaO-Al2O3 slag at 1600 C was measured with the mogravimetric method.

An example of crystallization phenomena of the sample at 1240 C under a water vapor atmosphere is shown in Figure 20. Figure 20 shows that a nucleus precipitates at the tip of thermocouple and grows as a plane front to the tip of another thermocouple. During the growth process, 2-3 gas bubbles precipitate in front of the plane front and move along with the plane front. The gas bubbles should be H2O gas because the water dissolved in the liquid slag sample is released as a gas due to the decrease in solubility of water in the slag with temperature and segregation as the crystalline phase of the precipitated solid has no water in its chemical composition.

From the isothermal experiments, the TTT diagram can be created from crystallization time and crystallization temperature, as shown in Figure 21. The variation in the water vapor atmosphere between 0.031-0.054 atm didn't show a significant difference in the position of TTT curve as the change of the content of the water dissolved in the sample is a very small change (0.032-0.042 wt%) for the variation of the water vapor between 0.031-0.052 atm (Figure 21).

A comparison of the TTT curve from Ar and Ar + water vapor atmospheres shows that TTT curve from Ar + water vapor condition is above and in front of the TTT curve from pure Ar condition. It can be concluded that the water dissolved in slag enhanced nucleation phenomena. In the isothermal experiments, the growth velocity can be measured under constant temperature conditions by assuming a constant growth velocity. Figure 22 shows the relationship between growth velocity and isothermal temperature of the sample from Ar and Ar + water vapor conditions.

The growth velocity increased with increasing isothermal temperature (Figure 22). So, growth in both conditions is probably controlled by diffusion. The growth rate from slags in a humid atmosphere is higher than growth rate from dry atmospheres. However, in humid conditions the gradient of growth rate change with temperature is less than that from dry conditions. Consequently, it can be concluded that water vapor enhances the growth process. From the continuous cooling experiments, the CCT diagram can be created from crystallization time and crystallization temperature, as shown in Figure 23. From Figure 23, the CCT curve from water vapor condition is higher than the CCT curve from without water vapor condition by approximately 30-50oC. Then, at the same cooling rate crystallization in humid atmospheres occurs at a higher temperature and before the crystallization in dry conditions. Furthermore, the critical cooling rate in water vapor condition (10 C/sec.) is twice of the critical cooling rate in dry atmospheres (5 C/sec).

Quite different results were discovered when studying slags containing MgO. From the isothermal experiments, the TTT diagram was created from crystallization time and crystallization temperature, as shown in Figure 24. From Figure 24, TTT curves from moisture atmosphere have a double nose, which is same as TTT curve from the moisture free atmosphere. The effect of a moist atmosphere on CaO-Al2O3-MgO slag is not as prominent as in CaO-Al2O3 slag and the variation in water content does not clearly affect the TTT curve. For the nose of TTT curves at low isothermal temperature, TTT curves from moisture condition is in front of the TTT curve from moisture free condition, only when isothermal temperature is below 1100oC. However, if isothermal temperature is between 1100-1250 C, TTT curves from both moisture and moisture- free conditions are the same. For the nose of TTT curves at high temperature, the TTT curves from moisture condition are behind TTT curve from moisture free condition. Therefore, it can be concluded that water enhanced crystallization of a CaO-Al2O3-MgO slag when isothermal temperature is lower than 1100oC. Water has no effect on crystallization of CaO-Al2O3-MgO slag, when isothermal is between 1100-1250oC. For isothermal temperature above 1250 C, water hinders crystallization of CaO-Al2O3-MgO slag. However, these effects are small and to a first approximation water had no effect in the crystallization phenomena of the slag.

From the continuous cooling experiments, the CCT diagram can be created from crystallization time and crystallization temperature, as shown in Figure 25.

From Figure 25, at the lower part of CCT curves (cooling rate more than 7 C/sec.), moisture has no effect on crystallization time and temperature. For the top part of CCT curves (cooling rate less than or equal to 7 C/sec.), crystallization under moisture free conditions start at a higher temperature and shorter time than crystallization under moisture conditions. Therefore, water hinders crystallization of CaO-Al2O3-MgO slag when cooling rate is less than or equal to 7 C/sec. In addition, water did not affect the critical cooling rate. If the effect of water on TTT and CCT is compared, it was found that water hinders crystallization at the high temperature only.

Figure 26 shows the relationship between growth velocity and isothermal temperature of the sample from moisture and moisture- free conditions. Water enhances growth ve locity only when isothermal temperature is lower than 1200 C (Figure 26). The major finding of this study is the elimination of the effect of water vapor on solidification by the addition of MgO.
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