## 3 Results and Discussion
### Effect of Carbonation by SCD on Converter Slag
#### 3.1.3 Thermogravimetric Analysis  
Figure 4 shows the TG/DTA analysis results of converter slag before and after reaction with SCD. Figure 4a is a TG graph obtained from as received converter slag, and a weight loss of 0.34% was observed at around 450 \({}^{\circ}\)C [34]. This peak is associated with the decomposition of portlandite. Same to the results from XRD patterns, the presence of portlandite in converter slag was also verified from TG. The presence of calcite was also verified by TG. There was 98.02 (before decomposition) \(-\) 97.61 (after decomposition) = 0.41% weight loss event associated with decomposition of CaCO\({}_{3}\).  
Figure 4b shows the TG/DTA graph of converter slag after reaction with SCD. From the TG graph of reacted specimens, the weight loss in the 450 \({}^{\circ}\)C region observed from as received converter slag disappeared due to the carbonation by SCD, and a weight loss of 96.47 (before decomposition) to 89.11 (after decomposition) = 7.36% was observed in the 600-800 \({}^{\circ}\)C region, which is associated with the decomposition of CaCO\({}_{3}\) polymorphs [32].  
Figure 4: Thermogravimetric analysis of converter slag reacted with supercritical carbon dioxide.  
It was found that the amount of CO\({}_{2}\) in CaCO\({}_{3}\) produced by reaction with SCD was 7.36 (after reaction) to 0.41 (before reaction) = 6.95%. Considering the final weight after TGA as a weight of unreacted and uncarbonated converter slag, the amount of CO\({}_{2}\) uptake that converter slag can provide by the reaction with SCD can be calculated. It was found to be 7.80 g CO\({}_{2}\) per 100 g converter slag. If there was no carbonation on as received converter slag, this could have increased to 8.26 g CO\({}_{2}\) per 100 g converter slag. It would be 10.40 g CO\({}_{2}\) per 100 g fresh converter slag supposing that there was neither reaction nor carbonation.  
According to the work of [PERSON] et al. [40], CO\({}_{2}\) uptake by blast furnace slag varied from changes in testing condition. They tested under a pressure range of 10 to 60 bar and temperature range of 20 to 80 \({}^{\circ}\)C, and the maximum amount was reported to be 2.95 g CO\({}_{2}\) per /100 g blast furnace slag at 50 \({}^{\circ}\)C 30 bar. The amount of CO\({}_{2}\) uptake by converter slag after reaction with SCD was greater than what was found earlier from the blast furnace slag. Such difference might be related to higher CO\({}_{2}\) bearing capacity of converter slag than blast furnace slag because converter slag used in this work has higher amount of CaO than typical blast furnace slag. However, it is also strongly associated with increased reactivity of CO\({}_{2}\) in supercritical condition.  
It should be also noted that particle size of the steelmaking slag can significantly affect the amount of CO\({}_{2}\) uptake. [PERSON] and [PERSON] [41] reported that basic oxygen furnace can absorb CO\({}_{2}\) up to 0.47-46.5 g of its original weight depending on its particle size. In their other work [42], total CO\({}_{2}\) uptake was reported to be around 6.7-53.6% CO\({}_{2}\) depending on the changes in temperature, pressure, and particle sizes. In this work, the size of the aggregate had to be controlled for mortar bar expansion tests, and the temperature and pressure condition was set to a single parameter. Such factors will need to be considered for further experimental works because considering the results from earlier works, it is reasonable to expect that CO\({}_{2}\) uptake by converter slag could have been increased if particle size was reduced.