## 3 Results and Discussion
### Aggregate Stability Evaluation  
Figure 8 shows the expansion rate of mortar bar using converter slag before and after reaction with SCD. According to Figure 8, for the first 10 days, the expansion rate of mortar using as received converter slag showed very similar expansion rate to that of mortar bar made of reacted converter slag. However, at 12 days, a crack was found from the surface of the mortar bar made of as received converter slag. Although the specimen did not show significant amount expansion during first 14 days of measurement, a large  
number of cracks was produced at 17 days accompanied by rapid increase in expansion rate. Finally, at 19 days, mortar bar was completely destroyed showing expansion rate of 0.13%. The photographic images of mortar bar made of as received converter slag are shown in Figure 9. The result proves instability of unreacted converter slag for its application as source of aggregate in concrete. Figure 5: pH of converter slag reacted with supercritical carbon dioxide. Figure 6: SEM image of as received converter slag. Figure 7: SEM image of converter slag reacted with supercritical carbon dioxide. From expansion of mortar bar made of reacted converter slag, two mortar bar specimens were not damaged whereas the last one was broken during expansion test. With respect to failed mortar bar, the first crack was observed at 17 days of measurement. The specimen was broken at 26 day, but showing only 0.04% of expansion. It was found that the reaction with SCD either removed the failure or delayed the time of failure, but it did not completely prevent the failure of the mortar bar. It is considered that the failure of mortar bar is mainly associated with periclase that was observed from XRD pattern of reacted specimen. The expansion seemed to be generated at the place where free periclase was located. It is evidenced by the observation of white powder, the color of magnesium hydroxide (brucite), on the surface of converter slag (Figure 9d). It can be concluded that due to the expansive reaction that occurred within converter slag, the mortar bar expanded, and a crack was generated. However, failure mode of the mortar bar with converter slag was somewhat different from what was generally observed from reactive aggregate. It should be noted that the failure of the mortar bar occurred much earlier than the expansion rate designated for reactive aggregate criterion (0.2%) in ASTM C 1260 [35]. Some samples were even broken before potentially reactive aggregate criterion (0.1%), which is set to classify less reactive aggregate. In general, mortar bar expansion from reactive aggregate can easily be higher than expansion rate that was designated for reactive aggregate criteria [44; 45; 46; 47]. Such a difference is associated with the fact that the reaction of reactive aggregate occurs mostly at the outside rim of the reactive aggregate (only small portion of reaction occurs inside the aggregate) [45; 47] whereas all the reaction from free lime and periclase occurs inside the converter slag. When free lime or periclase reacts inside converter slag, it expands and generate cracks on converter slag (local expansion), which eventually led to the breakage of the mortar bar without matter of how much reaction has taken place. This can explain how the mortar bar with reacted converter slag was destroyed without showing any notable amount of expansion. The only difference between mortar bar with and without reaction with SCD was on the fact that expansion with as received converter slag occurred at many different locations whereas expansive reaction with reacted converter slag occurred only at some limited spaces showing very small amount of expansion. The results from mortar bar expansion test clearly state a very important fact that any presence of free lime or periclase remaining on the steel slag, even in very small amount, can cause breakage of concrete specimen due to the expansion within converter slag.