## 4 Results
### Mortar Application of BOF Slags  
Figure 15 illustrates the expansion characteristics of mortar bars made of fresh and stockpiled BOFS submerged in water under a controlled temperature of 80 \({}^{\circ}\)C. Pre-determined reactive siliceous river sand was used for a comparative study in the expansion test in both water and 1 M NaOH solution. The expansion of the mixture containing river sand at 80 \({}^{\circ}\)C water was 0.01% at both 14 and 28 days, respectively. This river sand is innocuous in normal conditions, and not in a high-alkali environment. However, the mortar bars containing fresh and stockpiled BOFS had higher expansions at 14 and 28 days. For example, the mortar bars containing fresh BOFS exceeded the threshold value of 0.1% based on ASTM C 1260/C 1567 (14-day expansion) and were broken even before 14 days. The expansion of the stockpiled BOFS mortar was below 0.1% at 14 days. However, when the expansion period was extended to 28 days, the expansion of all BOF specimens also exceeded the threshold value of 0.1%, except the processed BOF (lab-harvested fresh BOF, which was water-submerged). This volumetric expansion was due to the formation of expansive portlandite minerals. During the hydration process, the reaction of free calcium oxide (f-CaO) with water induced the formation of Ca(OH)\({}_{2}\) and CaCO\({}_{3}\), leading to excessive expansion and cracks in the mortar bar specimen (Figure 16). Figure 17 presents the expansion characteristics of the mortar bars containing fresh and stockpiled BOFS, which were submerged in 1 M NaOH solution under a controlled temperature of 80 \({}^{\circ}\)C. As expected, the expansion of all mortar mixtures was beyond a limit value of 0.1% expansion at 14 days, indicating reactive and volumetric expansion field performance behaviors. In this case, both portlandite and alkali-silica reaction gel formations occurred together and resulted in more severe expansion. Again, the expansion  
Figure 15: Mortar bar expansion of BOFS and river sand in 80 \({}^{\circ}\)C water. of stockpiled BOFS was less than that of fresh BOFS. Therefore, when BOFS materials are used as an aggregate, it seems necessary to conduct an aging process that eliminates or minimizes deleterious expansion. As stated previously, one of the critical challenges facing the utilization of BOFS as an aggregate in construction applications is its volume instability due to the formation of Ca(OH)2 or magnesium hydroxide (Mg(OH)2) caused by the reaction between f-CaO or f-MgO and water. Mg(OH)2 crystalline products along with Ca(OH)2 arise. Figures 15 and 17 present the expansion characteristics of mortar bars containing stockpiled BOFS. Interestingly, the expansion of all mortar mixtures was below a limit value of 0.1% expansion at both the 14- and 28-day periods, indicating innocuous field performance behavior. This result indicates that the formation of portlandite had already taken place in the stockpile. There was no more deleterious portlandite formation, or less formation occurred, leading to less expansion of the mortar bar. However, when all mortar mixtures were submersed in 1N NaOH solution, all samples exceeded a limit value of 0.1% expansion at 14 days, indicating reactive and volumetric expansion field performance  
Figure 16: Cracked mortar bar after testing. Figure 17: Mortar bar expansion of BOFS and river sand at 80 °C in 1 M NaOH solution. behavior. This indicates that they exhibited reactive and volumetric expansion in this aggressive alkaline environment. This expansion was attributed to alkali-silica reaction (ASR), which is a chemical reaction between alkalis in the solution and reactive siliceous aggregates in the mortar or concrete. To minimize the expansion caused by ASR in these conditions, the text suggests the use of supplementary cementitious materials, such as fly ash or ground granulated blast furnace slag. These materials can help mitigate the expansion by reacting with the alkalis in the solution and reducing the potential for ASR.