## 2 Materials and Methods  
### Materials and Mixture Proportions  
In this study, cement and AOD slag were used to fabricate various mixtures of cement mortar and paste. The cement and AOD slag used in this test have been described previously [21]. Table 1 shows chemical composition of the ordinary Portland cement (OPC) and AOD slag. Density and Blaine fineness of OPC were 3.14 g/cm\({}^{3}\) and 332 m\({}^{2}\)/kg, respectively. The AOD slag was produced by Korean steel company POSCO, with a density and fineness of 2910 kg/m\({}^{3}\) and 359 m\({}^{2}\)/kg, respectively.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & & & \multicolumn{6}{c}{**Chemical Compositions (wt. \%)**} \\ \cline{2-10}  & **CaO** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **MgO** & **Cr\({}_{2}\)O\({}_{3}\)** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **SO\({}_{3}\)** \\ \hline OPC & 62.4 & 20.8 & 4.9 & 3.5 & 1.6 & - & 0.9 & 0.3 & 2.2 \\ AOD slag & 54.3 & 17.7 & 6.4 & 3.0 & 9.2 & 2.8 & 0.2 & 0.1 & 0.4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition of the OPC and AOD slag [21].  
The AOD slag was slowly cured in the air, which generated a large quantity of \(\gamma\)-C\({}_{2}\)S in the slag. The average particle size of the AOD slag and OPC were 25.5 \(\upmu\)m and 15.5 \(\upmu\)m, respectively. The mineral composition of the AOD slag was calculated using Rietveld refinement analysis based on an X-ray diffraction (XRD) pattern, as shown in Figure 1. \(\gamma\)-C\({}_{2}\)S accounted for approximately 38.1% of AOD slag, the highest proportion. Other mineral compositions of AOD slag included gehlenite (Ca\({}_{2}\)MgSi\({}_{2}\)O\({}_{2}\)), rollastonite (CaSiO\({}_{3}\)), magnesite (MgCO\({}_{3}\)), and calcite (CaCO\({}_{3}\)), which represented 10.4, 10.8, 15.6, and 12.4% by weight, respectively [21].  
The mixture proportions of paste specimens are summarized in Table 2. The water to binder ratio was set at 0.5 with varying amounts of the AOD slag. The replacement levels of the AOD slag were varied as 0, 30, and 60% of the OPC by weight, respectively. For the mixture proportions of the mortar specimens, the mass proportions of water, binder, and fine aggregate were 0.6:2:3. Standard sand shown in ISO 679 was used as the fine aggregate in the production of the mortar [26].  
AOD slag has non-hydraulic properties but generates silica gel and calcite through CO\({}_{2}\) curing [21; 23]. Consequently, cementitious materials containing AOD slag show different mechanical properties and microstructure depending on the CO\({}_{2}\) curing condition [27]. In this study, two kinds of experiments were conducted, based on CO\({}_{2}\) curing conditions, using a CO\({}_{2}\) curing chamber with adjustable temperature and CO\({}_{2}\) concentrations, as shown in Figure 2[21]. First, CO\({}_{2}\) curing concentrations of 0, 5, 10, and 15%, respectively, were used, and the specimens were cured under constant temperature and humidity (23 \({}^{\circ}\)C, RH = 60%). Second, curing was performed under various temperatures (23, 40, and 50 \({}^{\circ}\)C; RH = 60%), under CO\({}_{2}\) concentrations of 0 and 5%, respectively.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Water (g)** & **OPC (g)** & **AOD slag (g)** \\ \hline STP0 & 50 & 100 & - \\ STP30 & 50 & 70 & 30 \\ STP60 & 50 & 40 & 60 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mixture proportions of the paste specimens.  
Figure 1: XRD pattern of the AOD slag.