## 3 Preparation of Samples and Data Used  
### Materials and Mix Proportion  
Geopolymer concrete (GPC) was designed for a specified compressive strength grade (f'c = 25, 30, 35 MPa) based on the ACI 211-91 standard. For the preparation of GPC, fly ash, steel slag, sodium silicate, and sodium hydroxide were used. Fly ash (ASTM C618-03 Class F) was obtained from Pha Lai Thermal Power Plant (Hai Duong - Viet Nam). The typical chemical composition and the fineness of fly ash are shown in Tables 1 and 2, respectively. Anhydrous NaOH (purity, P = 98.5%; density, D = 2.1 g/cm\({}^{3}\)) and Na\({}_{2}\)SiO\({}_{3}\) water glass solution (relative density, RD = 1.42; 39-40.03 wt.% Na\({}_{2}\)SiO\({}_{3}\)) were purchased from Viet Tri Chemical Factory. The activated alkaline solution was prepared by (i) dissolving NaOH powder in water to obtain NaOH solutions with different molar mass (M), and (ii) mixing with Na\({}_{2}\)SiO\({}_{3}\) solution in different concentrations. The dissolution and the mixing process are exothermic reactions (temperature \(\approx\) 70 \({}^{\circ}\)C). In order to correctly activate fly ash, the solutions were prepared at least one day before mixing into GPC.  
Steel slag, served as both fine and coarse aggregates, was achieved from Thai Nguyen Iron and Steel Industrial Park (Thai Nguyen - Viet Nam). Prior to the preparation of samples, steel slag was crushed into different particle sizes and dried under vacuum, whereas excess metal was recovered by electromechanical equipment. The mixture of steel slag coarse aggregates (nominal maximum aggregate size, D\({}_{\text{max}}\) = 19 mm) was sieved following the ASTMC 136 standard and was mixed together to satisfy the ASTMC33 standard. Steel slag aggregates were then divided into two groups: (i) particle size of 4.75-19 mm, acting as coarse aggregates, and (ii) particle size in a 0.15-4.75 mm range, serving as fine aggregates. It is noteworthy that the purchased steel slag aggregates do not naturally satisfy the ASTMC33 standard. The sieved aggregates at 9.5 mm feature 65.83 wt.%, which is out of the range of the ASTMC33 standard (i.e., 20-55%). Therefore, coarse aggregates need to be partially removed from each sieve range, then blended again in order to redistribute the raw material to satisfy such a standard. The aggregate distribution size before and after the mixing process is highlighted in Figures 2 and 3.  
The mixing process was carried out with a forced mixing concrete mixer with 60-L capacity, supplied by Daiwa Kenko Co., LTD, Tokyo, Japan. The mixture was mixed following the ASTM C192 standard with a dry mixing time of 3 min and a wet mixing time of 5 min. After mixing, the slump of the mixture was measured, and then casted for the testing procedures. The mixture was added into the cylindrical mold and compacted on a vibrating table for about 1.5 min, which is two times longer than the process for traditional concrete. The sample surface was then covered by glossy paper to avoid evaporation. After 48 h, samples were removed from the mold and stored in the laboratory at a temperature T = 22 \(\pm\) 4 \({}^{\circ}\)C and a humidity w = 60 \(\pm\) 20% (Figure 4).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **CaO** & **MgO** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **TiO\({}_{2}\)** & **SO\({}_{3}\)** & **LOI \({}^{*}\)** \\ \hline
51.74 & 24.53 & 5.59 & 0.81 & 1.95 & 4.42 & 0.11 & 0.76 & 0.31 & 8.98 \\ \hline \end{tabular} \({}^{*}\)LOI: loss on ignition.  
\end{table}
Table 1: Main chemical compositions of fly ash (wt.%).  
Figure 2: Gradation curves of fine aggregate.