## 2 Materials and Methods  
### Materials  
The red mud used in this study was collected from the waste storage of the Tan Rai alumina plant of Lam Dong Aluminum Company Limited, Lam Dong, Vietnam. Kaolin is commercially available from the Truc Thon clay mine in Hai Duong province, Vietnam. The fly ash was provided by Pha Lai thermal power plant, Hai Duong, Vietnam. The red mud was dried, then ground by a ball mill and sieved to collect the fraction passing a 90 \(\upmu\)m mesh screen. Calcium hydroxide (analytical grade, \(\geq\)95% purity) was purchased from Xilong Science Co., Ltd., Shantou, China, as a white powder consisting of a small amount of impurities such as sulfate, magnesium salt, and substances precipitated from ammonium hydroxide. Sodium hydroxide was analytical grade (\(\geq\)98% purity, China). All the raw materials were dried at 105 \({}^{\circ}\)C for 24 h before use.  
The chemical compositions of raw materials (kaolin, fly ash, and red mud) were analyzed using X-ray fluorescence (XRF: S4 Pioneer, Bruker, Billerica, MA, USA) and are given in Table 1. Silicon dioxide and aluminum oxide are the main components of fly ash and kaolin, while iron oxide and aluminum oxide are the dominant components of red mud. The X-ray diffractograms of raw materials in Figure 1 demonstrate that kaolin, fly ash, and red mud mostly contain amorphous phases indicated by low-intensity peaks and diffracted patterns in the range of 20 = 10-17\({}^{\circ}\)[16; 31]. Kaolin consists of crystalline phases of kaolinite, quartz, and illite. The crystalline phases in the red mud are mostly composed of hematite, gibbsite, and sodalite. Fly ash contains crystalline phases of quartz and multite. Fly ash has the highest Si/Al ratio of 1.9. The Si/Al ratios of red mud and kaolin are 0.3 and 1.2, respectively. The admixing of kaolin with fly ash results in an increase in the Si/Al ratio of materials, which is believed to improve the geopolymerization process and increase the mechanical properties of the materials.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Chemical** & **Kaolin** & **Fly Ash** & **Red Mud** \\
**Composition** & **(wt\%)** & **(wt\%)** & **(wt\%)** \\ \hline SiO\({}_{2}\) & 48.85 & 40.87 & 6.01 \\ Fe\({}_{2}\)O\({}_{3}\) & 0.64 & 6.44 & 48.29 \\ Al\({}_{2}\)O\({}_{3}\) & 35.45 & 18.44 & 16.16 \\ TiO\({}_{2}\) & 0.03 & 0.77 & 6.44 \\ MnO & 0.18 & 0.06 & 0.08 \\ Na\({}_{2}\)O & 0.05 & 0.20 & 4.27 \\ K\({}_{2}\)O & 1.47 & 3.32 & 0.09 \\ CaO & 0.06 & 1.18 & 3.35 \\ MgO & 0.04 & 1.22 & 0.13 \\ P\({}_{2}\)O\({}_{5}\) & 0.04 & 0.11 & 0.19 \\ SO\({}_{3}\) & \textless{}0.01 & 1.02 & 0.54 \\ LOI (Loss on ignition) & 12.98 & 26.27 & 14.30 \\ \hline \end{tabular}
\end{table}
Table 1: Chemical composition of red mud, kaolin, and fly ash.  
Figure 1: XRD diffractograms of red mud (RM), kaolin (Kn), and fly ash (Fa) (Q: quartz; M: multite; K: kaolinite; I: illite; H: hematite; S: sodalite; G: gibbsite).  
The particle size distributions of kaolin, red mud, and fly ash were determined using a LA-960 laser scattering particle size distribution analyzer (Horiba Scientific, Kyoto, Japan), as shown in Figure 2a. The particle size diameter distribution of RM, Kn, and FA is in the range of 0.45-200 \(\upmu\)m, 0.15-89 \(\upmu\)m, and 0.67-229 \(\upmu\)m, respectively. The median particle size (D\({}_{50}\)) of red mud, kaolin, and fly ash, which indicates that 50% of total particles are smaller than this size, is 2.5 \(\upmu\)m, 2.6 \(\upmu\)m, and 26.1 \(\upmu\)m, respectively. The D\({}_{90}\) values of red mud, kaolin, and fly ash are 51.5 \(\upmu\)m, 29.9 \(\upmu\)m, and 58.9 \(\upmu\)m, respectively. The morphology of kaolin, fly ash, and red mud is presented in SEM pictures in Figure 2b-d. Fly ash exhibits a sphere shape, while kaolin has a plate-like structure. Red mud consists of irregularly shaped aggregates formed from smaller particles.