## 2 Materials and Methods
### Physicochemical Characterization  
The elemental composition analysis was carried out using the ARL Quant'X (Thermo Scientific Inc., Waltham, MA, USA) EDXRF spectrometer analyzer. The general chemical  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Mix** & \multicolumn{3}{c}{**Solid**} & \multicolumn{3}{c}{**Liquid (1:2)**} & \multicolumn{2}{c}{**L/S Ratio**} & **Curing Condition** \\ \hline  & **Hap** & **MK** & **Total Solid** & **8 M NaOH** & **Na2 SiO3** & **Total Liquid** & & **Temperature/Time** \\  & **(g)** & **(g)** & **(g)** & **(g)** & **(g)** & **(g)** & & \\ \hline HM10-1.00 & 100 & 10 & 110 & 36.67 & 73.33 & 110.00 & 1.00 & 60 \({}^{\circ}\)C/2 d \\ HM10-1.25 & 100 & 10 &  
composition of the starting materials obtained from the EDXRF spectrometer analyzer is presented in Table 2. The oxide ratio of Ca:P is 3.29 while the Si:Al ratios are found to be 1.41 and 1.52 for kaolin and metakaolin, respectively. The mineralogical analysis of the raw materials and activated HAp were determined by a Bruker D2 Phaser X-ray diffractometer (XRD) (Bruker, Germany) with Cu K\(\alpha\) radiation (40 kV and 30 mA). Figure 2 shows the XRD patterns of kaolin, metakaolin, and hydroxyapatite. As can be seen, the XRD pattern of kaolin (Figure 2a) exhibits two prominent reflections at 12.33\({}^{\circ}\) and 24.90\({}^{\circ}\) 20, and other reflections at 19.84\({}^{\circ}\), 34.93\({}^{\circ}\), 38.43\({}^{\circ}\), and 55.11\({}^{\circ}\) 20. These reflections were matched with the PDF card number 01-089-6538, as reported by [PERSON] et al. [36]. The results indicate that kaolinite was the predominant phase in the powdery bulk. Heat-treated kaolin at 750 \({}^{\circ}\)C was converted into metakaolin, where the X-ray pattern showed a diffuse halo peak with 20 between 18-38\({}^{\circ}\) (Figure 2b), which is the characteristic of the amorphous phase as presented in metakaolin [40,41]. The main components of metakaolin were SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), although it contained roughly 1.10 wt\(\%\) of TiO\({}_{2}\) as a major impurity which is identified as anatase in the XRD analysis [42]. Meanwhile, Figure 2c shows the XRD pattern of the raw HAp powder in which the pattern matched the hydroxyapatite phase as per PDF card number 00-009-0432 (25.85\({}^{\circ}\), 31.71\({}^{\circ}\), and 32.90\({}^{\circ}\) 20) [43]. There was no apparent secondary phase observed from the pattern which determined the high purity of the HAp powder. The Fourier transform infrared spectroscopy (FT-IR) measurement was carried out to confirm the dihydroxylation of water molecules after the heat treatment of raw kaolin. The test was conducted on an FT-IR analyzer (Perkin Elmer, Waltham, MA, USA) using normal potassium bromide (KBr) KBr pellets, and about 2 mg of the specimen was ground and pressed into disc form to evaluate the shifting of functional groups. The analyses were performed in the spectral range from 4000 to 400 cm\({}^{-1}\) and the number of scans was 60. Figure 3 shows the FT-IR analysis of kaolin and metakaolin. The FT-IR spectra of kaolin showing the absorption bands at 3695-3621 cm\({}^{-1}\) expressed the stretching vibrations of -OH groups of kaolinite. The bands located at 3436 and 1631 cm\({}^{-1}\) corresponded, \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Chemical Composition** & **CaO** & **P\({}_{2}\)O\({}_{5}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **Na\({}_{2}\)O** & **TiO\({}_{2}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **MgO** & **K\({}_{2}\)O** \\ (**Compound**) & & & & & & & & & \\ \hline HAp & 61.35 & 18.63 & 14.00 & 0.38 & 0.10 & - & - & 5.20 & - \\ Metakaolin (MK) & - & - & 38.70 & 59.00 & 0.40 & 1.10 & 0.60 & - & 0.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Composition of HAp, kaolin and metakaolin. Figure 2: The XRD patterns of: (**a**) kaolin, (**b**) metakaolin, and (**c**) hydroxyapatite.