## 3 Results and Discussion  
### Phase Analysis  
Figure 4 shows t the XRD spectra for all the specimens compact sintered at 900 \({}^{\circ}\)C, including the as-received HAp as a reference. The patterns indicated the presence of well-crystallized phases in all specimens, where higher crystallinity is observed in the as-received HAp compared to others. No apparent structural transition or peak shifts occur in the specimen as-received HAp, HM10-1.00 and HM30-1.25, indicating a major HAp phase was present without apparent secondary phases. The HAp reference specimen and HM10-1.00 displayed main character peaks at 23.26\({}^{\circ}\), 32.56\({}^{\circ}\), 33.32\({}^{\circ}\), and 47.06\({}^{\circ}\) 20, which correspond to the hexagonal structure of the hydroxyapatite phase (Powder Diffraction File-PDF No. 01-086-0740). HM30-1.25 specimen indicates the formation of the monoclinic HAp phase through the peak's determination at 23.49\({}^{\circ}\), 31.95\({}^{\circ}\), 34.12\({}^{\circ}\), 46.98\({}^{\circ}\), and 68.24\({}^{\circ}\) 20 (Powder Diffraction File-PDF No. 01-089-4405) [45]. To better elucidate the formation of the monoclinic HAp and taking account that the XRD characteristic peaks (at dominant peaks) of this phase overlap with the hexagonal by geometric similarity, it was necessary to carry out thorough observations on the identification of the peaks at high angles (20) [45; 46]. Reference peaks from the hexagonal HAp PDF Card indicate that no peaks can be observed within the 20 range of 67\({}^{\circ}\) to 69\({}^{\circ}\). However, peaks with lower intensity still can be observed within the 20 range indicating the formation of a monoclinic structure. Figure 5 shows the magnified region of all HAp phase specimens, where HM20-1.25 and HM30-1.25 indicate the presence of a peak at 67.71\({}^{\circ}\) and 68.38\({}^{\circ}\) 20 respectively. [PERSON] [45] reported that 97% of the related scientific works observed that almost all the crystallographic analyses referring to natural and synthetic HAp have shown, exclusively, the presence of the hexagonal crystalline phase, while only a few articles reported the probable presence of the monoclinic HAp. The monoclinic phase is thermodynamically more stable and mechanically superior to others, making it more appropriate for various medical applications. A similar finding was also reported by [PERSON] et al. who obtained the monoclinic HAp phase alongside the hexagonal phase on the specimen sintered at 900 \({}^{\circ}\)C [47]. Figure 4: Phase diffractogram of mix design inputs of MK/HAp sintered at 900 \({}^{\circ}\)C.  
Specifically, the specimen HM20 prepared at the 1.25 and 1.50 L/S ratios indicates the existence of the HAp phase alongside the calcium phosphate (Ca\({}_{2}\)P\({}_{2}\)O\({}_{7}\)) crystalline phase (Powder Diffraction File--PDF No. 00-033-0297), and is considered as a byproduct of the chemical reaction [45], while other specimens indicate that the peak shifted when sintered at 900 \({}^{\circ}\)C. The pattern in HM10-1.25, HM10-1.50, and HM20-1.00 indicates the presence of a well-crystallized structure belonging to sodium calcium phosphate silicate (PDF No. 00-033-1229) associated with the presence of the calcium phosphate byproduct, while HM30-1.50 shows the phase transformed to calcium hydrogen phosphate hydrate (Powder Diffraction File--PDF No. 00-011-0184) which may not be suitable for medical application especially in bone regeneration because of its lower C/P ratio of 1.33. These phase formations and phase transformations occurred due to the incomplete activation reaction during the initial preparation of the activated-HAp. This phenomenon refers to the situation where not all of the reactive silicate species were transformed into activated MK/HAp that might be contributed from chemical compositions of the metakaolin, activator, and HAp. Furthermore, the mix design input also influences the structural phase of the monoclinic and hexagonal hydroxyapatite as well as other phases.