## 4 Discussion  
In the present study, we prepared TCP and ACP powders from coral-based raw material and added the powders into commercially available toothpaste in concentrations of 10% and 25%. The quantity of free calcium released from the toothpaste was then measured. It was found that the raw material had almost completely changed from a CaCO\({}_{3}\) structure to a CaP structure by means of a two-step sintering method. It is known that the raw material made from coral is mainly comprised of CaCO\({}_{3}\) and calcium oxide (CaO) and that CaCO\({}_{3}\) and CaO are the main chemical compounds supporting the bone and tooth remineralization process [7; 19]. Previously published studies indicated that high temperature and high pressure could replace the CaCO\({}_{3}\) of coral with HAP or TCP, which is a process known as replaminerform [20; 21]. In accordance with the previous studies, here, we found that high temperature and high pressure promoted the conversion of HAP to TCP following the previous chemical reaction. We also found that the structure of the ACP was changed to \(\alpha\)-TCP and \(\beta\)-TCP at 800 and 900 \({}^{\circ}\)C [22; 23]. A similar result was also seen in previous studies discovering that ACP could be converted to \(\alpha\)-TCP and \(\beta\)-TCP after sintering at 800 and 900 \({}^{\circ}\)C [24; 25]. We also found that the mixed TCP and ACP powders released a stable amount of free calcium in the pH 5 to pH 9 range. A previous study demonstrated that pH has a significant effect on ACP precipitation during the synthesis process, in which some crystalline CaP forms at pH 8 to pH 9 and ACP forms at pH 10 to pH 11 [23]. The oral environment, on the whole, is composed of saliva and gingival fluid, which are rich with calcium ions, phosphate ions, and fluoride ions. In a neutral pH, these components bring a dynamic equilibrium between the HAP in the teeth and in the oral fluid [26; 27]. When the pH decreases to acidic (pH below 5.5), the oral environment becomes undersaturated, causing HAP crystals to release from the teeth into the oral biofilm, resulting in mineral dissolution. In conditions where the pH increases to over 4.5, the increased release of PO\({}_{4}\)\({}^{3-}\) and Ca\({}^{2+}\) ions causes supersaturation in the oral fluid. This process leads to the precipitation of the minerals back to the enamel surface [28; 29]. Four commercial toothpastes were tested to evaluate the effect of free calcium release after being combined with TCP powder and TCP/ACP powder. We observed that the free  
Figure 5: Free calcium concentration of different brands of toothpaste containing (**a**) TCP powder only and (**b**) TCP/ACP mixed powder (*** \(p<\) 0.001). The toothpaste containing only TCP powder had a significantly higher calcium concentration than the normal toothpaste. Furthermore, the toothpaste containing the TCP/ACP mixed powder had a higher calcium concentration than the toothpaste containing only TCP powder. calcium concentration increased significantly no matter which brand of toothpaste was combined with the 10% or 25% TCP powder. The concentration of free calcium increased even more in toothpaste containing the mixed TCP/ACP powder. It was previously believed that ACP could be converted to \(\alpha\)-TCP and \(\beta\)-TCP through the sintering process [24; 25]. Some previously published studies discovered that ACP has a potential role in promoting the HAP assembly into highly-ordered structures [30; 31]. High-order HA architectures were detected when the initial nanosphere particles were aggregated, with the structure consisting of ACP shells and HA cores. Similarly, [PERSON] et al. [32] demonstrated that ACP contains a well-defined local structural unit with a compositional constancy covering the broad conditions of preparative solutions, indicating the presence of a core structure. Based on previous findings, we presume that a core-shell structure might have been formed during the sintering process in the present study.