## 1 Introduction  
The human oral environment undergoes a dynamic process of demineralization and remineralization every day. The oral environment changes from neutral to acidic due to metabolites produced by bacteria in the mouth. An acidic oral environment causes demineralization of the tooth surface, leading to caries, periodontal disease, and tooth sensitivity. In the demineralization process, calcium phosphate (CaP) crystals on the surface of teeth are dissolved by the acidic environment, causing a loss of calcium ions. However, the oral environment can be made to return to neutral by performing oral cleaning, thus halting tooth surface demineralization. Subsequently, the free calcium and phosphate in the saliva will re-enter the tooth surface to form a protective layer of CaP crystals in a remineralization process [1; 2; 3]. Therefore, calcium ions are a crucial element of the oral environment since their presence can promote the formation of CaP crystals inside the teeth during the remineralization process.  
It is well known that the outer enamel of human teeth consists of micron-sized calcium carbonate (CaCO\({}_{3}\)) deficient hydroxyapatite (HAP) microcrystals [4]. The main component of the outer enamel is HAP (around 97%) [5; 6]. Recent studies have found that CaP, such as amorphous calcium phosphate (ACP), beta-tricalcium phosphate (\(\beta\)-TCP), and alpha-tricalcium phosphate (\(\alpha\)-TCP), are similar to the natural enamel [7; 8; 9]. Among all compounds of CaP crystals, \(\beta\)-TCP and ACP are the most commonly used in oral care applications [10; 11]. Although most CaP additives need to release calcium ions under acidic conditions, \(\beta\)-TCP can provide the best calcium ion release effect even in a neutral pH environment [12]. Accordingly, \(\beta\)-TCP is regarded as a stable tricalcium phosphate at room temperature. [PERSON] et.al. [12] also demonstrated that using \(\beta\)-TCP can increase calcium levels in saliva and promote the remineralization of tooth surface. In addition, it was also found that ACP presented as a unique artificial HAP [4]. Previously published studies have shown that ACP has good bone conduction and biodegradability, enhances cell proliferation ability, and promotes cell adhesion [13; 14]. Moreover, ACP and its composites are considered to be an effective remineralization agent that is often used for remineralization in the dental fields [15; 16; 17; 18].  
As stated above, an adequate increase in the amount of free calcium and phosphate in the oral environment plays an important role in facilitating tooth surface remineralization during oral cleaning. From an oral care point of view, it would be beneficial if daily use toothpastes could be developed with the addition of various remineralization agents to prevent the formation of caries without sacrificing any useful properties. Hence, the present study reports on the synthesis and characterization of \(\beta\)-TCP and ACP derived from coral-based raw material through a two-stage sintering method. We aimed to investigate the concentration of calcium in toothpaste incorporated with \(\beta\)-TCP and ACP powders. We hypothesized that adding \(\beta\)-TCP and ACP to toothpaste would increase the release of free calcium in the oral environment, which might be potentially beneficial for the tooth remineralization process.