## 4 Discussion  
Several studies have worked on stabilizing zirconia with CaO [19; 20; 21; 22] and on obtaining CaO from cockle shells [23; 24; 25; 26; 27]. However, to the authors' knowledge, there has not been any work on the use of CaO obtained from cockle shells to stabilize zirconia for dental application. In the current study, XRD analysis showed that the ZrO\({}_{2}\) was successfully stabilized with both the nano-CaO derived from cockle shells and also commercial nano-CaO (Ca-SZ). Different sintering temperatures (1200 \({}^{\circ}\)C, 1300 \({}^{\circ}\)C, 1400 \({}^{\circ}\)C, and 1500 \({}^{\circ}\)C) were used to prepare Ca-SZ pellets. Physical and mechanical properties of Ca-SZ(CS) were compared with Ca-SZ(CC). The morphological micrographs of Ca-SZ were characterized by SEM in order to deduce its surface structure and the average particle size. The particle sizes of Ca-PSZ(CS) and Ca-SZ(CC) obtained in the current study were 267 nm and 272 nm, respectively. The particle size of CaO derived from cockle shells ranged from 22 to 70 nm [17] and the particle size of the commercial CaO was <160 nm as compared to the 0.6-\(\upmu\)m grain size of yttria used in a previous study [28]. The smaller particle size of the CaO derived from cockle shells might provide a larger surface area for CaO to be uniformly dispersed on the surface of ZrO\({}_{2}\) and therefore may enhance the mechanical properties of the material [29], hence making the material more resistant to degradation or transformation [28]. The application of external heat during sintering enables the Ca-PSZ particle to be closely packed, with pores or voids removed, leading to shrinkage of the materials and forming more compact structures [18]. The voids and spaces shown on the polished surface of both Ca-PSZ pellets when sintered at lower temperatures suggests insufficient heat energy supplied in order to densely pack the materials, leading to an irregular surface upon polishing. Ca-PSZ(CS) sintered at 1400 \({}^{\circ}\)C showed the glossiest appearance with minimum surface crack, in addition to the smooth texture, which is indicative of sufficient and optimum heat energy during the sintering of the material. Figure 11: Vickers hardness of cross-section of Ca-SZ(CS); red line and Ca-SZ(CC); blue line when sintered for 2 h at different temperatures of 1200, 1300, 1400 and 1500 \({}^{\circ}\)C.  
The density of the materials is inter-correlated with the porosity of the microstructure. The incorporation of the Ca particle in ZrO\({}_{2}\) solid solution was probably the main reason the density of the prepared material increased for both groups. The increase in density of the prepared materials as the sintering temperature increased was in agreement with other researchers [18; 30; 31]. The slight enhancement of the density of Ca-SZ(CS) was in accordance with the SEM image in Figure 6, showing a compact structure with smooth morphology. However, the decrement of the density of Ca-PSZ for both specimens over 1400 \({}^{\circ}\)C was probably due to surface diffusion phenomenon as previously reported [32; 33]. The highest flexural strength was observed in Ca-SZ(CS) and Ca-SZ(CC) sintered at 1400 \({}^{\circ}\)C with the values of 1165 and 1152 MPa, respectively. These values were higher than another study, which reported the flexural strength value of 800-1000 MPa [3]. The higher flexural strength of the Ca-SZ in the present study was in accord with [PERSON] et al. who reported that the optimal sintering temperature for the third generation of ZrO\({}_{2}\) is in between 1400 \({}^{\circ}\)C and 1550 \({}^{\circ}\)C [34]. Therefore, the high flexural strength of Ca-SZ enables the material to afford restorations with less susceptibility to fracture, thus may be suitable for dental application. The highest compressive strength was achieved when the Ca-SZ was sintered at 1400 \({}^{\circ}\)C, with similar values of 4914 and 4913 MPa for Ca-PSZ(CS) and Ca-SZ(CC), respectively. The current result is in agreement with a previous report by [PERSON] et al. [35].