## 1 Introduction  
Zirconia-based ceramics have been used as dental implants [1] in addition to framework underneath the brittle veneering porcelain in the fabrication of fixed dental prostheses, such as crowns and bridges [2]. As the aesthetic properties of zirconia-based ceramics improve, they have also been used to construct the whole crown and bridge works in combination with computer-aided design/computer-aided manufacturing (CAD/CAM) [3]. Pure zirconia has three polymorphic forms at atmospheric pressure: monoclinic from room temperature until 1170 \({}^{\circ}\)C, tetragonal (1170-2370 \({}^{\circ}\)C) and cubic (2370-2680 \({}^{\circ}\)C) [4]. However, pure zirconia encounters a setback whereby upon cooling of the polymorphs, large volume change occurs, which would result in the cracking of the brittle products. Incorporation of a small percentage of cubic oxide stabilizers such as yttria (Y\({}_{2}\)O\({}_{3}\)), ceria (CeO\({}_{2}\)), magnesia (MgO), and calcia (CaO) to zirconia can delay the creation of the transformation phase [5, 6]. Subsequently, the zirconia crystals in their tetragonal or cubic shape at room temperature remain stable. Zirconia-containing ceramic systems currently used in dentistry are yttrium cation-doped tetragonal zirconium polycrystals (3Y-TZP), magnesium cation-doped partially stabilized zirconium (Mg-PSZ), zirconium-toughened alumina (ZTA) and ceria-stabilized zirconia-alumina (CeTZP/A) [3, 7, 8]. Yttrium partially stabilized tetragonal zirconia polycrystalline (Y-TZP) bridge framework with a high fracture toughness, from 5 to 10 MPa m\({}^{1/2}\), and a flexural strength of 900-1400 MPa [9, 10] can be fabricated by milling either fully sintered blocks or partially sintered blocks/green blocks using computer-aided design/computer-aided manufacture (CAD/CAM) procedures [11]. However, yttrium-stabilized zirconia has the disadvantage of inferior machining associated with wear of the tool due to its superior hardness. Partially sintered Y-TZP produces extensive sintering shrinkage during the post-sintering process, which might affect the final fit of the zirconia framework [12]. Y-TZP also suffers from low-temperature aging degradation (LTAD) caused by phase transformation [13]. Ce-TZP/A is the toughest dental ceramic material available, with a fracture toughness of 19 MPa m\({}^{1/2}\), and a flexural strength of 1400 MPa [8]. Although alumina and ceria, as stabilizing oxides, reduce the susceptibility of zirconia to LTAD [14], they can cause wear of the milling machine if milled fully sintered, due to their superior hardness. Therefore, other partially and fully stabilized zirconia, for example Ca- PSZ, may be used as suitable replacement materials for dental application. Studies have been conducted to produce calcia-doped zirconia in biomedical application such as for hip and knee replacement. [PERSON] in 2008 sintered Ca-PSZ for a hip replacement implant. The material exhibits modest fracture toughness (6 MPa m\({}^{1/2}\)). This property is better than commercial grade alumina, but lower than Y-TZP and Ce-TZP/A [15]. Therefore, Ca-PSZ is a possible material that has a better combination of mechanical properties than other commercially available biocercamics, as it can give optimum strength but at the same time does not wear the CAD/CAM milling machine. [PERSON] and [PERSON] in 2016 reported that calcia-doped zirconia exhibits all the polymorphism seen in the yttria-doped zirconia ceramics, but can be produced at lowered costs and in greater abundance due to the accessibility of calcium precursors in comparison to yttrium [16]. In most cases, the particles sizes of stabilizers are not clearly specified. Moreover, the reports have mostly focused on the synthesis of the calcia-doped zirconia, but the use of nano-CaO derived from cockle shells to stabilize zirconia for dental application has not been reported. In our previous study, spherical nano-CaO was successfully extracted from cockle shells [17]. Cockle shells are a natural source of nano-CaO and are readily available as waste.