## 1 Introduction  
Calcium carbonate is an inorganic calcium salt which can be derived from shelled mollusks, limestone, coccolithophores, plant ashes, chalk, and marble [1; 2]. It is considered to be a primary source of CaO, which has been identified as a biological constituent of human bone. CaCO\({}_{3}\) is found abundantly in three natural polymorphs: calcite, vatrite, and aragonite. CaCO\({}_{3}\) nanoparticles with desired sizes, shapes, and morphologies has been extensively researched for their vast application in industrial, electronic, and agricultural fields [3]. Cockles are native to the coast of South East Asia (Thailand, Malaysia, and Indonesia) and are readily available as waste. Cockle shells are a natural reservoir of CaCO\({}_{3}\), which is important for biomedical purposes [4]. In recent years, many reports have been documented on the application of CaCO\({}_{3}\) nanoparticles derived from cockle shells waste product, such as in drug delivery [5], polymer industries [6], biomedical scaffolds [7], and in nanomedicine [8]. The exploitation of solid waste such as the cockle shell makes the process more ecological, green, and cost-effective. Nanoparticles are important for producing materials with good properties [9]. The small particle size provides a larger surface area for the particles to be uniformly dispersed, and therefore may enhance the mechanical properties of the material [10], hence making the material more resistant to degradation [11]. The physical and mechanical properties of these nanomaterials are dependent on their morphology [12]. Therefore, various methodologies have been devised to synthesize appropriate morphologies of these CaCO\({}_{3}\) nanoparticles. Spherical nanoparticles behave quite differently from layered silicates and fibers [13]. Their large surface area results in strong interfacial interactions between the filler and the polymer matrices in dental composite material [14], and provides them with their superior diffusion properties in various solvents [15]. Spherical nanoscale CaCO\({}_{3}\) fillers are most used in the preparation of nanocomposites and cementing materials for various uses [16]. The spherical particle structure is also important in producing a material with a higher glossiness and enamel-like reflection/refraction [17]. In dentistry, zirconia-based ceramics have been used as dental implants [18], and in crowns and bridges [19]. Pure zirconia has the disadvantage of cracking upon cooling, which can be avoided by adding a small percentage of oxide stabilizers like yttria (Y\({}_{2}\)O\({}_{3}\)), ceria (CeO\({}_{2}\)), magnesia (MgO), and calcia (CaO) [20; 21; 22; 23]. Yttrium-stabilized zirconia is tough, with a fracture toughness of 5 to 10 MPa m\({}^{1/2}\) and a flexural strength of 900-1400 MPa [24; 25], and it suffers from low-temperature aging degradation (LTAD) [26]. Although ceria-stabilized zirconia-alumina (Ce-TZP/A) can overcome the problem of LTAD, it is also among the toughest dental ceramic materials available, with a fracture toughness of 19 MPa m\({}^{1/2}\) and a flexural strength of 1400 MPa [24]. Both previously mentioned zirconia have the disadvantage of inferior machining associated with wear of the tool, due to their superior hardness. Therefore, other partially and fully stabilized zirconia, for example calcium oxide (CaO), may be used as replacement materials for dental application. A study has been conducted to produce calcia-doped zirconia for hip and knee replacement with modest fracture toughness (6 MPa m\({}^{1/2}\)) [27]. Therefore, calcia-stabilized zirconia is a possible material to give optimum strength, and at the same time it does not wear the milling machine. A study was also carried out to stabilize zirconia using CaO [28]. However, the particle size of stabilizers was not clearly specified. Furthermore, the shape of the CaO stabilizers produced was not spherical.