## 3 Effect of Ca\({}^{2+}\) Source and Addition Method on Mineralized Products
### Results and Discussion
#### 3.2.4 SEM Observations  
The SEM images of samples from each group are shown in Figure 11.  
Figure 11: _Cont._  
The morphologies and structures of samples from each group can be observed in Figure 11.  
Group A1: Most of the mineralized products were spheres; some were single spheres and some were twin spheres. The size of a single sphere ranged between 5 and 20 \(\upmu\)m, while a few large spheres reached 30 \(\upmu\)m. The surfaces of the spheres were rough and showed clusters of triangular'scales', indicating that these spheres were formed by the growth of these triangular crystals from the center to the surface. In addition, there were a few hexahedral crystals with a size of 5 \(\upmu\)m.  
Group B1: Most of the mineralized products were triangular flat plates with sizes ranging from 5 to 20 \(\upmu\)m. The surfaces of most of the triangular crystals were smooth, although there were irregular crystals attached to some of the triangular crystals. In addition, there were some irregular flat plates between 15 and 20 \(\upmu\)m, slightly larger than the triangular flat plates, with irregularly shaped crystals attached to their surface. When the magnification was increased, it was observed that the triangular flat plates were not actually flat plates but rather blocks.  
Group A2: The mineralized products mainly took on three shapes: sphere, hexahedron, and irregular. The spheres were 5-20 \(\upmu\)m, exhibiting rough surfaces with irregular crystals attached. The hexahedrons were 10-20 \(\upmu\)m with smooth surfaces; some also had irregular  
Figure 11: SEM images of samples from each group. (**A1**) Ca(NO\({}_{3}\))\({}_{2}\), post-calcium-source method. (**B1**) Ca(NO\({}_{3}\))\({}_{2}\), pre-calcium-source method. (**A2**) CaCl\({}_{2}\), post-calcium-source method. (**B2**) CaCl\({}_{2}\), pre-calcium-source method. (**A3**) Ca(CH\({}_{3}\)COO)\({}_{2}\), post-calcium-source method. (**B3**) Ca(CH\({}_{3}\)COO)\({}_{2}\), pre-calcium-source method. (**C**) CaCl\({}_{2}\) + Na\({}_{2}\)CO\({}_{3}\).  
crystals attached. The irregular crystals exhibited different shapes and sizes ranging from 1 to 10 \(\upmu\)m; some of them were floccule, while others were 'cotton' shaped.  
Group B2: The mineralized products were mostly polyhedral crystals between 20 and 40 \(\upmu\)m with smooth surfaces. In addition, there were some individual tetrapyramidal crystals with sizes of 10-25 \(\upmu\)m; the surfaces of these crystals were also smooth.  
Group A3: The mineralized products were mostly spheres with sizes of 0.5-2.5 \(\upmu\)m. The surface of these crystals was smooth, where some were twin spheres and others formed spherical aggregates.  
Group B3: The mineralized products were mostly single spheres with a size of 2-40 \(\upmu\)m and rough surfaces, and irregular crystals were also attached to some spheres. A broken sphere was also observed, from which spherical crystals were formed via radiative growth from the center to the surface. In addition, we also observed a hexahedron surrounded by a sphere of 10 \(\upmu\)m with a rough surface and irregular crystals attached.  
Group C: The mineralized products were mostly square flaky aggregates with a size of 2-3 \(\upmu\)m and a smooth surface. In addition, there were a few 2 \(\upmu\)m cubes and 25 \(\upmu\)m pyramids.  
According to the conclusions from Sections 2.2 and 3.2, the mineralized products in Group B1, B2, B3, and C were all calcite; the mineralized products in Group A1 and A2 were calcite and vaterite; and the mineralized products in Group A3 were all vaterite. The calcite exhibited different morphologies, including triangle, polyhedron, sphere, square flaky aggregates, and pyramids, while the vaterite crystals were mostly spherical aggregates.  
In conclusion, Ca(CH\({}_{3}\)COO)\({}_{2}\) was found to be an optimal Ca\({}^{2+}\) source while the pre-calcium-source method was determined as the most appropriate addition method.