## 3 Results
### Characterization of Diatomic Powders, Diatomite/Sodium Alginate Dispersions and Resultant Granules
The diatomite-derived porous granules were obtained by dripping droplets from the diatomite/sodium alginate dispersions into a gelling bath of CaCl\({}_{2}\), as shown in Figure 1. The process begins with liquid droplets that are detached from the capillary, initially having a tear-like shape. During the granulation-falling process, these droplets evolve into an egg-like shape (ellipsoid) before eventually assuming a spherical shape due to the surface tension effect [40,41]. When considering specific granulation conditions, the ability to maintain a consistent shape during the process of droplet granulation and falling is likely determined by the properties of the diatomite/sodium alginate dispersion, particularly the viscous forces exerted by the fluid [40,41]. Upon contact between the sodium alginate in the liquid droplets and the Ca\({}^{2+}\) cations in the gelling bath, localized cross-linking rapidly initiates on the droplet surface, forming an external primary membrane. This was facilitated by the rapid gelation kinetics in the ion exchange process between Na\({}^{+}\) ions interacting with the COO\({}^{-}\) units of sodium alginate and Ca\({}^{2+}\) ions in the gelling bath [11]. Each Ca\({}^{2+}\) cation bound to four guluronate monomers [30,31,32,33], leaving encapsulated water, SiO\({}_{2}\) particles interacting with the monomer units of sodium alginate [61], and the remaining sodium alginate chains towards the core of the droplets. Subsequent layers of sodium alginate chains became cross-linked through the continuous inward diffusion of Ca\({}^{2+}\) (aging process), promoting homogeneous gelation [11,62], particularly at high gelling bath concentrations (minimum 0.3-2 wt.% CaCl\({}_{2}\)[11,40]). Additionally, the particles were brought closer together during the gelling process, reducing the inter-particle space and the mass of bound water (Figure 1b) [11]. In previous studies, the extent of the ion exchange equilibrium between Na\({}^{+}\) and Ca\({}^{2+}\) was qualified by the degree of substitution, which is related to the interactions between COO' groups and Ca\({}^{2+}\) cations forming COOCa units over the crosslinking period (immersion time in CaCl\({}_{2}\) bath). Research conducted with sodium alginate films (prepared from a casting solution of 1.76 wt.% sodium alginate) cross-linked using a 0.55 wt.% CaCl\({}_{2}\) solution revealed that the degree of substitution reached a plateau after 10 min or cross-linking. This indicated a constant presence of COOCa units, leaving approximately 10% of sodium ions unreplaced while maintaining a constant concentration of Ca\({}^{2+}\) cations in the bath [33]. Moreover, it is suggested that the initial stages of the gelation are accelerated by the accessibility of the alginate chains to Ca\({}^{2+}\) cations. It is important to note that extended aging times (ranging from 30 min to 12 h) were employed to ensure the hardening and proper handling of the resulting granules, depending on the experimental conditions [11,34,40]. After undergoing the aging and cleaning processes (Figure 4a), the resulting gel granules from the D04 and D44 dispersions exhibited distinct shapes with tear-like and pear-like characteristics, respectively. As the diatomite solid content in the dispersions increased (i.e., higher viscosity), the gel granules transitioned from tear-like (D04) to egg-like shapes (D08-D40,) and eventually approached a more spherical shape in the case of the D43 dispersion. However, during the subsequent processing steps, the final shape of the granules was slightly modified. This was observed when examining the D12 specimens, which appeared as ellipsoid-shaped granules with a small protrusion after the drying and heat treatment processes (Figure 4b,c), despite the absence of the protrusion in the gel state (Figure 4a).