## 3 Results
Strut walls with a circular cross-section  
Figure 1: Optical images of (**a**) sacrificial polymer templates (in this study, polyurethane foam) with pore densities of 10, 25, 45, 60, and 80 pores per inch (PPI, from left to right), used in a typical replica method. (**b**) Recitulated porous diatomite–kaolin composite specimens with pore densities of 10, 25, 45, 60, and 80 PPI (from left to right). of the sacrificial polymer template can be prepared by three-dimensional (3D) printing techniques associated with computer-aided design (CAD) software. Because adopting an expensive 3D printing technique would be inconsistent with the aim of introducing a low-cost, natural-based diatomite-kaolin material into a reticulated porous ceramic, in this study we focused on the optimization of processing conditions such as solid loading, average particle size, and pore density. Figure 2: Scanning electron microscopy (SEM) images of diatomite particles ball milled for (**a**) 0 h (as-received), (**b**) 4 h, (**c**) 8 h, and (**d**) 24 h; (**e**) particle size distributions of diatomite particles ball-milled for 0, 4, 8, and 24 h.  
Moreover, it is reasonable that the porous and irregular nature of the strut walls of a reticulated porous diatomic-kaolin composite would severely reduce the compressive strength. Therefore, it is interesting to investigate whether the more refined strut walls (obtained by increasing the solid loading and reducing the average particle size of the diatomic-kaolin composite slurry) of the reticulated porous diatomic-kaolin composite can enhance the compressive strength. This aspect will be discussed later. It is difficult to judge whether a reticulated porous diatomic-kaolin composite specimen has been prepared successfully or not. It is important to determine whether the density would contribute to the overall strength (when the diatomic-kaolin composite slurry is properly coated on the strut walls of the sacrificial polymer template) or not (when the slurry is distributed non-uniformly as debris located deep inside the reticulated porous ceramic specimen). Therefore, in this study, non-destructive micro-CT analysis was used to investigate the reticulated porous ceramic specimens. Figure 4a shows the three-dimensional microstructure of a typical reticulated porous diatomic-kaolin composite specimen (pore density = 45 PPI), reconstructed by micro-CT. A representative cross-section from the micro-CT reconstructions of the specimen is shown in Figure 4b. The image clearly shows that the reticulated porous diatomic-kaolin composite had well-defined inter-connected pore channels, as intended. Moreover, the figure does not show any evidence of voids or of a non-uniform distribution of strut walls inside the composite, which would be frequently observed when using the un-optimized ceramic slurry in the preparation. Figure 4c shows the three-dimensional microstructure of a typical reticulated porous diatomic-kaolin composite specimen (pore density = 80 PPI), reconstructed by micro-CT. A representative cross-section from the micro-CT reconstruction of the specimen is shown in Figure 4d. The image clearly shows that the reticulated porous diatomic-kaolin composite had a well-defined inter-connected pore structure, similar to the sample with a pore density of 45 PPI. The pore channels beneath the surface were not substantially blocked, which shows the effectiveness of the diatomic-kaolin composite slurry that had penetrated deep inside the sacrificial polymer template. Moreover, the presence of the blocked pore channels is not a key factor, unlike in conventional reticulated porous ceramics, because the residual diatomic-kaolin particles cannot actually block the pore channels (even if they were intended to) owing to their porous and irregular nature. Generally, when the pore density of the sacrificial polymer template reaches approximately 60 PPI, the processing window of a ceramic slurry composition (in terms of parameters such as solid loading and viscosity) becomes narrow.