## 3 Results
milled for 24 h with solid loadings from 50 to 80 wt.% (Figure 6d), The comparison shows that at the same solid loading, the viscosities of the diatomite-kaolin composite slurries ball milled for 24 h were higher, because of the average particle size of the diatomite-kaolin composite (which is directly related to their surface area). As the viscosity of a diatomite-kaolin composite slurry can be tailored easily with the aid of an appropriate surfactant, the absolute value of the viscosity may not be very meaningful. Therefore, we investigated the effect of processing conditions such as solid loading, pore density, and average particle size on the mechanical strengths and densities of the reticulated porous diatomite-kaolin composites. As discussed above, if the viscosity of a diatomite-kaolin composite slurry is too high, it is difficult for the slurry to completely fill the internal pores of a sacrificial polymer template. It is also very difficult to remove the excess diatomite-kaolin composite slurry while maintaining the formation of a thin and uniform ceramic coatings over the strut walls of the sacrificial polymer template. Therefore, we estimated that the conditions corresponding to the maximum viscosity (24 h ball milling of the diatomite-kaolin composite slurry with 80 wt.% solid loading) would not yield the highest compressive strength. To confirm the above hypothesis, we systematically measured the densities and compressive strengths of the diatomite-kaolin composite specimens. Figure 7a shows the densities of the reticulated porous diatomite-kaolin composite specimens prepared with different solid loadings (from 50 to 80 wt.%) and ball-milling times (from 0 to 24 h), which had a pore density of 45 PPI. As the solid loading amount or ball milling time increased, the densities of the specimens showed a corresponding increase. This shows that if the pore density is low enough, the increased viscosity can result in a linear increase in the applied mass (the diatomite-kaolin composite slurry), regardless of the compressive strength. Figure 7b shows the densities of the reticulated porous diatomite-kaolin composite with 80 PPI pore density, prepared with different solid loadings (from 50 to 80 wt.%) and ball-milling times (from 0 to 24 h). Unlike the samples with a pore density of 45 PPI in Figure 7a, as the ball-milling time reached 24 h, the densities of the specimens decreased. This suggests that it is difficult for the internal pores of the sacrificial polymer template to be completely filled with the highly viscous diatomite-kaolin composite slurry ball milled for 24 h.  
Figure 8a shows the compressive strengths of the reticulated porous diatomite-kaolin composite specimens with a pore density of 45 PPI, prepared with different solid loadings (from 50 to 80 wt.%) and ball-milling times (from 0 to 24 h). Similar to the trend of the densities, when the solid loading amount or ball-milling time increased, the compressive strengths of the specimens showed a corresponding increase. It was thus inferred that when the pore density was 45 PPI, the density could contribute  
Figure 7: Densities of reticulated porous diatomite–kaolin composites prepared from the composite particles ball milled for 0, 4, 8, and 24 h, at solid loadings ranging from 50 to 80 wt.%, with pore densities of (**a**) 45 PPI and (**b**) 80 PPI. to the compressive strength of the specimens. The compressive strengths of the reticulated porous diatomite-kaolin composite specimens with different solid loadings (50 to 80 wt.%) and ball-milling times (0 to 24 h), which had a pore density of 80 PPI, are shown in Figure 8b. Similar to the trend of the densities, when the ball milling time reached 24 h, the compressive strength of the specimen decreased. This suggests that when the pore density was 80 PPI, the density increases did not fully contribute to the compressive strength of the specimens.