## 2 Materials and Methods
The average particle size of the diatomite-kaolin composite was controlled by wet ball-milling using alumina balls (diameter 10 mm, ball to powder ratio = 1:2) for 0, 4, 8, and 24 h. Typical polyurethane foams (SKB Tech, Ulsan, Korea) with pore densities of 45 and 80 PPI along with dimensions of 50 mm \(\times\) 50 mm \(\times\) 20 mm were used as sacrificial templates. A batch of diatomite-kaolin composite slurry was prepared to coat the sacrificial polymer template, and consisted of 50-80 g of diatomite (Celite 499, Celite Corp, Barbara, CA, USA), 15-24 g of kaolin (Kaolin, Sigma-Aldrich, St. Louis, MO, USA) as a strength enhancer, 100 mL of distilled water as a solvent, 10 g of polyvinyl alcohol (PVA) as an organic binder (PVA 500, Junsei Chemical, Tokyo, Japan). The mixing ratio of kaolin was set to 30 wt.% of diatomite. The effect of the kaolin mixing ratio on the properties of the reticulated porous diatomite-kaolin composite will be reported in a separate study. To uniformly mix the diatomite-kaolin composite with the other additives, the diatomite-kaolin composite slurry was ball-milled again for 4 h using alumina balls. The reticulated porous diatomite-kaolin composite specimens were prepared by the conventional replica method. A sacrificial polymer template (polyurethane foam) was soaked in the diatomite-kaolin composite slurry until its internal voids, cells, and pores were completely coated with diatomite-kaolin composite particles. Then, the impregnated polyurethane foam was squeezed to remove any excess diatomite-kaolin composite slurry that did not participate in the coating of the strut walls, and to enable the formation of a thin diatomite-kaolin composite coating layer over the struts of the polyurethane foam. After that, the impregnated polyurethane foam was fully dried for 24 h at 25 \({}^{\circ}\)C. Subsequently, the dried specimen was heat-treated at 400 \({}^{\circ}\)C (heating ramp was 5 \({}^{\circ}\)C/min) for 1 h to burn off the organic binder and the sacrificial template, and finally sintered for 1 h at 1200 \({}^{\circ}\)C (heating ramp was 5 \({}^{\circ}\)C/min). After sintering, the polyurethane foam coated with the diatomite-kaolin composite slurry transformed into the reticulated porous diatomite-kaolin composite. The average particle sizes of diatomite and kaolin were determined by a particle size analyzer (LSTM 13 320 MW, Beckman Coulter, Brea, CA, USA). The pore structures and characteristics of the reticulated porous diatomite-kaolin composites were analyzed using scanning electron microscopy (SEM, JSM-5800, JEOL, Tokyo, Japan) and mercury porosimetry (Autopore IV 9510, Micromerities, Norcross, GA, USA). The viscosity was measured using a rotational rheometer (Discovery HR-1, TA Instruments, New Castle, DE, USA) at 25 \({}^{\circ}\)C. The three-dimensional microstructure was investigated by micro-computed tomography (micro-CT, XT H 160, voxel size = 3 \(\upmu\)m, Nikon, Tokyo, Japan). The compressive strengths of the reticulated porous diatomite specimens were measured with a fixture using an Instron 4206 (Instron, Norwood, MA, USA) testing system. Specimens with dimensions of 20 mm \(\times\) 20 mm \(\times\) 20 mm were machined for the compressive strength tests.