## 3 Results and Discussion
### Strength and Permeability of Geopolymer Ceramic Shell  
For flexural strength and permeability tests, geopolymer ceramic shells of are prepared with six layers of coating and sprinkling using a wax pattern of 170 mm \(\times\) 40 mm \(\times\) 10 mm. After dewaxing, two ceramic shell plates can be produced for every wax pattern. The first layer uses only zircon slurry and zircon sand sprinkling and the second layer uses zircon slurry and mulite sand sprinkling. For the third to sixth layer of the ceramic shell, the original four layers of mulite coating and sprinkling are replaced with one to four layers of geopolymer slurry (G5 MK5 and 0.57 SiO\({}_{2}\)/K\({}_{2}\)O alkali solution) and WS sand sprinkling, as shown in Figure 1.  
Figure 8 shows the green and fired strength of the ceramic shell specimens made with various geopolymer coating layers. The flexural strength of the specimens increases from 8.46 MPa to 11.2 MPa as the number of geopolymer coating layer increases from one to four layers. The flexural strength of the ceramic shell prepared with original mulite coating and sprinkling can only reach 6.35 MPa. The flexural strength improvement can be attributed to the formation of a geopolymer gel that cements the WS sand and forms a denser matrix. The more geopolymer coating layers applied to the ceramic shell specimens, the higher the flexural strength obtained. On the contrary, the fired strength (sintered at 1125 \({}^{\circ}\)C for 30 min) of ceramic shell specimens decrease from 9.1 MPa to 5.61 MPa as the geopolymer coating layers increase from none to four layers, as shown in Figure 7. The decrease of flexural strength is due to the outgassing of CO\({}_{2}\) during CaCO\({}_{3}\) (CC) decomposition, and thus increases the porosity of the ceramic shell. [PERSON] et al. indicated that the decomposition of CaCO\({}_{3}\) takes place between 635 \({}^{\circ}\)C and 865 \({}^{\circ}\)C and accompanies outgassing of CO\({}_{2}\)[32]. According to [PERSON]'s study, about 5.5% weight loss was observed at 825 \({}^{\circ}\)C in a CaCO\({}_{3}\) thermal analysis [30]. Therefore, ceramic shell specimens with more geopolymer coating layers have lower fired flexural strength reached around 6 MPa.  
Permeability of the ceramic shell is required to promptly evacuate air and other entrapped gases during metal filling (the method to determine hot permeability and strength of ceramic shell moulds [14]). Ceramic shells with low permeability may entrap the air, and thus, casting defects occur For permeability measurement, the ceramic shell specimen was fixed and sealed to a chamber that has a constant airflow at 1 bar pressure. After turning off the airflow, the pressure inside the chamber and the time were recorded until the ambient pressure was reached. The effect per unit of mould shell thickness on the time required for the pressure to drop from 1 bar to 0 bar is shown in Figure 9. The experimental results show that as the number of replacement layers was replaced by only the outside half layer (5L-G0 HL) to three and a half layers (2L-G3 HL), the time required for the pressure drop from 1 bar to 0 bar for a certain thickness of mould shell had a shorter trend and the same trend as  
Figure 8: Flexural strength of shell mould at room temperature and after sintering at 1125 \({}^{\circ}\)C.  
the pressure drop time. The high permeability of ceramic shells with geopolymer coating is attributed to the decomposition of CaCO\({}_{3}\) that increases the porosity of the specimen. Since the geopolymer coating has better permeability, the more layers it replaced, the shorter the pressure relief time required, and the better the permeability.