## 3 Results and Discussion
Because G9 MK1 geopolymer only contents 2% MK in the mixture, there are insufficient Al and Si ions dissolved from MK to form silico-aluminate structure, and thus, results in the decrease of flexural strength. For G5 MK5 and G7 MK7 samples, the flexural strength increase from 5.2 MPa to 5.7 MPa and 2.4 MPa to 4.1 MPa,  
Figure 4: Setting time of geopolymer slurry prepared with different SiO\({}_{2}\)/K\({}_{2}\)O molar ratio solutions. Figure 3: The apparent viscosity of geopolymer slurry. respectively, as the SiO\({}_{2}\)/K\({}_{2}\)O modulus of alkali solution increases from 0.57 to 0.82. It is well known that the activation of metakaolin by alkali solution can form AlO\({}_{4}\) and SiO\({}_{4}\) tetrahedral units [25]. During the condensation of tetrahedral aluminum-silicate units, alkali metal ions (Na\({}^{+}\) or K\({}^{+}\)) will be needed to balance the charge associated with tetrahedral Al [26]. Therefore, the lower concentration of K\({}^{+}\) ions in the higher SiO\({}_{2}\)/K\({}_{2}\)O modulus solution can better form the 3-D alumino-silicate structure [27]. Figure 6 shows the fired flexural strength of geopolymer slurries after sintering at 1200 \({}^{\circ}\)C for 30 min. The fired strength of geopolymer slurries generally has higher flexural strength than those without sintering process, as shown in Figure 4. This is because the semicrystalline behaviour of geopolymer will change into crystalline ceramic phases upon heating [28]. At sintering temperature, the formation of the liquid phase produces joining of the particles and reduction of the porosity and result in the improvement of flexural strength [29]. The fired strength of G5 MK5 prepared with 0.57-0.82 SiO\({}_{2}\)/K\({}_{2}\)O solutions are higher than G7 MK3 and G9 MK1 geopolymer slurries due to higher metakaolin content. According to the study conducted by [PERSON] [30], the porosity of a sintered geopolymer binder is directly related to the calcium carbonate content in the original mixture. The fired strength reduction of the ceramic shell is believed to be due to the outgassing of CO\({}_{2}\) through the thermal decomposition process, and thus increases the porosity of the shell. Figure 5: Green flexural strength of geopolymer slurry. Figure 6: Flexural strength of geopolymer slurry after sintering at 1200 \({}^{\circ}\)C.  
Figure 7 shows the morphologies of unheated and sintered geopolymers prepared with a 0.57 SiO\({}_{2}\)/K\({}_{2}\)O alkali solution. Generally, sintered geopolymer slurries have a much denser surface than that of unheated geopolymer slurries for each individual CC and MK mixing ratio. For the green specimens, no obvious cementing can be observed and a loose and porous structure with clear particle interface can be clearly identified. However, the green G5 MK5 exhibits a denser surface with less fractures than that of the G7 MK3 and G9 MK1 specimens. This is believed to be due to the high MK content that results in high degree geopolymeric reaction. For sintered specimens, particle interface disappears, and cementing surface can be observed. These dense surfaces shown in Figure 7d-f can be attributed to the liquid phase produced at a temperature higher than 900 \({}^{\circ}\)C joining particles and forming cementing surfaces, as shown in Figure 7a-c. [PERSON] et al. indicated that the structural change of MK-based geopolymer at a temperature above 850 \({}^{\circ}\)C is attributed primarily to viscous sintering [31]. [PERSON] et al. also observed an exothermal due to leucite crystallization from a sol-gel-based precursor over the range of approximately 1000-1200 \({}^{\circ}\)C [31]. The crystallization of geopolymer gel should also exist in sintered specimens and results in porosity reduction during the crystallization process. Figure 7: Scanning electron microscopy (SEM) micrograph of different carbonate/metakaolin ratios of geopolymer before (**a**–**c**) and after (**d**–**f**) sintering at 1200 \({}^{\circ}\)C.