*3.3. Microstructure Analysis*

An SEM revealed the morphological features of kaolin-based geopolymer ceramic samples at sintering temperatures of (a) unsintered, (b) 900, and (c) 1100 ◦C, as shown in Figure 5. The unsintered kaolin showed the presence of well-defined clay platelets and an incomplete reaction of kaolin, as shown in Figure 5a. After sintering at 900 and 1100 ◦C, the images clearly showed the presence of pores and cracks in all of the heated kaolin-based geopolymer ceramic samples. The pores formed a network, which resulted in increased internal porosity. The kaolin-based geopolymer surface became glassy and glossy when sintered at 900 ◦C (Figure 5b). This microstructure change was attributed to moisture hydration and phase transformation, as reported by Dudek et al. [17]. It can also be seen

in Figure 5b that the kaolin-based geopolymer ceramic samples sintered at 900 ◦C had a higher porosity, alongside cracks and voids.

**Figure 4.** Surface area and pore volume of kaolin geopolymer samples versus sintering temperature.

Increasing the sintering temperature up to 1100 ◦C increased the number of large pores. The pore size distribution of kaolin-based geopolymer was ~50 μm for the unsintered samples. After sintering at 1100 ◦C, the pore size increased to 80 μm, similar to the findings from the tomography analysis. The pore sizes in kaolin directly affect its mechanical strength. The SEM images also showed significant cracks due to moisture evaporation and shrinkage during the sintering process. The loosely grained structure of kaolinite can also cause cracks, and the presence of voids at the interface of loosening grains can result in increased total porosity.

#### *3.4. Neutron Tomography Imaging Analysis*

Segmentation was carried out in a small area to analyze porosity data in the kaolinbased geopolymer samples quantitatively, and the 3D reconstruction images are shown in Figure 6. The kaolin-based geopolymer samples' widths, lengths, and thicknesses, shown in Figure 6a–c, were 2900, 1740, and 1740 μm, respectively. The white color indicates the solid kaolin-based geopolymer, while blue indicates the air (pore) space. The total number of pores for this region was estimated to be 197, and after sintering at 900 and 1100 ◦C, the total number of pores decreased to 182 and 125, respectively. Neutron tomography made imaging very small pores at high resolutions possible, and the results are shown in Figure 6d–f. In the case of the unsintered kaolin, the pore size was ~50 μm3, and when sintered at 900 and 1100 ◦C, the pore size increased to 68 and 82 μm3, respectively. Figure 6g displays pore numbers and sizes. These sizes are in agreement with those measured in the SEM images shown in Figure 4.

**Figure 5.** SEM micrograph of (**a**) unsintered, (**b**,**d**) sintered at 900 ◦C, and (**c**,**e**) sintered at 1100 ◦C kaolin-based geopolymer.

When sintered, the small pores merged to become large(r) pores due to moisture hydration after sintering. Our images show the isolated closed pores in the 3D volume, and it was, in fact, a network of fully connected open pores in 3D. Interestingly, after sintering, the pore distribution of the kaolin-based geopolymer became layered, as shown in Figure 6b,c. The layer distance between porosities was estimated to be ~120-130 μm when sintered at 900 and 1100 ◦C because the kaolin-based geopolymer exhibited low reactivity with the alkaline silicate solution.

A layered structure was caused by the sintering of the kaolin-based geopolymer at a higher temperature. The layered structure was indicated by the transformation of pore appearance, as shown in Figure 7. The pore transformation was attributed to the larger surface area causing necking reactions between particles (Figure 7b). During sintering, atoms diffuse from an area of higher chemical potential to an area of lower chemical potential. Small pores then merge to form larger pores. The layered grain structure represented the disorganized kaolinite structure (grey color) that was due to dehydroxylation. The dehydroxylation of kaolin resulted in the destruction of the crystalline structure and the transformation of the mullite phase, as confirmed by an XRD analysis. These findings are consistent with ElDeeb et al. [18], who posited that the hydroxylation of clay sheets occurs with high-temperature sintering.

**Figure 6.** Tomography imaging of (**a**) unsintered and sintered geopolymer at (**b**) 900 and (**c**) 1100 ◦C. (**d**–**f**) Tomography imaging with zoom and higher resolution and (**g**) total pore numbers and average pore sizes.
