*3.5. Elemental Distribution Analysis*

The kaolin-based geopolymer ceramic samples were further characterized using micro-XRF mapping to understand their elemental distribution vis-à-vis sintering temperatures. Figure 8 illustrates the localized micro-XRF mapping of the kaolin-based geopolymer ceramic samples that were (Figure 8a) unsintered or heated to 900 (Figure 8b) or 1100 ◦C (Figure 8c), signifying where the (main) elements Si and Al were critically located within the samples. The distributions of the main elements within the geopolymer ceramic edifice were confirmed using synchrotron micro-XRF. The distribution of Si combined with the Al map allowed for the identification of the kaolin-based geopolymer ceramic backbone (kaolinite). The red, green, and blue spots represent the high, medium, and low intensities, respectively, for each distribution element at the integrated area.

**Figure 7.** Sintering mechanism of pore transformation in various environments: (**a**) unsintered, (**b**) 900 ◦C, and (**c**) 1100 ◦C.

**Figure 8.** Micro-XRF elemental distribution maps of Si and Al in kaolin geopolymer ceramic at various sintering temperatures.

The various sintering temperatures resulted in significant changes in the Si and Al element distributions, edging the material towards phase transformation. A high concentration of Si (Figure 8a) represented the kaolinite grain. Upon obtaining the pore microstructure of the kaolin-based geopolymer ceramic (Figure 8b), the Si and Al regions showed higher intensities, reflecting the presence of the minerals quartz and nepheline, as depicted in Figure 8 and the next section. At 1100 ◦C, Si and Al were of higher intensities in a localized area, reflecting the formation of mullite. This Si–Al-rich crystalline mineral contributed to the pores' microstructure appearance, as shown in Figure 5b,c.

#### *3.6. Mineral Phase Transformation*

Figure 9 shows an XRD diffractogram of the kaolin-based geopolymer ceramic when (a) unsintered or heated to (b) 900 or (c) 1100 ◦C. The unsintered kaolin-based geopolymer showed the presence of crystalline phases such as kaolinite, quartz, and tridymite. A geopolymerization reaction was initiated by the dissolution of aluminosilicate materials in an alkali activator (combination of NaOH and Na2SiO3 solutions). Next, the products of dissolution underwent nucleation growth and polymerization processes before hardening

at the polycondensation stage. There have been several findings obtained with a similar method for producing kaolin-based geopolymer at room temperatures [19,20]. Additionally, kaolinite was traced as a major mineral in spectra of kaolin-based geopolymer samples [21]. Owing to the lower activity of pure kaolin, a number of distinctive kaolinite peaks remained in the diffractogram of the kaolin-based geopolymer [22]. However, these kaolinite peaks decreased at high sintering temperatures, as shown in Figure 9b.

**Figure 9.** Phase transformation of kaolin geopolymer when (**a**) unsintered, (**b**) sintered at 900 ◦C, and (**c**) sintered at 1100 ◦C. M, mullite; C, cristobalite; Q, quartz; K, kaolin; N, nepheline; T, tridymite.

Sintering temperatures up to 1100 ◦C introduced the formation of the mullite phase (Figure 8c). The mullite phase is present in this sintering region, manifesting superior thermochemical stabilities [23,24]. Furthermore, the appearance of cristobalite was due to unreacted quartz (SiO2) after the decomposition of kaolinite at 900 ◦C [25]. The liberation of SiO2 corresponded to the kaolinite–mullite transformation, which yields to Al–Si spinel phase [26]. This was corroborated with the elemental distribution analysis obtained using micro-XRF, as the sintered kaolin geopolymer ceramic samples showed a high intensity at the Si and Al regions at 1100 ◦C (Figure 7c). The transformation of mullite is described by the chemical reaction in Equation (1) [27,28]:

$$2\text{Si}\_3\text{Al}\_4\text{O}\_{12}\text{Al}\cdot\text{Si}\text{ spinel} \to 2\text{Al}\_2\text{O}\_3\text{.}2\text{SiO}\_2\text{ nullite} + 5\text{SiO}\_2\tag{1}$$

#### **4. Conclusions**

This manuscript summarizes the effects of sintering temperature on the pore structure of an alkali-activated kaolin-based geopolymer ceramic. Sintering temperatures significantly affected the size and number of pores in the kaolin-based geopolymer. The material's density and water absorption confirmed the presence of pores after the sintering process. Microstructural analyses showed that sintering at 1100 ◦C resulted in large pore sizes relative to the material's unsintered counterpart. Tomography imaging also confirmed the presence of a layered pore structure after sintering. The pore size at 900 ◦C was 50 μm3, and after sintering at 900 and 1100 ◦C, the pore size increased to 68 and 82 μm3, respectively.

**Author Contributions:** Conceptualization and methodology and writing, M.I.I.R. and D.D.B.N.; supervision and resources, M.A.A.M.S.; methodology and formal analysis and investigation, I.H.A., T.C.Y., W.K., A.F. and N.F.S.; interpretation and review and editing, M.M.A.B.A., A.V.S., P.V. and J.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** The neutron tomography studies of the geopolymer ceramic used for reinforcement materials in a solder alloy for a robust electric/electronic solder joint were financed by Ministry of Education Malaysia under reference no: JPT.S (BPKI)2000/016/018/019(29). This work was supported by the TUIASI's internal grants program (GI\_PUBLICATIONS/2021), financed by the Romanian Government. This work also supported by Ministry of Higher Education Malaysia regarding the use of the ISIS Neutron and Muon Source and funded by the UK Department of Business, Energy and Industrial Strategy (BEIS) and Malaysia and delivered by the British Council.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors gratefully acknowledged the Ministry of Higher Education Malaysia regarding the use of the ISIS Neutron and Muon Source. A grant was funded by the UK Department of Business, Energy and Industrial Strategy (BEIS) and Malaysia and delivered by the British Council.

**Conflicts of Interest:** The authors declare no conflict of interest.
