*4.3. Morphological and Phase Analysis*

Scanning electron micrographs of kaolin's microstructure were examined. These show that morphology changes as the calcining temperature rises, and it also gradually affects the strength, hardness, apparent density, and volume shrinkage ratio. Figure 7 shows plate like kaolin SEM micrographs. It is evident that the kaolin morphology consisted of crystals with sharp edges, hexagonal shapes, rods, plates with corrosion, and irregular shapes [22,49,111]. The geopolymer made from kaolin has the benefit of being reliably created, with known properties during both preparation and development. However, the rheological issues caused by its plate-shaped particles make the system more complex to process and require more water [51,111].

**Figure 7.** SEM images of the plate like kaolin (**a**) [111] (**b**) [79].

Figure 8 shows the SEM images for the room-temperature cured binders (a), paste specimen after immersed in water (b), room-temperature cured binders with 10% NaOH (c), and for the room temperature cured binders with 10% CaO (d). When the specimen was cured at room temperature, several microcracks with discontinuous gel developed, however a rather big crack was identified in the paste specimen following immersion in water. As illustrated in the figure, this produced a dense and strong alkali aluminosilicate matrix (c). The formation of such cracks could explain why the compressive strength of geopolymer specimens decreased following immersion in water. A solid gel with a well-packed structure was observed in the CaO paste, with visible amounts of crystalline or weakly crystalline C-S-H phase. The inclusion of C-S-H phases may provide stiffness to the geopolymer paste, improving the mechanical properties of a kaolin-based geopolymer [79,99,112]. The addition NaOH to the system could enhance the leaching of Si and Al from the kaolin particles to the solutions and resulted in increased geopolymerization and formation of sodium silicate hydrate gel [23,36,113,114].

**Figure 8.** SEM images for (**a**) a room-temperature cured paste, (**b**) paste specimen after immersed in water, (**c**) paste with 10% NaOH and (**d**) paste with 10% CaO [79].

The Si/Al ratio has a strong influence on the microstructure of geopolymers, and the other three parameters (Al/Na, water/solids, and H2O/Na2O ratios) have less of an impact. This is shown by the fact that geopolymers with the same Si/Al ratio have similar microstructures, but there are big differences when the Si/Al ratio changes [25,115,116]. The geopolymer system is a two-phase gel made of water and an aluminosilicate binder. The water acts as a reaction intermediate and is released when the gel solidifies to create pores and a two-phase structure [15,105,117]. In contrast, water plays an active role in cement hydration and ultimately affects paste porosity in the OPC system [49,56,118]. Porosity in geopolymers is determined by solution chemistry during geopolymerization, which is primarily a function of Si/Al ratio and alkali metal cation type. Absolute pore volume is governed by nominal water content [50,64,119].

Only three of the kaolin's classic phases, quartz, muscovite, and kaolinite, can be seen in the XRD diffraction pattern depicted as in Figure 9 [22,51,108]. One of the crystalline phases found in kaolin is kaolinite, which makes up 35% of the crystalline phases and is converted to metakaolin through calcination [35,117,120]. The decomposition of the mineral calcite into CaO and CO2, which is evidenced by the mass loss at around 677 ◦C in the thermogravimetric analysis (TGA), is what causes the calcite reflections in the XRD pattern of the calcined ceramic industrial sludge to vanish [51].

The XRD patterns for calcined kaolin and the comparable hydrates with and without additions are shown in Figure 10, where, relative to calcined kaolin, the kaolin-based geopolymer paste has less crystalline peaks. Several crystalline hydrated phases, including quartz (Q), muscovite (M), and gypsum, were discovered (G). The addition of NaOH or CaO has only a minimal impact on the crystalline phases of the resulting hydrates.

**Figure 10.** X-ray Diffraction Analysis for kaolin-based geopolymer [79].

Kaolin is highly recommended for use in ceramic geopolymers based on its excellent properties as a ceramic. In addition, the properties of the ceramic geopolymer can be enhanced by addition of reinforcement. There is a critical need for ceramic reinforcement to enhance its physical and mechanical properties. Geopolymers have several positive properties, including high strength, high density, few pores, an elastic modulus, and little shrinkage; yet, brittle and can easily break. Reinforcement or addition in kaolin geopolymers may solve this problem.
