**Table 8.** Addition as a reinforcement in ceramic composite.

Wu and Tian [98] reported on rubber addition, which significantly improved deformation and yield strength, as is the case for matrices with higher SiO2/Al2O3 molar ratio and 40 and 50 parts per hundred rubber (phr) rubber. The tensile strength, elongation at break, and hardness of nitrile butadiene rubber (NBR) filled with 40 phr kaolin were satisfactory, and the minimal wear indicated the optimum wear resistance. Additionally, 50 phr kaolin-filled EPDM and CR have adequate elongation at break, hardness, and wear but not tensile strength, which is lower than that of 40 phr kaolin-filled EPDM. However, the durability of rubber is not particularly long. Additionally, rubber cannot handle high temperatures because it may cause a tendency for the material to rupture and degrade, which reduces the composite's ability to endure tensile strain. In order to create new geopolymer materials from a blend of commercial metakaolins and calcined clays, Selmani et al. [121] focused on valorizing naturally existing clays. Metakaolin MK1 was replaced by metakaolin MK2, which produced different compositions with the following codes: G1, G2 (16%), G2 (33%) and G2 (50%). However, because of impurities, adding natural clay reduced the compressive strength of the geopolymer composites (illite, calcite, iron).

According to Jamil et al. [22], the sluggish rate of the Al content's dissolution makes it necessary to spend more time to produce kaolin with a strong chemical interaction. A kaolin-GGBS ceramic geopolymer was created by adding ground granulated blast furnace slag (GGBS) to reduce the rate of dissolution. However, research in measuring compressive strength to gauge the brittleness of kaolin geopolymer composites is still limited. In addition, Tiffo et al. [50] reported that, to give the kaolin geopolymer its physical and mechanical properties, researchers substituted amorphous aluminium hydroxide and aluminium oxyhydroxide in varying amounts. As a result, the replacement successfully contributed to the development of heated kaolin-based geopolymers that are thermally stable and have a high compressive strength. The result is not visible, though, until 28 days into the curing process. This is crucial to demonstrate that the geopolymer system has no additional reaction mechanisms because of the removal of kaolin.

Coudert et al.'s [122] study was primarily concerned with the application of an alkali activated fly ash-based binder to improve the engineering properties of soft clay-rich soils and to replace conventional stabilisers (lime or cement). By using optical microscopy, microstructural measurements of the alkali activated fly ash binder treated soil over time were made. In a way like the alkali-activated fly ash binder, after 24 h of curing there are scattered dark patches that look like calcium-rich nodules all over the sample. After 28 days, these nodules are surrounded by larger black zones that are made up of newly formed compounds. At 28 days, hollowed grey nodular structures can be identified as the binder and linked to the breakdown of calcium-rich particles. However, combining micro-indentation with scanning electron imaging would also make it possible to measure regional variations in hardness. Therefore, the degree of calcium particle reactivity can be used to understand how important local microstructural differences are for macroscopic mechanical performances.

In the fresh-state, alkali-activated slurry, Perumal et al. [67] examined how surfactants function as stabilisers for the gas-liquid interface, enhancing the establishment of interconnected porosity utilising impure kaolin. Depending on the paste characteristics, surfactant type, and content, the pore structure produced by direct foaming can have a wide size distribution. Lower strength is generally the result of a less homogenous pore structure. The effect of three different molarities of alkali activator (5 M, 10 M, or 15 M NaOH) and water binder ratio (0.55 and 0.65) on the mechanical property of kaolin-based geopolymer has been described, however the research on the effects of Si/Al ratio and ageing duration has not been covered.

The effects of Micro additions of Fe2O3 and MgO on the mechanical and physical characteristics of the geopolymer binder were investigated by Kaya et al. [49]. The binder was developed by substituting zeolite for kaolin at percentages of 10%, 20%, and 30%. Additionally, by replacing 4%, 6%, and 8% of the Fe2O3 and MgO in the binder with zeolite, the quantities of Fe2O3 and MgO were enhanced. The binder was activated using NaOH

that contained 15% Na by weight (Na/binder). Because kaolin has a denser structure due to its lower porosity than zeolite, replacing zeolite with it causes an increase in the unit weight, compressive strength, flexural strength, and UPV of the geopolymer specimens. However, to correspond with the development of hematite (Fe2O3) and periclase, the formation of sodium aluminosilicate and calcium silicate hydrates as hydration products was not further discussed (MgO). Therefore, these authors were the first to realize the potential of nanoparticles to impart toughness and strength of geopolymer structure.

#### *Addition of Zirconia in Ceramic Geopolymers*

To increase the strength and toughness of ceramics, for instance, zirconium dioxide (ZrO2) may be added. This would be done by taking advantage of the tetragonal to monoclinic phase transformation that is brought on by the presence of a stress field before a break. On the other hand, zirconia brings improvement in compressive strength, fracture toughness, crack deflection, crack bridging, and micro-cracks.

Due to the better chemical and thermal properties to standard additives, nanosized particles are one possibility to increase the mechanical performance of such geopolymers [121,123,124]. Additionally, nanoparticles can function as a filler to lower the nanoporosity at the level of the interfacial transition zone between aggregated particles as well as a catalyst to speed up the geopolymerization reaction [55,124,125]. Table 9 shows the properties of zirconia, including melting point, boiling point, density, and molar mass.

**Table 9.** Zirconia properties [126].


Temperature and time during the sintering process should be studied because they directly affect the grain size, yttrium segregation, and amount of cubic phase in zirconia, which in turn affects its physical, mechanical, and optical properties [14,45,125]. Increasing the sintering temperature increases the grain size of zirconia, which may improve its physical qualities but makes it more susceptible to low-temperature irradiation (LTD). Although Al2O3 has good hardness, abrasion resistance, and chemical inertness at elevated temperatures, it has relatively low toughness [56,121,123], which leads to early failure. To boost its fracture toughness, Yttria-Stabilized Zirconia (YSZ) is used as a strengthening agent. The product of this mixture is Zirconia-reinforced Alumina (ZTA). It undergoes a phase transformation from tetragonal to monoclinic that results in a transformation strengthening process [45,127].

Greater ZrO2 content in a kaolin-based mullite ZrO2 composite yielded greater density and flexural strength. Due to the decreased viscosity of the produced glassy phases in the sintered samples [56,128], the presence of ZrO2 may have increased the thermal shock resilience of the sample. The proposed method involves adding zirconia to kaolin or metakaolin (calcined kaolinite) in order to produce at high temperatures mullite and zircon (ZrSiO4)-based ceramics, according to the following Equation (1) [56]:

$$\text{3(Al}\_2\text{O}\_3\cdot2\text{SiO}\_2) + 4\text{ZrO}\_2 \rightarrow 3\text{Al}\_2\text{O}\_3\cdot2\text{SiO}\_2 + 4\text{ZrSiO}\_4 \tag{1}$$

Zircon, which does not suffer any structural change until its dissociation at around 1500 ◦C, possesses a number of desirable qualities, including a high resistance to alkali corrosion and an extremely low thermal expansion coefficient (4.1 × <sup>10</sup>−<sup>6</sup> ◦C−1) between room temperature and 1400 ◦C, and a low heat conductivity [56,121,124]. Table 10 shows the impacts of the addition of zirconia in past research.


**Table 10.** Impact of the addition of zirconia in past research.

Phair et al. [129] demonstrated that the incorporation of just 3% mass of zirconia to a geopolymeric matrix significantly increased the compressive strength by 30%. Incorporating 5% or more zirconia, caused considerable brittleness due to the adverse bulk physical effects of extra filler on the 3D polysialate network. However, no clear evidence exists to establish that the absence of zeolite crystallisation is primarily attributable to the high CaO level. Furthermore, Mecif et al. [56] discovered that ZrSiO4 production, which occurs at temperatures above 1150 ◦C, is promoted by the presence of fusing impurities such as K, Fe, Ca, and Mn in clays, as well as a reduction in zirconia particle size. It was also discovered that the rise in the porosity ratio of the final products for zirconia levels more than 20 wt percent was dictated by a decrease in the flux amount due to the reduced clay content. Sintering a mixture of 38 wt% of fine zirconia powder and 62 wt% of the more reactive clay at 1400 ◦C for 2 h produced ceramics that are mostly composed of zircon and mullite.

Kenawy et al. [130] hypothesized that the comparatively lower density with greater ZrO2 contents could be the result of thermal expansion mismatches between ZrO2 and the mullite matrix. This may cause interior fissures and a weakened matrix, resulting in a reduced density. Moreover, the higher the ZrO2 content, the greater the viscosity of the produced glassy phases and, consequently, the lower the particle diffusion and rearrangement. Regardless, this researcher did not explore the effect on compressive strength. Moreover, the previous researchers theorised that zirconia promotes a 3D polysialate grid structure through the creation of insoluble sodium polysialate, based on research by Zawrah et al. [124] to determine the chemical foundation for the increase in compressive strength. This 3D polysialate minimises the mobility of sodium while maintaining the matrix's structural integrity and charge balance. To clarify the grain/particle sizes, phases, chemical species, and yttrium distribution, as zirconia materials behave differently at different sintering temperatures, additional research is required.

Geopolymers, which combine some characteristics of organic polymers, cements, and ceramics due to the unusual polycondensed network structure, have attracted a great deal of interest from researchers as a green cementitious material due to the advantageous and distinctive properties. Additional research is necessary to comprehend the properties of kaolin ceramic geopolymers reinforced with zirconia for use in ceramic technology.
