**1. Introduction**

Geopolymer manufactured raw materials are extremely rich in silica and alumina, which is an advantage given that over 65% of the Earth's crust is composed of alumina and silica minerals [1,2]. Geopolymer consists of a three-dimensional network of aluminosilicate tetrahedral atoms that are covalently bound to one another [3–6]. Geopolymers are a relatively recent type of construction material created from industrial by-products and cementitious materials with high alumina and silica content [1,6–8]. Due to climate change

**Citation:** Mohd Mortar, N.A.; Abdullah, M.M.A.B.; Abdul Razak, R.; Abd Rahim, S.Z.; Aziz, I.H.; Nabiałek, M.; Jaya, R.P.; Semenescu, A.; Mohamed, R.; Ghazali, M.F. Geopolymer Ceramic Application: A Review on Mix Design, Properties and Reinforcement Enhancement. *Materials* **2022**, *15*, 7567. https:// doi.org/10.3390/ma15217567

Academic Editor: Valentina Medri

Received: 18 August 2022 Accepted: 24 October 2022 Published: 28 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

strategic initiatives, the market for geopolymer products has expanded dramatically in recent years. In addition, green systems are a key problem in the building industry, and the use of geopolymers through the geopolymerisation process has piqued the interest of scientists worldwide [5,6,9,10]. It is a "new" category of materials that has attracted a great deal of interest and risen gradually in research article investigations over the past decade.

Geopolymers have traditionally been considered an alternative to Portland cementbased materials with significant environmental and durability benefits. These advantages need to be compensated for by many brief mix designs and technologies when compared to conventional Portland cement [7,11,12]. Blended cements use a wide variety of nonconventional ingredients, such as geopolymer binders and pozzolan-based compounds. However, geopolymers have many additional potential uses; for example, that are advantageous due to thermal stability in the fabrication of thermally resistant structural elements [5,13–15], as adhesives [16,17], for the solidification of hazardous wastes [18–21], or as catalytic support [7,8,22].

Blended cements are stronger and less likely to crack than conventional cement, not to mention being eco-friendlier. Inorganic geopolymers are synthesised in an alkaline environment from silica–alumina gels [6,7,9,13,23]. When viewed with scanning electron microscopy (SEM), the structure is composed of interconnected chains or networks of inorganic molecules that are held together by covalent bonds. One atom of silicon or aluminium is connected to four atoms of oxygen to form a tetrahedron. These tetrahedrons form a three-dimensional network with one oxygen atom in common between each of the tetrahedrons [17,18,23–25]. The most used raw materials are natural minerals, such as kaolin [9,24,26,27] and calcined clays [28–32], and industrial wastes, such as fly ash [4,13,33–36], slag [35–37], red mud [28,38,39], and waste glass [40–42].

Kaolin converts to a pozzolan material named metakaolin (MK) after high temperature of thermal treatment. Regarding the issues of sustainability, kaolin as a geopolymer material can satisfy the world demand for ceramic industries. This review also discovered findings on the potential use of kaolin as a raw material with and without thermal treatment. However, there have only been a few research studies conducted on the use of kaolin as a raw material in a ceramic geopolymer application. This article also discussed a comprehensive review of the characterization of kaolin, addition of kaolin geopolymer, and the potential of zirconia reinforcement in ceramics. Furthermore, the experimental results by the researchers regarding the percentage ratio of zirconia addition to improve the properties of the ceramic geopolymers are also presented. At the conclusion of this review, the feasibility of future research into the low-cost manufacture of ceramics from geopolymer derived from kaolin is evaluated. Therefore, it is necessary to undertake a thorough literature analysis on current understandings regarding the functionality of geopolymer regarding its application on ceramic.

#### **2. Mix Design and Manufacture Method for Kaolin Geopolymer in Ceramics**

A new type of building materials with improved strength, durability, and other qualities entered the market in the nineteenth and twentieth centuries [43,44]. Ceramic components can be made from a wide variety of metallic and non-metallic atom combinations, and each atom combination typically lends itself to a number of structural configurations [30,40,45,46]. To address the rising needs and requirements in a wide range of application fields, scientists were compelled to develop numerous novel ceramic materials.

Inorganic solid powders with carefully controlled purity, particle size, and particle dispersion are used to create ceramic geopolymers [9,17,22]. To create a ceramic with specific material properties, various precursors are mixed in the process. This powdered mixture is mixed with a binder so that it can be machined in a "raw" state, moulded to exact specifications, and then sintered in a controlled furnace [40,41,47]. The raw ceramic must be heated to a temperature below its melting point to be sintered. By removing the moisture and binder, fine ceramic products with high hardness and density are created by

condensing the microscopic gaps between the particles and fusing them together [30,46,48]. The formulation of geopolymer materials for ceramic applications is shown in Table 1.

**Table 1.** Mix design of geopolymer materials in ceramic application.


According to Jamil et al. [22], the phase transition of the sintered kaolin-ground granulated blast surface slag (GGBS) geopolymer was aided by the addition of GGBS to kaolin, which accelerated the geopolymer's setting time. Kaolin's structural alterations were influenced by the high alkalinity of NaOH (8 M), which made it capable of reacting with GGBS. The sintered kaolin-slag geopolymer's characteristics alter as the solid to liquid (SL) ratio rises. Akermanite and albite are two new phases that are formed when the solid content is at its highest (SL:2). The morphology of the sintered kaolin-GGBS geopolymer indicates enhanced densification and pore creation with increasing solidto-liquid ratios. Additionally, two steps of the sintering profile, as shown in Figure 1, mitigated the beginning of fractures as the dihydroxylation mechanism is retarded. In this research, the use of kaolin as a raw material without calcination gives good feedback on energy consumption and green method by skipping the sintering stage. The effect of kaolin geopolymer at post-sintering temperatures, however, is not explored further in the thermal gravimetric and thermal analysis.

**Figure 1.** Two-steps sintering profile of kaolin-GGBS geopolymer [22].

Ma et al. [46] revealed that the flexural strength SiC whiskers (SCWS) reinforced geopolymer composites (SCWS/KGP) composites could be improved with the presence of SiC whisker and reached the peak value when the SCWS content was 2 wt%. The production process for the composite of SiC whiskers (SCWS) and KGP (Kaolin Geopolymer) is shown in Figure 2. The improvement in the KGP composites' flexural strength is mostly attributable to the strong interface bonding between the SiC whiskers and the geopolymer matrix. When its content reached 4 wt%, whiskers aggregation was observed, which negatively impacted the mechanical performance of SCWS/KGP composites. Additionally, geopolymer evolved into high density, twin-structure leucite ceramics after being heated to 1100 ◦C and 1200 ◦C. While this was going on, there was no interfacial reaction between the leucite matrix and the SiC whisker, which preserved its chemical stability. Due to leucite formation and a strong interfacial contact between the whisker and matrix, the composite treated at 1200 ◦C with 2 wt% SiC whisker demonstrated a 124.8% higher flexural strength than the composites before high temperature treatment. Nevertheless, this research does not compute the compressive strength, which is the interfacial zone between the whisker and the matrix, because shrinkage can be determined by the whisker that is subjected to compressive stresses.

Yun Ming et al. [48] confirmed the existence of zeolite Y in metakaolin-based geopolymer powder-based geopolymers with one-part mixing. Figure 3 depicts the production procedures for geopolymer powder, one-component geopolymer, and ceramic geopolymers. The one-part mixing geopolymers attained a maximum compressive strength of 10 MPa after 28 days. The sintering of the compressed geopolymer powder changed the amorphous phases into nepheline phases without passing through intermediate phases. At 1200 ◦C, the greatest flexural strength of ceramic geopolymers was 90 MPa. This method reduced the probability of cracking in geopolymers that had already been cured. However, it was recommended to reduce the sintering temperature to produce nepheline ceramic geopolymers, as the sintering temperature indicated in this study was too high.

**Figure 2.** Preparation procedure for SCWS (SiC whiskers)/KGP (Kaolin Geopolymer) composite [46].

**Figure 3.** Steps to produce geopolymer powder, one-part-mixing geopolymer and ceramic geopolymer [48].
