*3.3. Sintering Temperature*

Sintering is the process of producing a solid mass of material under pressure and heat without fully melting it. In this process, atoms in raw materials diffuse across particle boundaries and fuse to form a single solid object. The sintering process flow is shown in Figure 4.

**Figure 4.** Sintering process flow.

Zhang et al. [57] revealed, possibly for the first time in China, the alkali activation reactivity of calcined kaolin from Guangxi province. The thermal treatment of kaolin is typically necessary to obtain more reactive precursors that result in geopolymers with high strength was discovered. Significant attempts have been made to identify the best heating temperature for the increase of the kaolin geopolymer's strength. Past research findings regarding the sintering temperature are presented in Table 5.


#### **Table 5.** Sintering temperature from past research.

Naghsh and Shams [89] demonstrated that as the calcination temperature increased from 400 ◦C to 800 ◦C, the MK dissolution extent in NaOH solution increased continuously. Sornlar et al. [90] discovered that dehydroxylation of kaolin to metakaolin occurred upon calcination at 600 ◦C, resulting in a large increase of amorphous phase with some crystalline phases (illite and quartz) remaining in the resulting metakaolin powder. According to reports, the amount of amorphous phase in the metakaolin had a significant effect on the curing and strength development of the geopolymer. Due to the relatively wide specific surface area, which may necessitate a high water-to-binder ratio to obtain satisfactory workability, MK is not utilised in the majority of construction situations.

Alexandre and Lima [26] conducted additional research using KOH as an alkaline activator. After calcination, minerals such as anatase and quartz in samples sintered at 750 ◦C remain stable or metastable. In addition, the de-hydroxylation of goethite has led to the neoformation of hematite in these samples. However, current knowledge suggests that the use of KOH as an alkali activator does not optimise geopolymerization, as the K-O link is weaker than the Na-O bond.

Majdoubi et al. [91] showed that the crystalline phase of kaolinite disappears between 300 ◦C and 1300 ◦C of sintering temperature. The constant emergence of several characteristic peaks of kaolinite suggests that calcination at temperatures between 700 ◦C and 800 ◦C was not performed perfectly. At 800 ◦C, it is readily apparent that the full absence of the halo characterises the amorphization of our material, indicating that the three-dimensional network of geopolymers is no longer in place and has experienced a significant transformation from amorphous to completely crystalline. When the temperature approaches 1100 ◦C, the aluminium phosphate phosphocristobalite phase and SiO2 cristobalite dominate the X-ray powder diffraction (XRD) graph. This phase is the most resistant at high temperatures, which explains why the geopolymer's resistance is very weak. The slight variation between the three halos is a result of the increase in calcination temperature: the higher the temperature, the greater the intensity. After heat treatment, it was also noticed that the typical peaks of quartz and muscovite become more distinct and intense.

Merabtene et al. [92] discovered that calcined kaolin at 800 ◦C for 24 h, followed by quick air cooling and the selection of 800 ◦C possessed an excellent precursor for geopolymer synthesis. The heating scheme of 800 ◦C confirms the existence of carbonates such as calcite (CaCO3), as indicated by XRD and Fourier transform infrared (FTIR) investigations. Nevertheless, the sintering temperature must be increased to 900 ◦C due to the increasing kaolin reactivity between 800 ◦C and 900 ◦C. It appears to be connected to the altered oxygen atom environment during dehydration.

Villaquirán and Meja [93] determined the sintering temperature to be between 300 ◦C and 1500 ◦C. In the absence of dehydration, the compressive strength of the geopolymer reached its maximum value as the calcination temperature rose to 900 ◦C and decreased drastically at 1000 ◦C. However, the 300 ◦C sintering temperature range is not really significant because past research has shown that kaolinite gradually loses OH cation between 700 ◦C and 900 ◦C during the sintering temperature.

Jamil et al. [22] emphasised from the outset that 6 M to 8 M of NaOH is sufficient to achieve alkalinity. The partial conversion of Al from its original 4-coordinated state to its 6-coordinated state is known as the phase transformation from monoclinic to tetragonal. As a result, the sintering temperature ranges between 200 ◦C and 1200 ◦C. The kaolin geopolymer after pre-sintering, which can be explained by the beginning of crystallisation of the amorphous geopolymer network, the total disappearance of the gehlinite phase, and the beginning of the appearance of the akermanite and albite phase, is due to the phase change from the more stable hexagonal aluminium phosphate. This phase is responsible for the observed colour change; the geopolymer has turned white due to the presence of crystalline Al, which indicates the presence of the akermanite and albite phases. Previous geopolymer research [20] had already revealed this modification.

Liew et al. [48] evaluated the reactivity of kaolin calcined at 900 ◦C, 1000 ◦C, 1100 ◦C and 1200 ◦C in a furnace at a heating rate of 5 ◦C/min and soaking duration of 3 h and discovered that the ceramic geopolymers had a maximum flexural strength of 90 MPa at 1200 ◦C. This study discovered the highest sintering temperature among previous studies. There is, in reality, no universally ideal temperature for MK production. Nevertheless, it is acceptable to select various calcination schemes (temperature and duration) because the mineral composition and particle size of kaolin, as well as the heating procedure, all have a role (stable or fluidized).

The porosity and apparent density variation with kaolin content have different aspects according to the sintering temperature. The sintering reactions between kaolin and alkali activator absolutely influences the chemical and mechanical properties of kaolin ceramic geopolymers, respectively. Although calcination temperature had a positive impact on aluminium alloys, the high calcination temperature had a significant negative impact on the sustainability of the environment. As a comparison, the calcination of kaolin to obtain metakaolin takes place from 600 ◦C to 800 ◦C. Concerning the environmental impact and sustainability of geopolymeric cements produced from natural kaolin, the use of non-calcined kaolin aids in the reduction of manufacturing costs and environmental implications, resulting in a green ceramic [2,22,94].

## **4. Properties of the Kaolin Ceramic Geopolymers**

Figure 5 shows that the chemical structure of kaolin in the 3D network structure of geopolymers consists of a Si2O2Al framework spatially connected chains of [SiO4] and [AlO4] tetrahedral. The Si and Al share oxygen corners for each other and produce the charge-balancing metal cations.

**Figure 5.** Chemical structure of kaolin [95].

Kaolin is a common mineral found in soils and sediments, and it has a wide range of applications. This clay mineral is a 1:1 layer aluminosilicate in which an alumina octahedral sheet and a silica tetrahedral sheet are fused to produce a layer held together by hydrogen bonding [73,93,96]. This clay possesses no exchangeable cations [89,97,98], because isomorphic substitution and cationic vacancies are near to zero.

Theoretically, any pozzolanic compound with a high alumina and silica concentration is acceptable for geopolymer synthesis under highly alkaline circumstances [18,49,56,76]. However, several considerations must be made for the geopolymerization reaction following the addition of an alkaline activator. The measurement of the physical properties and chemical composition of the raw material is one of the most essential variables in this category, as it determines the alkalinity level of the activator [19,22]. It is essential to completely analyse the samples and, based on this, to optimise the composition and amount of the activating solution and curing conditions due to the diversity of the raw materials, which may vary from batch to batch, whether mineral or waste products, for example [17,18,99,100].

Clays are hydrous aluminium silicates with a composition of approximately Al2O3– 2SiO2–2H2O [67,101]. In order to lower costs, contemporary research on the manufacture of ceramic support has centred on the use of less expensive raw materials, such as apatite powder, fly ash, natural raw clay, dolomite, and kaolin. Among these ceramic materials, kaolin has emerged as a potential raw material that is frequently employed for separation applications at a lower cost [57,73]. Moreover, kaolin is one of the least expensive and most abundant support raw materials, and it is readily accessible [22,33,102].

Geopolymers derived from kaolin has demonstrated great promise in the construction and building industries as well as engineering applications. Previous research indicates that changes in the reactivity of source materials employed in the synthesis of wastebased geopolymers have a substantial impact on the final characteristics of the ceramic geopolymer. At the raw materials selection stage, the attributes of kaolin correspond to its mineralogical compositions and thermal treatment histories. Consequently, it merits additional research into its compressive strength, chemical and mineralogical composition, morphological development, and phase analytic features.
