**4. Metakaolin**

Thermal treatment of kaolinite leads to the transformation of crystalline phases into reactive amorphous phases [7], which is the active constituent that determines the final strength of geopolymers. The thermal treatment is usually carried out at temperature in the range of 550–800°C which accompanied by dehydroxylation of strongly bounded hydroxyl ions on the Al-constitutive layer. Thus, kaolinite is transformed into metakaolin.

Metakaolin also has layered structure as kaolinite even after the thermal treatment process. However, the layer structure appeared more open than kaolinite (**Figure 3**) [23, 24].

Also, the thermal treatment destroys the hexagonal layer of kaolinite and causes atomic arrangement that converted the hexa-coordinated Al ions of kaolinite are converted into penta- and tetra-coordinated Al ions [25]. The amount of penta- and tetra-coordinated Al ions reflects the reactivity of metakaolin [24].

#### **4.1. Clay-based geopolymers**

Clays are frequently used as the source materials in geopolymer formation. They have a total composition of Al2 O3 and SiO2 in the range between 70 and 90% (**Table 1**) wherein the composition of clay is dependent on the origin and the geology of the location. Initially, in the early stage of geopolymer development, kaolin/kaolinite is mostly used as the aluminosilicate sources [2, 6, 32, 33]. Later, the experimental work has expanded to calcined clays, ashes and slag. This is because kaolin/kaolinite shows low reactivity with alkaline silicate solution causing low strength products. It is deemed that the near zero charge between layers and the layered structure that does not permit the exchange of ions or other element. Hence, kaolin/kaolinite has low surface area for geopolymerization reaction. According to Heah et al. [34], the low surface area limits the dissolution of kaolin/kaolinite to provide Si4+ and Al4+ ions for further reaction. Comparatively, fly ash has greater surface area as they have spherical-shaped particles.

Summary of the compressive strength of geopolymers based on clay/clay minerals is tabulated in **Table 2**. The strength achieved by geopolymers based on clay/clay minerals is low. The addition of kaolinite as secondary source of aluminosilicate is necessary in order to achieve strength. Unfortunately, the use of kaolinite alone in geopolymer is not preferable as it will produce weak structure [35]. The statement is further supported by van Jaarsveld et al. [38] who concluded the strength of fly ash geopolymers degraded as the result of high kaolinite content (41%) addition. The main reason for the deterioration in strength is because

Kaolinite [31] 49.35 36.03 0.20 0.02 0.02 — 0.04 0.02 2.29 — — 11.94 Kaolinite [31] 40.86 39.87 0.39 0.46 0.12 — 0.01 0.12 0.17 — — 17.91 Kaolinite [31] 42.66 40.92 1.12 0.45 0.04 — 0.14 0.14 0.09 — — 14.13 Halloysite [31] 48.12 36.33 0.33 0.16 — — 0.05 0.04 0.03 — — 14.8

Metakaolin [26] 51.35 44.24 0.98 0.90 0.48 0.45 0.16 0.13 0.08 0.01 — 0.72 Metakaolin [27] 52.1 43.0 0.7 — 0.3 — 0.12 2.5 — — 1.0 Metakaolin [28] 59.7 34.1 0.9 — — 0.2 0.1 — 0.1 1.2

47.5 15.6 6.7 — 2.4 — 0.3 10.2 1.9 — — 15.4

50.0 15.9 5.7 — 1.9 — 0.3 6.9 1.7 — — 17.5

48.92 25.16 7.52 0.86 0.21 0.16 0.21 0.68 1.4 0.01 2.94 11.93

**O3 TiO2 MgO P2O5 Na2O CaO K2O MnO SO3 LOI**

Clay-Based Materials in Geopolymer Technology http://dx.doi.org/10.5772/intechopen.74438 243

If the clays/clay minerals are heat-treated, the mechanical strength of the final products would increase [35, 39]. Pre-treatment is crucial to increase the reactivity of clays/clay minerals. The pre-treatment methods include mechanochemical, chemical and thermal treatments. MacKenzie et al. [40] reported that typical characteristic geopolymers are produced with heat-treated (200–1000°C for 2 hours) halloysite. Mechanochemical-treated (high-energy grinding for 20 hours at 400 rpm) halloysite showed less complete geopolymer formation. For acid-treated (0.1 M HCl) halloysite, the resulting geopolymers were poorly set while alkaline-treated (0.1 M NaOH) halloysite caused the formation of crystalline zeolites. Thermal treatment is the most used methods. Successfully calcined clays lead to highly pozzolanic amorphous phase. For instance, geopolymers from clay sediments treated at 750°C for 2 hours showed greater compressive strength (6–12 MPa) than those treated at 400°C (1–4 MPa) [29].

not all kaolinite reacted in the reaction.

**Table 1.** Chemical composition of clays from different origins.

**Clay/clay mineral**

Clay sediment from Occhito reservoir, Italy

Clay sediment from Sabetta reservoir, Italy

Kaolinite from Hiswa, Jordan

[29]

[29]

[30]

**SiO2 Al2**

**O3 Fe2**

**Figure 3.** SEM micrograph of metakaolin (800°C for 2 hours) [23].


**Table 1.** Chemical composition of clays from different origins.

**4. Metakaolin**

242 Cement Based Materials

Thermal treatment of kaolinite leads to the transformation of crystalline phases into reactive amorphous phases [7], which is the active constituent that determines the final strength of geopolymers. The thermal treatment is usually carried out at temperature in the range of 550–800°C which accompanied by dehydroxylation of strongly bounded hydroxyl ions on the

Metakaolin also has layered structure as kaolinite even after the thermal treatment process.

Also, the thermal treatment destroys the hexagonal layer of kaolinite and causes atomic arrangement that converted the hexa-coordinated Al ions of kaolinite are converted into penta- and tetra-coordinated Al ions [25]. The amount of penta- and tetra-coordinated Al ions

Clays are frequently used as the source materials in geopolymer formation. They have a total

position of clay is dependent on the origin and the geology of the location. Initially, in the early stage of geopolymer development, kaolin/kaolinite is mostly used as the aluminosilicate sources [2, 6, 32, 33]. Later, the experimental work has expanded to calcined clays, ashes and slag. This is because kaolin/kaolinite shows low reactivity with alkaline silicate solution causing low strength products. It is deemed that the near zero charge between layers and the layered structure that does not permit the exchange of ions or other element. Hence, kaolin/kaolinite has low surface area for geopolymerization reaction. According to Heah et al. [34], the low surface area limits the dissolution of kaolin/kaolinite to provide Si4+ and Al4+ ions for further reaction. Comparatively, fly ash has greater surface area as they have spherical-shaped particles.

in the range between 70 and 90% (**Table 1**) wherein the com-

However, the layer structure appeared more open than kaolinite (**Figure 3**) [23, 24].

Al-constitutive layer. Thus, kaolinite is transformed into metakaolin.

reflects the reactivity of metakaolin [24].

O3

and SiO2

**Figure 3.** SEM micrograph of metakaolin (800°C for 2 hours) [23].

**4.1. Clay-based geopolymers**

composition of Al2

Summary of the compressive strength of geopolymers based on clay/clay minerals is tabulated in **Table 2**. The strength achieved by geopolymers based on clay/clay minerals is low. The addition of kaolinite as secondary source of aluminosilicate is necessary in order to achieve strength. Unfortunately, the use of kaolinite alone in geopolymer is not preferable as it will produce weak structure [35]. The statement is further supported by van Jaarsveld et al. [38] who concluded the strength of fly ash geopolymers degraded as the result of high kaolinite content (41%) addition. The main reason for the deterioration in strength is because not all kaolinite reacted in the reaction.

If the clays/clay minerals are heat-treated, the mechanical strength of the final products would increase [35, 39]. Pre-treatment is crucial to increase the reactivity of clays/clay minerals. The pre-treatment methods include mechanochemical, chemical and thermal treatments. MacKenzie et al. [40] reported that typical characteristic geopolymers are produced with heat-treated (200–1000°C for 2 hours) halloysite. Mechanochemical-treated (high-energy grinding for 20 hours at 400 rpm) halloysite showed less complete geopolymer formation. For acid-treated (0.1 M HCl) halloysite, the resulting geopolymers were poorly set while alkaline-treated (0.1 M NaOH) halloysite caused the formation of crystalline zeolites. Thermal treatment is the most used methods. Successfully calcined clays lead to highly pozzolanic amorphous phase. For instance, geopolymers from clay sediments treated at 750°C for 2 hours showed greater compressive strength (6–12 MPa) than those treated at 400°C (1–4 MPa) [29].


**5. Geopolymerization mechanism**

hydroxide (**Figure 6**) [47].

and B—CSH phases) [43]**.**

silicates in highly alkaline medium.

Geopolymer formation involves chemical reaction that transforms partially or totally amorphous aluminosilicates into three-dimensional polymeric networks. The geopolymerization reaction is exothermic. Under strong alkaline medium, the aluminosilicate sources dissolve into SiO4 and AlO4 tetrahedral units which later on participate in the polycondensation process [44, 45].

**Figure 4:** SEM micrographs of (a) pure metakaolin; and (b) metakaolin-50% slag geopolymer (a—Geopolymer matrix

Clay-Based Materials in Geopolymer Technology http://dx.doi.org/10.5772/intechopen.74438 245

The chemical attack of kaolinite starts from the surface and edge and continues layer by layer inside the structure (**Figure 5**) [46]. The Al-substituted silicate layers formed and the structural deformed Al sites transformed into tetra-coordinated Al sites after attack by alkali

Davidovits proposed the reaction mechanism as shown in **Figure 7**. The reaction aluminosili-

In general, the geopolymerization mechanism is similar for all types of aluminosilicates. Most researchers agreed that the geopolymerization reaction involves dissolution, polycondensation and hardening process. The dissolution of aluminosilicates is initiated by presence of hydroxyl ions in the alkaline silicate solution which releases Si and Al species for further polycondensation reaction [9, 49]. The geopolymerization reaction is deemed occurs in multistep simultaneously [38, 42, 50] such as reorganization and diffusion of dissolved ions with formation of small coagulated structures, solid state transformation and hardening to form hard solid polycondensation to form aluminosilicate gel phases and dissolution of alumino-

In addition, Xu and Van Deventer [51] suggested that geopolymer is formed through Eqs. (2)–(4). Eq. (1) represents the mixing of aluminosilicates with alkali silicate solution. Geopolymer gel is

cates and alkali silicate solution produced geopolymers with Si-O-Al backbone.

formed in Eq. (3), while Eq. (4) shows the formation of geopolymer rigid solid.

**Table 2.** Strength result of clay/clay minerals geopolymers.

According to the author, thermally treated clay sediments exhibited improved surface area toward dissolution and geopolymerization reaction. Apart from the purely clay geopolymers, blended geopolymers are also produced with the addition of other materials such as calcium hydroxide, slag and ashes with the clay materials as the starting source material. When calcium hydroxide is added, the strength of the blended geopolymers does not degrade [41, 42]. Similarly, the addition of 30% slag in metakaolin geopolymers showed improvement in the mechanical strength. Slag acted as filler in the geopolymer structure and enhanced the mechanical properties. However, the slag addition is limited to below 50% as it will greatly deteriorate the strength at content beyond 50% [43]. The high calcium content in both calcium hydroxide and slag caused the formation of geopolymer matrix as the main phases and the calcium silicate hydrates (CSH) phases as the secondary phases [27, 43]. This can be clearly shown in **Figure 4**.

**Figure 4:** SEM micrographs of (a) pure metakaolin; and (b) metakaolin-50% slag geopolymer (a—Geopolymer matrix and B—CSH phases) [43]**.**
