**3. Geopolymerization**

*Geopolymers and Other Geosynthetics*

Geopolymers can be defined as covalently bonded noncrystalline Si▬O▬Al networks in which SiO4 and AlO4 tetrahedral frameworks are linked by shared oxygen to form a dense amorphous to semicrystalline three-dimensional framework. These are termed as geological polymers for the reason that their starting raw materials is of geological origin, and formation of geopolymer proceeds via

Prof. J. Davidovits in 1978 introduced the term geopolymer and described it as cement-free green cementitious material. These are the inorganic polymers obtained from alkali activation of aluminosilicate materials like fly ash. These are structurally and chemically comparable to natural rocks and are synthesized by the condensation mechanism similar to thermosetting organic polymers therefore termed as geopolymers. Earlier these were considered as a special case of 'soil cement/silicates' or alkali-activated aluminosilicate cement and termed as 'geocements' as it consists of

Geopolymers have the potential to replace ordinary Portland cement (OPC) and to overcome the limitations associated with OPC. The production of OPC requires high temperature for calcinations which is not a requisite for the production of geopolymers. Unlike OPC, the production mechanism of geopolymers does not produce greenhouse gas CO2 and possess extraordinary chemical properties and mechanical strength. Thus, geopolymers are environment-friendly substitutes for

Geopolymers can be produced from sources of geological origin (e.g. kaolinite, clay) or industrial by-products such as fly ash, granulated blast furnace slag, red mud, waste paper sludge, rice husk ash, wheat straw ash, etc. [4, 5]. The choice of source material in geopolymerization technology depends upon the competitive cost, availability, and specific application. Fly ash [Class F fly ash]-based geopolymerization is getting intense research interest in past few years. It is a coal combustion residue generated from thermal power plants extensively rich in silica and alumina content [6]. Alkali activation of reactive silica- and alumina-rich raw materials produces an intense 3D▬Si▬O▬Al▬O▬polymerization network [7]. The compact 3D framework thus formed after hardening is known as geopolymer,

inorganic polymerization and condensation reactions [1].

OPC and are frequently referred to as 'green cement'.

three-dimensional framework of cross-linked polysialate chains [2, 3].

and the complete process is termed as geopolymerization (**Figure 1**).

**2. Geopolymers**

**12**

**Figure 1.**

*Schematic representation of conventional geopolymerization.*

Geopolymerization is the process of transforming aluminosilicate raw material into covalently bonded 3D network consisting [▬Si▬O▬Al▬O▬]n bonds. In other words, geopolymerization process refers to geosynthesis, i.e. synthesis of chemically integrated minerals. The geopolymerization reaction results in the formation of viscous cementitious slurry which upon hardening forms strong, durable, and compact geopolymeric material [6, 8, 9]. Moreover much has been known about geopolymers and their chemistry in the last two decades, and efforts are still going on to uncover and discover some new scientific aspects of these wonder materials including some innovative applications besides construction sector [10, 11]. Considering this numerous scientists and researchers are engaged all over the world to extract out more potentiality in geopolymeric materials. The knowledge regarding geopolymer science during the last two decades indicated that the inorganic geopolymers are prepared by using starting raw materials which should essentially contain reactive silica and alumina in their structure, e.g. fly ash, and the alkaline activator solution containing mixture of sodium hydroxide and sodium silicate [12–14]. Conveniently we termed this process of developing geopolymer as conventional geopolymerization technology. It is to note that geopolymers can be prepared by utilization of different aluminosilicate sources such as red mud, blast furnace slag, kaolinite, rice husk ash, etc., and the starting material plays important role in deciding physicochemical and mechanical properties of geopolymeric material [15, 16]. The basic understanding of geopolymer formation and chemical reactions involved during conventional geopolymerization can be summarized as follows:


**Figure 2.** *Geopolymerization reactions proposed by [1–3].*

#### **Figure 3.**

*Illustration of silanol and aluminol linkages in geopolymerization via SN2-solution chemistry mechanism.*


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*Advanced Geopolymerization Technology DOI: http://dx.doi.org/10.5772/intechopen.87250*

species like Na+

In the geopolymeric chemistry, the negative charge on Al is balanced by cationic

tions which proceed with inversion of configuration. The rate of concerted mechanism depends on the concentration of both nucleophiles OH<sup>−</sup> and the molecule undergoing attack. The complete reaction sequence for SN2 mechanism is presented in **Figure 2**. On the other hand, **Figure 3** is the complete illustration of silanol and aluminol linkages in geopolymerization via SN2-solution chemistry mechanism.

In order to make large-scale commercialization of geopolymeric materials, it is essential to replace this conventional synthesis method because it is not a userfriendly process. The dissolution of NaOH in water is an exothermic reaction which leads to the formation of highly hazardous alkaline solution. The handling of alkaline solution is risky and prone to on-site hazardous accidents. Besides, the conventional method produced geopolymers with the properties suited for the confined commercial applications. With the conventional method, tailoring of properties of geopolymeric product is not possible. It is noteworthy that, for multifunctional applications of any product, tailoring of properties is highly desired for large-scale production. With this context, the conventional process seems to be ineffective regarding tailoring of properties for broad-range applications. Therefore to overcome these limitations, we introduce the concept of advanced geopolymerization

Advanced geopolymerization is a novel approach for the manufacture of geopolymers via innovative solid-state chemistry mechanism invented and patented by [27]. The advanced process is one part system, i.e. the tailored geopolymeric precursors in solid powder form are the one part requirement and only water is needed for its conversion to advanced geopolymeric material, in contrast to conventional process, where the first solution of alkali is prepared and then mixed with source materials. In the novel process, tailored geopolymeric precursors in solid powder form are obtained via mechanochemical dry grinding of raw materials for 8 hours. The basic raw materials include vitreous silica- and alumina-containing waste material, i.e. fly ash and NaOH with or without sodium silicate. This solid powder needs only the addition of water to form geopolymeric material. The advanced process comprises solid-state reactions during dry grinding process and enables tailoring of raw materials and sequencing of reactions among them for the preparation of geopolymeric materials. These geopoly-

meric materials can be used on site easily just like ordinary Portland cement.

Grinding is an important operation that is used industrially for particle size reduction and production of large surface areas or liberating valuable things from any mineral. It comprises of different steps including material transport to grinding zone, grinding action, initiation and propagation of cracks, breakage of particle, or initiation of solid-state reactions within. Size reduction and intergranular breakage are significantly achieved by subjecting particles to mechanical pressure for a prolonged period. Generally, the breakage and fracturing process during grinding involves

**6. Mechanochemistry/solid-state method**

**4. Drawbacks of conventional geopolymerization process**

and its basic reaction mechanism further in this chapter.

**5. Advanced geopolymers**

to maintain the electrical neutrality. These are stereo-specific reac-

#### *Advanced Geopolymerization Technology DOI: http://dx.doi.org/10.5772/intechopen.87250*

*Geopolymers and Other Geosynthetics*

**14**

**Figure 3.**

**Figure 2.**

*Geopolymerization reactions proposed by [1–3].*

*Illustration of silanol and aluminol linkages in geopolymerization via SN2-solution chemistry mechanism.*

• Similarly in aluminosilicate materials, oxides of Al get hydrated and form aluminol (>Al▬OH) containing one negative charge and represented as Al (OH)4−. Al3+ developed in fourfold coordination structure as it contributed only three electrons to bonding framework in place of four silicon atoms and

• Autopolycondensation of silanols and aluminols takes place, and oligomers are transformed into polymer with the release of water molecules, further hardened to compact, strong 3D structure of [▬Si▬O▬Al▬O▬]n framework.

because of this carry one negative charge [13].

In the geopolymeric chemistry, the negative charge on Al is balanced by cationic species like Na+ to maintain the electrical neutrality. These are stereo-specific reactions which proceed with inversion of configuration. The rate of concerted mechanism depends on the concentration of both nucleophiles OH<sup>−</sup> and the molecule undergoing attack. The complete reaction sequence for SN2 mechanism is presented in **Figure 2**. On the other hand, **Figure 3** is the complete illustration of silanol and aluminol linkages in geopolymerization via SN2-solution chemistry mechanism.
