**2. Geopolymers**

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 inorganic polymerization and condensation reactions [1].

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

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

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

**13**

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

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

• Chemically, the conventional process includes solution chemistry mechanism in which amorphous aluminosilicate, e.g. fly ash, reacts with the solution of sodium hydroxide with sodium silicate and forms geopolymeric gel. Therefore geopolymer formation follows the bimolecular nucleophilic substitution (SN2) mechanism [17]. Though geopolymerization reactions via solution chemistry mechanism cannot be fully understood, the complete mechanism can be understood under following heads including association, dissociation, oligo-

• Step I is association, that is, association of water molecules to siloxane bond (▬Si▬O▬Si▬) present in aluminosilicate raw material. This association leads to the formation of an intermediate silicon species. The structure of intermediate silicon species is pentavalent making it highly reactive. The intermediate pentavalent silicon possesses distorted trigonal bipyramid structure and due to highly reactive nature undergoes dissociation rapidly to

• Step II is dissociation; in this step of SN2 mechanism, intermediate pentavalent silicon undergoes dissociation in a concerted manner and forms silanol

SiO(OH)3 and silanetetraol Si(OH)4 species. It is to be noted that the hydrated silica behaves as an acidic oxide in the presence of alkaline solution and possesses tendency to go into solution. Further, OH<sup>−</sup> ions break siloxane bridges

• The silanols further reacts and forms silanediol SiO(OH)2, silanetriol

(>Si▬OH) and aluminol (>Al▬OH) groups [12, 18–22].

and result into the formation of alkaline silicates [23–26].

**3. Geopolymerization**

summarized as follows:

merization, and autopolycondensation.

from silanols [12, 18–20].

#### **Figure 1.**

*Schematic representation of conventional geopolymerization.*
