3.2.4 Activating alkali solutions

The efficiency of the alkali activation process of geopolymers is very much dependent on the addition of chemical activators (sodium/potassium hydroxide, soluble silicates, etc.) and also the curing regime (heat treatment) employed on the hardened geopolymer concrete [6]. Strength development of geopolymers fabricated without the addition of chemical activators or subsequent heat treatment is very slow, particularly during the early stages.

The most common alkaline liquid used in geo-polymerization is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate or potassium silicate [28].

Besides the aluminosilicate as raw material an alkaline activator is required to produce a geopolymer. The alkali component used as an activator is a compound from the elements of the first group in the periodic table. The common activators can be classified as follows:


Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production… DOI: http://dx.doi.org/10.5772/intechopen.82413

• Metal aluminate NaAlO2.

from different sources and processes also differ in composition and reactivity, meaning that this is rather a diverse class of materials which can provide alkali-

furnace slag is obtained by quenching molten iron slag from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder [19]. The main components of blast furnace slag are CaO (30–50%), SiO2 (28–38%), Al2O3 (8–24%), and MgO (1–18%). In general, increasing the CaO content of the slag results in raised slag basicity and an increase in compressive strength. The MgO and Al2O3 content show the same

improvement can be obtained [25]. GGBS has now effectively replaced sulfateresisting OPC on the market for sulfate resistance because of its superior

Particle size of the raw materials is also important factors in geopolymerization. Finer particle sizes of the RHA improve its reactivity and thereby higher degree of geopolymerization can be achieved. The finer the particle size the stronger the geopolymer [26]. Higher degree of geopolymerization makes the resulting

geopolymer more ductile and stronger. As the particle size of the RHA decrease the surface area increase [27]. The increased surface area also results in the formation of more ductile and stronger geopolymers. This suggests that the mechanical properties of geopolymers are depending upon the physical property, the particle size of the raw materials. The strength of geopolymer mortars are affected by the fineness of RHA. The increase in the fineness of RHA increases its reactivity and strength of mortars. RHAs with 1– 5% retained on No. 325 sieve are suitable for making

The efficiency of the alkali activation process of geopolymers is very much dependent on the addition of chemical activators (sodium/potassium hydroxide, soluble silicates, etc.) and also the curing regime (heat treatment) employed on the hardened geopolymer concrete [6]. Strength development of geopolymers fabricated without the addition of chemical activators or subsequent heat treatment is

The most common alkaline liquid used in geo-polymerization is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate

Besides the aluminosilicate as raw material an alkaline activator is required to produce a geopolymer. The alkali component used as an activator is a compound from the elements of the first group in the periodic table. The common activators

• Ground Granulated Blast furnace Slag (GGBS): Ground-granulated blast-

trend up to respectively 10–12% and 14%, beyond which no further

performance and greatly reduced cost.

Geopolymers and Other Geosynthetics

3.2.3 Particle size and reactivity of raw materials

geopolymer mortars.

3.2.4 Activating alkali solutions

or potassium silicate [28].

can be classified as follows:

84

very slow, particularly during the early stages.

• Alkali metal hydroxide: NaOH, KOH, LiOH.

• Alkali metal silicate (AMS) Na2SiO3, K2SiO3.

• Alkali metal hydroxide silicate: Na2SiO3 + NaOH.

activated products with a range of performance levels [24].

• Metal carbonate Na2CO3.

The Alkalinity of the solution is a widely investigated factor and the most significant factor controlling the compressive strength of geopolymer concrete. High alkalinity of the solution accelerates the dissolution of the raw materials, which shortens the setting time. It also enhances the compressive strength of geopolymers. The higher the alkaline concentration is, the higher the compressive strength is obtained.

#### 3.3 Concept and chemical mechanism of geopolymerization

Geopolymer is an inorganic polymer with SiO4 and AlO4 tetrahedra being the structural units [29]. Geopolymers composites are also defined as an Al- and Si-rich cementitious, amorphous binder, which is formed by polymerization of an alkali activated solid aluminosilicate precursor [30]. Geosynthesis is based on the ability of the aluminum ion to induce crystallographical and chemical modifications in a silica backbone. Usually polymerization reaction takes place in organic compounds, due to the tetra valancy of the carbon atom. Geopolymerization is an inorganic polymerization. It consists of dissolution and hydrolysis followed by a condensation step in an alkaline silicate plus alumino-silicate system. The chemistry of geopolymerization is similar with the synthesis of zeolites, although the resultant products are different in composition and structure [31]. It consists of chains or a 3D framework of linked AlO4 <sup>5</sup>� and SiO4 tetrahedra. The more general term inorganic polymer defines a super group with a deviation from the tetrahedral coordination of Al and Si and the aluminosilicate chemistry [32]. The dissolution of the mineral results with the formation of Si▬O▬Al species as monomers. Their reorganization leads to the GP network.

The schema below illustrates the reactions proposed for the polycondensation process [12]. In the reactions (8) and (9) the amount of Al–Si materials used depend on the particle size, the extent of dissolution of Al–Si materials and the concentration of the alkaline solution. The formation of [Mz(AlO2)x(SiO2)y�MOH�H2O] gel is a dominant step in the geopolymerization and essentially relies on the extent of dissolution of aluminosilicate materials (reaction (10)).

$$\text{Al Si material (s)} + \text{MOH (aq)} + \text{Na}\_2\text{SiO}\_3 \text{ (s or aq)}\tag{8}$$

$$\begin{array}{rcl} \text{A I Si material} & \text{(s)} \, ^+ [\text{M}\_x (\text{AlO}\_2)\_x \, (\text{SiO}\_2) \, \text{n} \, \text{MOH.m} \text{MOH}\_2\text{O}] \, \text{gel} \end{array} \tag{9}$$

$$\text{Al Si material (s)} + \overset{\text{J}}{[\text{M}\_4((\text{AlO}\_2)\_4 \ (SiO\_2)\_6 \ nMOH.mMOH\_2O}]} \tag{10}$$

The general empirical formula of geopolymer is as follows:

$$\rm{M} + n \left[ -(\rm{SiO\_2}) \rm{z-AlO\_2-} \right] \rm{n} \tag{11}$$

Where M+ = an alkali cation (K+ , Na<sup>+</sup> ) for balancing the negative charge of Al3+ in IV-fold coordination; n = degree of polymerization; and z = Si/Al ratio. The value of 'z' represents describe the Si/Al ratio, based on which three types can be distinguished: poly(sialate) with 1:1 Si/Al ratio, poly(sialate-siloxo) with 2:1 Si/Al ratio and poly(sialatedisiloxo) with 3:1 Si/Al ratio [7]. Geopolymers possess amorphous to semi-crystalline three dimensional silico-aluminate structures consisting of

linked SiO4 and AlO4 tetrahedra by sharing the oxygen atoms, which can be designated as poly-sialate (▬Si▬O▬Al▬O▬) (Si/Al = 1), poly-sialate-siloxo (▬Si▬O▬Al▬O▬Si▬O▬) (Si/Al = 2), poly-sialate-disiloxo (▬Si▬O▬Al▬O▬Si▬O▬Si▬O▬) (Si/Al = 3),and sialate links (Si:Al > 3). The sialate is an abbreviation for silicon-oxo-aluminate. The structures of the above types of poly(sialates) are schematically presented in Figure 6 [23, 29].

Aluminosilicate backbones are formed during geopolymerization process as shows Figure 5. Sialate is an abbreviate form for alkali silicon-oxo-aluminate, the alkali element being (Na, K, Li, Ca) and the term poly(sialate) covers all geopolymers containing at least one (Na,K,Li,Ca)(Si▬O▬Al) and (Na,K,Li,Ca) sialate unit. Sodalite frameworks and kalsilite frameworks have structural molecules Na-(▬Si▬O▬Al▬O▬) and K-(▬Si▬O▬Al▬O▬) respectively [13]. As shows Figure 6, they are chain and ring inorganic polymers that are the result of the polycondensation of the monomer, orthosialate (OH)3▬Si▬O▬Al▬(OH).

Sanidine frameworks, K-(▬Si▬O▬Al▬O▬Si▬O▬Si▬O▬) may be considered as the condensation result of orthosialate with two ortho-silicic Si(OH)3 [13]. The sialate unit may be at the beginning, in the middle or at the end of the sequence. There are six isomorphs: 2 linear, 2 branched and 2 cycles. Leucite frameworks with structural molecule K-(▬Si▬O▬Al▬O▬Si▬O▬) may be considered as the condensation result of orthosialate with ortho-silicic acid Si(OH). There are three isomorphs, a linear (▬Si▬O▬SiO▬Al▬O▬), mono-siloxo-sialate and 3 cycles. Anorthite frameworks containing 2 sialate unit, Ca-

(▬Si▬O▬Al▬O▬Si▬O▬Al▬O▬) are ring polymers that are result of the polycondensation of the monomer [13].

Crystalline alumina hydrate is extracted from the digestion liquor by hydrolysis.

$$2\text{NaAlO}\_2 + 4\text{H}\_2\text{O} \quad \xrightarrow{\text{---} \qquad \text{Al(OH)}\_3 + 2\text{NaOH}} \qquad (12)$$

After Duxson, the geopolymer structure consists of cross-linked, SiO4 and AlO4 tetrahedral species where the negative charge on Al3+ in IV-fold coordination is

ð14Þ

balanced with the positive charges of the alkali ions (Na+, K+). The geopolymerization reaction can be expressed as shown below:

Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production…

DOI: http://dx.doi.org/10.5772/intechopen.82413

Polymeric structures from polymerization of monomers [33].

Figure 6.

87

The formation reactions of the above geopolymer material are established by the following two reactions [18].

Figure 5. Schema of GP-monomers units [23, 29]. Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production… DOI: http://dx.doi.org/10.5772/intechopen.82413

Figure 6.

linked SiO4 and AlO4 tetrahedra by sharing the oxygen atoms, which can be desig-

Aluminosilicate backbones are formed during geopolymerization process as shows Figure 5. Sialate is an abbreviate form for alkali silicon-oxo-aluminate, the

geopolymers containing at least one (Na,K,Li,Ca)(Si▬O▬Al) and (Na,K,Li,Ca) sialate unit. Sodalite frameworks and kalsilite frameworks have structural molecules Na-(▬Si▬O▬Al▬O▬) and K-(▬Si▬O▬Al▬O▬) respectively [13]. As shows Figure 6, they are chain and ring inorganic polymers that are the result of the polycondensation of the monomer, orthosialate (OH)3▬Si▬O▬Al▬(OH).

Sanidine frameworks, K-(▬Si▬O▬Al▬O▬Si▬O▬Si▬O▬) may be considered as the condensation result of orthosialate with two ortho-silicic Si(OH)3 [13]. The sialate unit may be at the beginning, in the middle or at the end of the sequence. There are six isomorphs: 2 linear, 2 branched and 2 cycles. Leucite frameworks with structural molecule K-(▬Si▬O▬Al▬O▬Si▬O▬) may be considered as the condensation result of orthosialate with ortho-silicic acid Si(OH). There are three isomorphs, a linear (▬Si▬O▬SiO▬Al▬O▬), mono-siloxo-sialate and

(▬Si▬O▬Al▬O▬Si▬O▬Al▬O▬) are ring polymers that are result of the poly-

Crystalline alumina hydrate is extracted from the digestion liquor by hydrolysis.

The formation reactions of the above geopolymer material are established by the

n(Si2O5, Al2O3) + 4nH2O + NaOH/KOH Na+, K+ + n(OH)3-Si-O-Al-O-Si-(OH)3

n(OH)5 -Si-O-Al**—**O-Si-(OH)3 + NaOH/KOH (Na, K)-(-Si-O-Al-O-Si-O-) + 4nH2O

2NaAlO2 + 4H2O Al(OH)3 + 2NaOH ð12Þ

(OH)2

O O O (Geopolymer backbone)

(Geopolymer precursor

ð13Þ

nated as poly-sialate (▬Si▬O▬Al▬O▬) (Si/Al = 1), poly-sialate-siloxo

types of poly(sialates) are schematically presented in Figure 6 [23, 29].

alkali element being (Na, K, Li, Ca) and the term poly(sialate) covers all

(▬Si▬O▬Al▬O▬Si▬O▬Si▬O▬) (Si/Al = 3),and sialate links (Si:Al > 3). The sialate is an abbreviation for silicon-oxo-aluminate. The structures of the above

(▬Si▬O▬Al▬O▬Si▬O▬) (Si/Al = 2), poly-sialate-disiloxo

3 cycles. Anorthite frameworks containing 2 sialate unit, Ca-

condensation of the monomer [13].

Geopolymers and Other Geosynthetics

following two reactions [18].

(OH)2

Schema of GP-monomers units [23, 29].

Figure 5.

86

Polymeric structures from polymerization of monomers [33].

After Duxson, the geopolymer structure consists of cross-linked, SiO4 and AlO4 tetrahedral species where the negative charge on Al3+ in IV-fold coordination is balanced with the positive charges of the alkali ions (Na+, K+). The geopolymerization reaction can be expressed as shown below:

$$\text{\textbullet } \text{\textbullet} \text{\textbullet} \text{\textbullet}\_2 \text{\textbullet}\_2 \text{O}\_3 + \text{\textbullet} \text{\textbullet} \text{\textbullet}^- \text{\textbullet}^- \text{\textbullet}^+ \text{\textbullet}^- \text{\textbullet}^- \text{\textbullet}^- \text{\textbullet}^+ \text{\textbullet}^- \text{\textbullet}^- \text{\textbullet}^+$$

ð14Þ

Geopolymers are mainly represented in the models proposed by Davidovits and Barboza the as shown in Figures 7 and 8 [34, 35]. The two models have in common a space, three-dimensional disposition. The Davidovits structural model of GP is designed on the basis of a poly-sialate–siloxo type. It takes on a monolithic type of GP comparable to organic polymers. The water molecules surrounding the Na ion in the Barbosa model suggests the presence of pores in the structure of the GP.

#### 3.4 Characterization of geopolymers

With regard to the characterization of GPs, the use of common materials science techniques presents challenges because of the complex multiphase nature of precursors being structurally disordered: glassy (FA) or thermally disrupted layer (MK). The most widely used tools for microstructural analysis of GP-materials are scanning electron microscopy (SEM), X-ray diffraction (XRD), EDX.

#### 3.4.1 Microstructure of RM and RHA

The SEM diagram of the RM is shown in Figure 9 RM has relatively porous microstructure with the presence of dispersed particles. The figure presents unequal formed aggregates with smaller particles. The aggregates represent probably Fe2O3 particles and needle-shaped particles of CaSO4.

Figure 10 presents the morphological properties of the RHA detected by SEM. It shows a porous and multifaceted particle shape and size. The foremost constituents of rice husk comprise hydrated silica, cellulose and hemi cellulose component and lignin component of approximately. The porous and honeycomb morphology seen can be credited to the burning out of the organic component in the rice husk during

Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production…

The XRD diffractogram of RM (Figure 11) shows the presence of hematite Fe2O3, gibbsite Al(OH)3, Al2O3.H2O, lapidocrocte FeO(OH) and calcite CaCO3. RM displays some undisclosed peaks and a few sharp peaks that are mainly from hematite and calcite, but no observable broad humps [36]. This suggests that the amorphous phases are not present at large quantity. By comparison with its chemical composition, alumina mainly presents as amorphous phases. Thus, red mud provides mainly Al (in the form of amorphous Al2O3 or dissolved NaAlO2) and NaOH

There are three parent materials contributing in the synthesis of RHA/RM geopolymers: RM, RHA, and NaOH solution. However, only amorphous phases in raw materials contribute in geopolymerization reaction [36]. Among the three raw materials, the red mud provides NaOH, A12O3, and NaAlO2; rice husk ash provides

amorphous SiO2; sodium hydroxide solution provides NaOH.

combustion.

Figure 10.

89

SEM micrograph of RHA.

Figure 9.

but little Si to geopolymerization.

The SEM micrograph of RM from ACG Plant.

DOI: http://dx.doi.org/10.5772/intechopen.82413

3.4.2 Microstructure of RM-based GP

Figure 7. Davidovits model [34].

Figure 8. Barbosa model [35].

Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production… DOI: http://dx.doi.org/10.5772/intechopen.82413

Figure 9. The SEM micrograph of RM from ACG Plant.

Geopolymers are mainly represented in the models proposed by Davidovits and Barboza the as shown in Figures 7 and 8 [34, 35]. The two models have in common a space, three-dimensional disposition. The Davidovits structural model of GP is designed on the basis of a poly-sialate–siloxo type. It takes on a monolithic type of GP comparable to organic polymers. The water molecules surrounding the Na ion in the Barbosa model suggests the presence of pores in the structure of the GP.

With regard to the characterization of GPs, the use of common materials science techniques presents challenges because of the complex multiphase nature of precursors being structurally disordered: glassy (FA) or thermally disrupted layer (MK). The most widely used tools for microstructural analysis of GP-materials are

The SEM diagram of the RM is shown in Figure 9 RM has relatively porous microstructure with the presence of dispersed particles. The figure presents unequal formed aggregates with smaller particles. The aggregates represent probably Fe2O3

scanning electron microscopy (SEM), X-ray diffraction (XRD), EDX.

3.4 Characterization of geopolymers

Geopolymers and Other Geosynthetics

3.4.1 Microstructure of RM and RHA

Figure 7.

Figure 8. Barbosa model [35].

88

Davidovits model [34].

particles and needle-shaped particles of CaSO4.

Figure 10 presents the morphological properties of the RHA detected by SEM. It shows a porous and multifaceted particle shape and size. The foremost constituents of rice husk comprise hydrated silica, cellulose and hemi cellulose component and lignin component of approximately. The porous and honeycomb morphology seen can be credited to the burning out of the organic component in the rice husk during combustion.

The XRD diffractogram of RM (Figure 11) shows the presence of hematite Fe2O3, gibbsite Al(OH)3, Al2O3.H2O, lapidocrocte FeO(OH) and calcite CaCO3. RM displays some undisclosed peaks and a few sharp peaks that are mainly from hematite and calcite, but no observable broad humps [36]. This suggests that the amorphous phases are not present at large quantity. By comparison with its chemical composition, alumina mainly presents as amorphous phases. Thus, red mud provides mainly Al (in the form of amorphous Al2O3 or dissolved NaAlO2) and NaOH but little Si to geopolymerization.
