3.2.2 Industry wastes

• Fly ash: fly ash is defined as the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gasses from the combustion zone to the particle removal system. The characteristics of fly ash are loss on ignition (LOI), fineness and uniformity. LOI is a measurement of unburnt carbon remaining in the ash. Finer gradation generally results in a more reactive ash and contains less carbon. Fly ash is a very fine, powdery material, composed mostly of silica. It consists mostly of silt-sized and clay-sized glassy spheres. These are generally spherical in shape and range in size from 0.5 to 100 μm [20]. They consist mostly of SiO2, which Survey of Bauxite Resources, Alumina Industry and the Prospects of the Production… DOI: http://dx.doi.org/10.5772/intechopen.82413

is present in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and hazardous. Three classes of fly ash are defined by ASTM C 618; Class N fly ash, Class F fly ash, and Class C fly ash [15]. The chief difference between these classes is the amount of calcium, silica, alumina, and iron content in the ash.


This is stronger, less permeable and more resistant to chemical attack. Due to its insulating properties, RHA has been used in the manufacture of refractory bricks. Recently RHA has been incorporated in activated aluminosilicates.

• Catalyst residues: Petroleum refineries worldwide process crude oil in fluid catalytic cracking (FCC) units, and 160,000 tons of spent FCC catalyst residue are thus produced every year [23]. The spent catalyst is essentially an agglomeration of zeolite (faujasite) crystals held together by an aluminosilicate matrix including amorphous silica and clays. However, in using this type of residue as a precursor in alkali-activation, it is important to consider the significant heavy metal content of the catalysts, particularly nickel, vanadium and/or lanthanum, as these may impact the performance of the geopolymer materials, and are also potentially leachable under some conditions. Catalysts

Figure 4. (a) RM oven dried (b) RM ground (c) rice husk ash.

comprise natural minerals such as metakaolin, clays, which contains Si, Al and oxygen in their chemical composition [16]. By-product from other industries such as fly ash, silica fume, slag, rice-husk ash red mud (§ I.3.1.), etc. could be utilized alternatively as the source materials. Disposal, price, application and demand of the users are the main factors in the process of the selection of source materials [4]. In order to obtain a GP with desired properties e.g. high strength, low shrinkage, high acid resistance or low cost, a range of ratios need to be controlled: Si/Al ratio; Na/Si and K/Si ratio and water to solid ratio. It is therefore of major importance to characterize the aluminosilicate source and to determine their reactivity, in order to be able to evaluate the amount silicates and aluminates reacting. It should be noted that the particles size distribution or fineness is of importance regarding the reactivity of the aluminosilicate source. The mix can then be optimized by adjusting the

Most of the investigations have used alkali solutions for dissolution of raw materials to form the reactive precursors necessary for geopolymerization. It has been shown that silicate activation increases the dissolution of the starting materials

. Two groups of materials are required to make a geopolymer; one is source materials containing alumina and silica and other is an alkali that activates the polymerization reaction. Basing on their origin materials of the first group are

• Kaolinite (microsilica): is a clay mineral having the chemical composition Al2Si2O5(OH)4 Rocks that are rich in kaolinite are known as kaolin or china clay. Kaolinite is a clay mineral with the chemical composition Al2S2O5(OH)4, which means each particle has one tetrahedral silica layer and one octahedral alumina layer [15]. It is a soft, earthy, usually white mineral, produced by the chemical weathering of aluminum silicate minerals like feldspar. Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin. Kaolin is a fine, white, clay mineral that has been traditionally used in the manufacture of

• Metakaolin is a dehydroxylated form of the clay mineral kaolinite in the temperature range of 500–800°C. It is a highly pozzolanic and reactive material. Kaolinite is a clay mineral with the chemical composition

Al2Si2O5(OH)4, which means each particle has one tetrahedral silica layer and one octahedral alumina layer. It is a soft, earthy, usually white mineral, produced by the chemical weathering of aluminum silicate minerals like

• Fly ash: fly ash is defined as the finely divided residue that results from the combustion of ground or powdered coal and that is transported by flue gasses from the combustion zone to the particle removal system. The characteristics

of fly ash are loss on ignition (LOI), fineness and uniformity. LOI is a measurement of unburnt carbon remaining in the ash. Finer gradation generally results in a more reactive ash and contains less carbon. Fly ash is a very fine, powdery material, composed mostly of silica. It consists mostly of silt-sized and clay-sized glassy spheres. These are generally spherical in shape and range in size from 0.5 to 100 μm [20]. They consist mostly of SiO2, which

3.2.1 Natural minerals: Kaolinite, calcined kaolinite (metakaolin) and clays

type and the amount activators added.

Geopolymers and Other Geosynthetics

natural or industrial (mainly by-products).

porcelain.

feldspar.

3.2.2 Industry wastes

82

and gives rise to favorable mechanical properties [21].

from different sources and processes also differ in composition and reactivity, meaning that this is rather a diverse class of materials which can provide alkaliactivated products with a range of performance levels [24].

• Metal aluminate NaAlO2.

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

• Metal carbonate Na2CO3.

strength is obtained.

AlO4

85

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

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

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

<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

AI Si material (s) +MOH (aq) + Na2SiO3 (s or aq) ð8Þ

AI Si material (s) +[Mz (AIO2)x (SiO2) nMOH.mMOH2O] gel ð9Þ

AI Si material (s) + [Ma ((AIO2) <sup>a</sup> (SiO2) <sup>b</sup> nMOH.mMOH2O] ð10Þ

M + n [-(SiO2)z -AlO2 -]n ð11Þ

) for balancing the negative charge of Al3+

3.3 Concept and chemical mechanism of geopolymerization

dissolution of aluminosilicate materials (reaction (10)).

The general empirical formula of geopolymer is as follows:

, Na<sup>+</sup>

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

Where M+ = an alkali cation (K+

• Ground Granulated Blast furnace Slag (GGBS): Ground-granulated blastfurnace 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 trend up to respectively 10–12% and 14%, beyond which no further improvement can be obtained [25]. GGBS has now effectively replaced sulfateresisting OPC on the market for sulfate resistance because of its superior performance and greatly reduced cost.
