2.2.4 Suitability of RM for the production of GP

A polymer is a macromolecule, organic or inorganic, consisting of the repeated sequence of the same pattern, the monomer (from Greek monos: one or only one, and meros: part), connected to each other by covalent bonds. In the following macromolecule A▬A▬A▬A▬A▬A▬A … = [▬A▬]. The constituent unit is A; it is formed of a group of atoms that repeats itself. At the molecular level, most macromolecules are in the form of "long and flexible thread". The chemical reactions allowing to pass from a monomer A to the macromolecule [▬A▬] are called polymerization.

general and holds for silicates, Al▬Si (such as zeolites), alumino-phosphates and geopolymers. Aluminosilicate based geopolymers such as polysialate consist of SiO4 and AlO4 tetrahedra which are linked alternatively by sharing all the oxygen atoms [5]. Alkali-activated binders have either amorphous or nanocrystalline microstructures. This depends on the amount of alumino-silicate content in the precursor materials and on the mineralogical nature mineralogy of raw materials [17].

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

Depending on the composition of the starting materials, alkali-activated binders can

• Alkaline earth binding systems: Me2O▬MeO▬Me2O3▬SiO2▬H2O, the typical

• Alkaline binding system: Me2O▬Me2O3▬SiO2▬H2O, the typical example of

Glukhovsky classified the alkaline activators in six groups, where M is an alkali ion: alkalis, MOH; weak acid salts, M2CO3, M2SO3, M3PO4, and MF; silicates, M2O. nSiO3; aluminates, M2O.nAl2O3; aluminosilicates, M2O.Al2O3. (2–6) SiO2 and strong acid salts, M2SO4 [18]. Further GP are broadly classified as acid activated and alkali activated geopolymers as shown in Figure 1. Acid activated geopolymers were recently introduced, having properties comparable to alkali activated materials e.g. phosphoric acid activated metakaolin produced 30% higher cold crushing strength than their alkali activated counterparts [19]. The higher porosity of this group of polymers suggested their possible application in waste water treatment and as an adsorbent [20]. Acid activated geopolymers have not been explored and further research in this field is required. Alkali activated GP are materials of special interest in the past four decades due to their superior properties compared to OPC. Based on the alumino silicate matrix they are classified as sialate, sialate silaxo and sialate di

Starting materials are checked for pozzolanic content i.e. SiO2 + Al2O3. With the advanced mixing technologies, mix design comprised of pozzolanic and semi pozzolanic waste materials have been reported. The base materials found to be suitable

example of this group is alkali-activated blast furnace slag OPC.

this group is alkali-activated metakaolin or fly ash OPC [3].

be divided into two groups as shows Figure 3:

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

3.2 Raw materials for geopolymer production

silaxo.

Figure 3.

81

Classification of Geopolymers [17].

Geopolymer is a class of inorganic polymers generally formed by the chemical reaction between silica-rich and alumina-rich solids with a high alkaline solution. It is assumed that they result from the dissolution of alumina and silica into a silicate solution occurring the polycondensation of these monomers into aluminosilicate anions.

The general formula of polymer A▬A▬A▬A▬A▬A▬A … = [▬A▬] corresponds to M + n [▬(SiO2)z▬AlO2▬]n in geopolymer chemistry. A monomer A is equivalent to (▬Si▬O▬Al▬O▬), poly-sialate-siloxo (▬Si▬O▬Al▬O▬Si▬O▬), or poly-sialate-disiloxo (▬Si▬O▬Al▬O▬Si▬O▬Si▬O▬). As shows Table 2, the RM from the ACG plant in Fria contains about 15% Al2O3. In the chapter below the role of Al2O3 as precursor or monomers provider in geopolymerization will be highlighted. Taking into account the country's bauxite reserves and the subsequent quantity of RM resulting from it processing into alumina, geopolymer production may be a serious option for Guinea for the valorization of RM.

### 3. Geopolymerization

The GP technology has recently attracted increasing attention as a viable solution to reuse and recycle industrial solid wastes and by-products. It provides a sustainable and cost-effective development for many issues where hazardous residues have to be treated and stored under critical environmental conditions [15]. Generally, materials containing mostly amorphous silica (SiO2) and alumina (Al2O3) are a possible source for GP production. Furthermore Geopolymers appear to be a potential alternative to the classic hydraulic binders. Some research studies have been carried on to produce alkali-activated materials from RM. Due to Its low reactivity and low SiO2/Al2O3 molar ratio (<2.0), it has been combined with other higher grade precursors such as metakaolin and metakaolin to prepare alkaliactivated materials using sodium hydroxide (NaOH) and sodium silicate as alkaline activators solutions. The authors obtained 10.8 MPa compressive strength after 28 days curing [16].

#### 3.1 Classification of Geopolymers

Bragg used a method based on the theory of distinct silicate or aluminate anions as the basic unit of constitution. This central unit is a tetrahedral complex consisting of a small cation such as Si or Al that lies in tetrahedral coordination with 4 oxygen anions to produce SiO4 or AlO4. The silicon-oxygen bond should never be ionic; it should be polar and covalent [5]. This is since specific silicon and oxygen atoms Cannot move at liberty within the crystalline structure. Covalent bonding is more

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

general and holds for silicates, Al▬Si (such as zeolites), alumino-phosphates and geopolymers. Aluminosilicate based geopolymers such as polysialate consist of SiO4 and AlO4 tetrahedra which are linked alternatively by sharing all the oxygen atoms [5]. Alkali-activated binders have either amorphous or nanocrystalline microstructures. This depends on the amount of alumino-silicate content in the precursor materials and on the mineralogical nature mineralogy of raw materials [17]. Depending on the composition of the starting materials, alkali-activated binders can be divided into two groups as shows Figure 3:


Glukhovsky classified the alkaline activators in six groups, where M is an alkali ion: alkalis, MOH; weak acid salts, M2CO3, M2SO3, M3PO4, and MF; silicates, M2O. nSiO3; aluminates, M2O.nAl2O3; aluminosilicates, M2O.Al2O3. (2–6) SiO2 and strong acid salts, M2SO4 [18]. Further GP are broadly classified as acid activated and alkali activated geopolymers as shown in Figure 1. Acid activated geopolymers were recently introduced, having properties comparable to alkali activated materials e.g. phosphoric acid activated metakaolin produced 30% higher cold crushing strength than their alkali activated counterparts [19]. The higher porosity of this group of polymers suggested their possible application in waste water treatment and as an adsorbent [20]. Acid activated geopolymers have not been explored and further research in this field is required. Alkali activated GP are materials of special interest in the past four decades due to their superior properties compared to OPC. Based on the alumino silicate matrix they are classified as sialate, sialate silaxo and sialate di silaxo.
