3.2 Raw materials for geopolymer production

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

Figure 3. Classification of Geopolymers [17].

This chapter addresses specifically the use of RM for the production of

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

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

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

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

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

may be a serious option for Guinea for the valorization of RM.

geopolymers for construction and other engineering applications.

2.2.4 Suitability of RM for the production of GP

Geopolymers and Other Geosynthetics

polymerization.

3. Geopolymerization

28 days curing [16].

80

3.1 Classification of Geopolymers

anions.

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].

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,

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

• Red mud: RM (see § I.3.1.) is characterized by strong basicity even with a high water content, because of the presence of huge amount of sodium hydroxide used to extract silicates and alumina. Figure 4(a) shows oven dried RM, Figure 4(b) grounded RM; its color is due to the Fe2O3 or Fe3O4, which can

• Rice-husk ash: Rice husk is the natural sheath that forms on rice grain during its growth. Removed during the refining of rice, the rice husk ash RHA is generated after burning the rice husk in the boiler. At present, the most common method of disposal of RHA is dumping on waste land, thus creating an environmental hazard through pollution and land dereliction problems. The major compounds from rice husk are silica and cellulose which yields carbon when thermally decomposed [22]. Rice husk is unusually high in ash compared to other biomass fuels, close to 20%. The ash is 92–95% silica, highly porous and light weight, with a very external surface area RHA is an active pozzolan which when combined with line in the presence of water results in a stable and more amorphous hydrate (calcium silicate). Rice husk is unusually high in ash compared to other biomass fuels—close to 20%. Figure 4(c) shows RHA after burning of RH at ambient temperature. At higher temperatures the RHA color

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.

• Catalyst residues: Petroleum refineries worldwide process crude oil in fluid catalytic cracking (FCC) units, and 160,000 tons of spent FCC catalyst residue

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

are thus produced every year [23]. The spent catalyst is essentially an

silica, alumina, and iron content in the ash.

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

make up to 60% of the mass of the RM.

tends to white and its Si content increases.

Figure 4.

83

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

Recently RHA has been incorporated in activated aluminosilicates.

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 type and the amount activators added.

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 and gives rise to favorable mechanical properties [21].

. 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 natural or industrial (mainly by-products).
