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CHAPTER 3 GEOPOLYMER CONCRETE 3.1. Introduction This chapter is an attempt to review the available literature on Geopolymer in terms of its historical background, materials, chemistry, properties and applications as well as investigation into the geopolymerisation reaction and structure. The purpose of this is to identify the most suitable source materials for geopolymer binder which should be investigated in this research work, so that appropriate guidelines and limits can be established on the use of these materials to produce green concrete. Moreover, geopolymer is a new innovation in engineering materials, it is reasonable to have a sound knowledge of properties of the various component materials that make up geopolymer concrete and factors affecting them so that an experimental frame work can be carried out to determine the influence of these factors on the properties of geopolymer concrete. Thirdly, there is a need to have a good understanding of chemistry of geopolymer so that the effect s of various reagents in the geopolymer matrix can be determined. Moreover, there are diverse opinions on the nomenclature of polymers as it relates to its structural arrangement and origin, Singh S. (2013). Hence there is a need to have a clear distinction of nomenclature and structure of geopolymer which need to be established in this research work. Finally the application of geopolymer concrete is still an illusion to many people; this research work will identify the various areas in engineering and technology in which geopolymer concrete can be applicable. In conclusion, a comprehensive review of geopolymer concrete and its chemistry is beyond the scope of this research work. Detailed information can be found from "Geopolymer Institute". Moreover it must be acknowledged that geopolymer is a new innovation in engineering, hence further research is still required regarding its formation and chemical structure. 3.2. Historical background The concept of geopolymer concrete is as old as ancient Egyptian and Roman Empire, but was not known until recently. The mystery behind the ancient Egyptian and Roman mortars which has resisted deterioration and chemical attack for many decades gave rise for search of a new material binder that is more durable than the present ordinary Portland cement binder (Davidovits 1996). The way in which these ancient concretes and mortars has resisted deterioration for many years when compared with the present Portland cement mortar and concretes prompted the search into the mystery behind the composition of these ancient compounds(van Jaarsveld J.G.S. et. al 1996). In 1978, Davidovits, a French Scientist proved that the pyramid of Egypt was not built with OPC mortar being used in the present time, but with some aluminium silicate materials which he called "geopolymer". He noted that the durability of ancient mortars used in construction of pyramid of Egypt gave credence to the fact that the present Portland cement was not used in its construction. It is a known fact that the pyramid has lasted more than 4000 years and has resisted physical and chemical attack. Moroveer, Glukhovsky (1959,), after investigation on the properties of ancient construction 73

materials, proved that the presence zeolite or compounds of "aluminosilicate calcium hydrates "which are not different from the ones found in OPC must have been responsible for the durability of ancient concretes. Following the investigation, a binder emerges which he called "soil cement" which was named pattly because of its resemblance to earthy rock and partly due to it pozzolanic activity. The discovery triggered' the interest of other research scholars (Eitel 1966, Krivenko 1994). Therefore the presence of calcium silicate hydrates(CS-H) which is present in OPC was not the only compound responsible for the durability of ancient mortars as assumed by (Langton C.A et. al 1984).The long term durability of ancient mortars and concretes as seen in the Pyramid of Egypt fig. 2.1 triggered the interest of many other researchers. In his work on the resistance of ancient mortars to chemical attack, Malinowsky R. (1979) noted that the ―canals of underground and elevated aqueducts" which were built without joints by ancient builders were impermeable to water, without cracks and free from shrinkage. Campbell D.H et. al (1991), therefore proved that the resistance of ancient mortars to deterioration were as a result of zeolitic and amorphous compounds present in them.

Fig 3. 1. Pyramid of Egypt constructed more than 4000 years (Geopolymer Institute, 1994) Davidovits (1988, 1999), proposed a hypothesis on the possible use of geopolymer binder for the construction of pyramid of Egypt which has lasted up than 6000years. Other author reported that high amount of zeolitic compound in ancient mortars as being responsible for its long term durability (Frantisek et. al 2008, Freestone I.C et. al 2007, Contension H. et. al 1979, Perinet G. et. al 1980). In his experimental work to prove that the pyramids were built with alkali activated aluminosilicate binders instead of ordinary stones , Davidovits discovered a new binder which he called "Geopolymer", which was synthesised from metakaolin. 3.3 Geopolymer What is geopolymer?. It is a new type of binder produced by reaction of aluminium silicate with alkali. In this binder, no Portland cement is involved. It is only Al/Si material and alkali. Another way to have environmentally friendly concrete, which can lower CO2 emission, is the development of inorganic alumino-silicate polymer, called geopolymer, synthesized from materials of geological origin or by- product materials such as Fly ash that is rich in silicon and aluminum( Saeed A, et al 2012, Phair J.W 2006, Cox P.M, et al 2000). A lot of research 74

is going on in many research centers and institutions on geopolymer, its materials and properties ( Pradip N, et al 2014, van Deventer J.S.J. et al 2012, Duxson P. et al 2007, Panias D and Giannopoulou I.P. 2006, Saidi N. et al 2013, Bakri M.M.A. et al 2011). According to Davidovits J. (1994), geopolymer is an alkali- poly (Sialate-Siloxo) binders resulting from the inorganic polycondensation reaction yielding three- dimensional polymeric framework. It is essentially a mineral chemical compound or mixture of compounds, consisting a repeating units e.g. silicon-oxide(-Si-O-Si-O-), Silicon-aluminates(-Si-O-Al-O-), Ferro- Silicate aluminates(-Fe-O-Si-O-Al-O-), or Aluminium Phosphate(-Al-O-P-O-), created through a process of polymerisation(Davidovits J. 2013). Geopolymers can be grouped in the family of inorganic binders, which are formed by the reaction of solid aluminium silicate with high concentration of alkali or alkali Al-Si material produced from water glass solution (Duxson P, et. al 2006, Hardjito D. et. al 2005). Geopolymers have two major constituents which are used in its production. They are grouped into source materials and alkali. The source materials are either materials of geological origin which are rich in silicon and aluminium compounds or industrial waste products such as BFS, FA, S.F, Kaolin, Clay etc. The alkalis used are sodium silicate (Na2SO3) and sodium hydroxide (NaOH). Geopolymer concrete is a new innovation in construction in which Ordinary Portland cement (OPC) is totally absent but are replaced with pozzolanas that have abundance of Si and Al commonly found in Fly ash and other agro waste materials. These source materials are activated by high concentration of alkali solution to produce the binder which binds the composite materials at elevated temperature (Subhash V, et. al 2013). The synthesis of geopolymers are carried out by mixing aluminium silicate (Al2SiO3) material(source materials) and high concentration of alkali solution(NaOH, KOH etc). The alkali activators induce the geopolymerisation reaction yielding a large amount of gel phase which enhances the mechanical properties of the geopolymer (Xu H, et. al 2000). The reaction mechanisms occur through the reaction of alkali solution with the aluminium silicate material by breaking O linkages at the surface and leaching out free Silicon oxide (SiO4) and Aluminium oxide (Al2O4) tetrahedral units (Palomo A, et. al 1992).The SiO4 and Al2O4 tetrahedral units then polycondensate to form a gel phase which binds the aluminium silicate material together (Xu H, et. al 2000). The gel phase consists of a network of SiO4 and Al2O4 tetrahedral inter linked by shared O atoms. Thus, the main building blocks of the geopolymer gel consist of [Al-O-Si] and [Si-O-Si] bond. The Al3- in the gel network has four fold coordination, therefore the Al has negative charge which is balanced by metal cations such as K, Na etc (Davidovits J, 1991).The gel phase can be expressed empirically as Mn{ -(SiOz)AlOz)nWH2O} in which M is a cation and n is the degree of polymerisation(1,3,6).The 'Z' is the ratio of Si:Al which can be 1,2,3 or ˃˃3. 3.4 Source Materials As earlier stated, the two main ingredients of geopolymer are the source materials and alkali. The source materials are industrial waste products which have abundance of Si and Al in amorphous form e.g. FA, Kaolin, BFS, S.F etc are good for geopolymer materials (Hardjito D, et.al 2005). A lot of minerals of natural origin and industrial waste products are looked

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into by many scholars. The use of metakaoline as a geopolymer source material was studied by (Gourley 2003, Pinto T. et. al 2002, Davidovits 1999, Barbosa et. al 2000,). The following research scholars (Palomo A. et. al 1999, Swanepoel et. al 2002), investigated the use of low calcium fly ash as geopolymer source material, others looked into natural AlSi minerals (Xu and van Deventer 2002) and further insight into combination of GGBFS and metakaolin. The blending of blast furnace slag and metakaolin to produce fire- resistance geopolymers was studied by Chengs and Chiu (2003). High rate of dissolution of metakaolin in the reaction and color, coupled with easy control of Si/Al ratio made its usage in geopolymer concrere a priority (Gourley 2003). The limitation posed on the use of metakaolin as a source material for geopolymer is cost implications. Materials for geopolymer concrete achieve better result when the molarity ratio of SI-Al is 2 (Davidovits 1999). Higher compressive strength is usually obtained when source materials are calcined unlike using non- calcined material eg. Kaolin, clay, mine tailings and naturally occuring minerals (Barbosa 2000). In their research study, Xu and Deventer (2000), made a significant imput when they discovered in their result a great improvement in compressive strength and less reaction time in blending calcined and non- calcined material. 3.4.1 Fly Ash According to ACI committee 232, 2004, FA is a fine material obtained from the combustion of powdered coal from thermal station and transported by the flue gases and collected by electrostatic precipitator. It is commonly used pozzolanic material in the manufacture of high strength and high performance concrete. Its cementitious properties made its utilisation as PC replacement in concrete unique one, (Mindess S et. al 2003). The use of high volume fly ash to replace OPC was first developed by Malhotra who substituted OPC with 60% fly ash (Malhotra V.M, 1999). Since then, the use of fly ash as concrete admixture has grown tremendously, especially in making high performance concrete. Being an industrial waste material, it is disposed into landfills with its consequential effect of environmental pollution. However, the utilisation of FA in concrete production increases the rheological properties of concrete and reduces impact of environmental pollution. Moreover, reaction of FA with cement lowers the heat of hydration and contributes to the dense concrete texture, resulting in decrease in water permeability. Fly ash develops low initial strength in concrete due to slow pozzolanic reaction unlike port land cement concrete, but its continued reactivity increases the strength more than OPC concrete after 28 days,(BS EN 450 FA). XRD analysis of fly ash shows the structure of FA as spherical, which enhances the workability of concrete in fresh state (Hardjito D, et. al 2005). Fly ash has three main chemical constituents which gave it gave it its pozzolanic properties. They include, SiO2 (2560%), Al2O3 (10-30%) and Fe2O3 (5-25%). 3.4.1.1 Classification of fly ash ASTMC 618-99(1999) classified fly ash into two main categories, i.e., class F and class C. fly ash. Class F fly ash is categorised according to the percentage constituents of the three chemical compounds. If the sum of the three constituents is 70% or more, it is classified as 76

Class F fly ash, and if the sum of the three constituents is more than 50% it is classified as Class C fly ash. Pozzolanic activity of Class F fly ash is due to the presence of SiO2 and Al2O3 in amorphous form (Wesche, 1991). Class C fly ash contains high amount of calcium oxide (CaO), which gives it its cementitious properties. BS EN 450 recommends 25 to 40% application of fly ash in concrete for general purposes. Processing FA to greater fines than BS EN 450 specification makes it more effective in improving the performance of concrete, lowers carbon dioxide emission in cement and improves the durability of concrete(Dhir R.K, 2009). On the global ranking, China and India are the world largest generators of fly ash with a combined total of 212 metric tonne per annum(Rout, J.R 2011).Out of this number, China produced 112 metric tonnes per annum while India produces 100metric tonnes respectively. Regrettably, only 14% of this number is utilised, while the rest were disposed as waste (Malhotra 1999). 3.4.2 Kaolin Kaolin is a fine powdery substance obtained from clay. It is white in colour commonly found in the earth's crust. Its composition is mainly mineral kaolinite and clays. It also has other various minerals such as muscovite, quartz, feldspar and amatase. The shape is roughly hexagonal platy crystals ranging from0.1 - 10µm in size when viewed on electron microscope. Like OPC, it has great affinity with water and when mixed together, it becomes plastic and can be moulded into desired shape. The density of kaolin is comparatively lower than that of OPC and its water demand is lower. Kaolin can be used in various areas such as paper, paint, rubber and other products. Its major application is in ceramic industry for the manufacture of porcelain or china clay, because of its high fusion temperature and white burning characteristics. When passed through a furnace, kaolin undergoes a thermal transformation. At a temperature of 500 -600oC, a disordered metakaolin,(Al2SiO2) is formed(Bellotto M, et.al 1995). The physical and chemical properties of kaolin make its demand for industrial uses high. Some of these properties that make kaolin unique are shape, particle size, colour, softness and non- abrasiveness. The chemical properties, such as comparatively low base exchange capacity and relative insolubility are important properties (Haydn Murray). Various other uses of kaolin are as follows: a) Used in paper industry: Kaolin is the most important raw material used in paper industry, which uses approximately 1.2 million tonnes in 1958 (de Polo, 1960). Using kaolin in paper industry gives better quality prints with whiter and smoother surfaces. It is used as filler in the interstices of the sheet to add ink receptivity and opacity to the paper sheet. When used to coat the surface of the paper sheet gives a sharp photographic illustrations and bright printed colours. It constitutes nearly one- third of the weight of today's slick sheet magazines. The significant properties of kaolin of great value to the paper industry are whiteness, low viscosity, non-abrasiveness, controlled particle size and flat hexagonal plates. Figure 2. 1 shows the relationship between the particle size of kaolin and opacity.

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Opacity is a very important property to paper industry. Opacity is the ratio of percentage reflectance of a sheet of paper measured over a black background divided by a percentage reflectance measured over a white background. According to Lyons (1958), brightness, gloss and viscosity properties are dependent on particle size. Flow properties of kaolin clays, especially the kaolin coating clays used in the paper industry are very important because of their influence on coat weight, smoothness, texture and other properties.

Fig 3.2. Relationship between the particle size of kaolin and opacity of coated paper (Haydn Murray 1962) b) Rubber: Kaolin has its major use in making many rubber products by acting as fillers. It increases the strength, abrasion, resistance and rigidity to natural and synthetic rubber products. The extrusion of rubber products is easier when kaolin is used. c) Ceramics industry: This is another area where kaolin is being used. Ceramic white ware products, insulators and refractory's (smooth) are areas where kaolin is used. In white wares, kaolin aids accurate control of molding properties, and adds dry and fired strength, dimensional stability and a smooth surface finish to the ware. The excellent dielectric properties and chemical inertness of kaolin make it well suited for porcelain electrical insulators. In refractory applications, the dimensional stability, high fusion point and low water content, along with high green strength, make kaolin an important constituent.

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Kaolin has other industrial applications, which include; ink, adhesion, insecticides, medicine, food additives, catalyst preparation, bleaching, adsorbent, cement, fertilizers, plasters etc. Improvement in the functionality of kaolin fillers after surface modifications have been noted in the ink, paint, rubber and plastic industries(Albert 1960).Various other uses of kaolin include, paint industry and plastic making. 3.4.3 Alkali solution Alkali liquid plays a very major role in the formation of geopolymer matrix. The aluminium (Al) and silicon (Si) in the source materials are dissolved in an alkali activation solution and subsequently polymerises into molecular chains and become the binder. The reaction of amorphous materials containing aluminium and silicon with an aqueous solution containing sodium hydroxide and sodium silicate in their mass ratio, results in a material with three dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds(Ngoyen V.B.C.D.T, et. al 2008).The most commonly used alkali liquid in geopolymer synthesis is mixture of NaOH or KOH and Na2SiO3 or K2SiO3(Davidovits1999,Palomo at. al 1999,Barbosa et. al. 2000,Xu and Deventer 2000, Swanepoel and Strydom2002, Xu and Deventer 2000). The type of alkali used in geopolymer plays a very important role in the polymerisation process(Palomo et.al 1999). According to them, when the alkali liquid contains soluble silicate of Na or K than the use of only alkali hydroxides, the rate of reaction becomes high. Xu et.al (2000) reported that the addition of Na2SO3 solution to NaOH solution as the alkali increase the reaction between the source materials and alkali solution. Motorwala A, et . al (2013) reported a low compressive strength with higher concentration of potassium hydroxide compared with low concentration of sodium hydroxide which gives higher strength. 3.4.3.1 Sodium Silicate This is the alkali liquid used to manufacture of geopolymer concrete. Na2SO3 is sold in the market as a solution according to grades, with different elements that make up the compound solution. The percentage compositions of different compounds that form NaSO3 determine the quality of the grade, eg the percentage of SiO2 to NaO2 and that of water.

3.4.3.2 Sodium Hydroxide Sodium hydroxide is the alkali hydroxide for making geopolymer. It is commonly available in pellets or flakes with 97- 98 5% purity. There are two types that are available in the market which are commercial and industrial grades. The commercial grade is cheaper and can serve the required purpose. Moreover, Potassium hydroxide (KOH) is another alternative alkali.

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3.4.3.3 Super plasticisers This is one of the chemical admixtures used in recent times to improve the quality of concrete. Its function in concrete is to reduce water demand and increase the workability of concrete. Super plasticisers are paramount in the production of flowing, levelling, self compacting and for production of high strength and high performance concretes. It would not have been possible to use SCMs such as S.F, BFS, and FA to make high performance concrete without super plasticisers. Its utilisation in concrete is important milestone in the advancement of concrete technology. There are many types of super plasticisers currently in use as chemical admixtures. They differ in quality depending on the polymer used as the base. The following super plasticisers are currently in use (Bradly and Howarth 1986; Ramachandran et. al 1998) a. Poly melamine-formaldehyde (PMF) b.

Naphthalene Sulfate(NS)

c.

Lignosulfates (LS)

d. Poly carboxyl ate ester (PC) e. Poly acrylates f. Multi carboxyl ate ester(MCE) g. Acrylic polymer based (AP) Among these superplasticers, naphthalene sulphate and melamine sulphate are mostly used commercially. Poly carboxyl ate ester is also commercially available but is more expensive (Aitcin, 2008). 3.5 Chemistry of Geopolymer Some research scholars have proposed the fundamental chemistry of alkali-activated binders which are referred for some time now (Glukhorsky et. al 1980, Krivenko 1994). A Ukranian scientist, Victor Glukhorsky (1957), undertook a research into the differences between the durability of ancient cement and modern concretes, which metamorphosed into the synthesis of materials of geological origin like from clay, felspar, volcanic ashes, metallurgical slags and fly ash (Glukorsky 1980). He noted the superior properties of these new materials compared with existing cementitious materials. Krivenko (1994) currently continues the pioneering work undertaken by Glukorsky. Several scholars have studied the use of wastes such as fly ash, slags, clay in the synthesis of geopolymers (Rahier et. al 1997, van Jaarsveld et. al 1997, 1998. 1999, Khahil and Merz 1994). 3.5.1 Terminology The following terminologies were proposed by Davidovits to refer to geopolymers based on silicon - aluminates. He suggested that the term 'Poly(sialate)' be used for chemical 80

designation of geopolymers based on Silico- aluminate (Davidovits 1988a, 1988b, 1991, van Jaarsveld et. al 2002a). "Poly(sialate) are chain and ring ring polymers with Si4+ and Al3+ in IV-fold coordination with oxygen and range from amorphous to semi-crystalline" with empirical formula as follows: Mn(-(SiO2)z - AlO2)n . wH2O. where "Z" is 1, 2, 3 or higher up to 32. M is monovalent cations such as Potassium or Sodium, and "n" are the degree of polycondensation (Wallah and Rangan 2005). Sialate is an abbreviation for alkali silico - aluminate, the alkali can be sodium, potassium, Lithium, Calcium poly(sialate) covers all geopolymers containing at least one (Na, K, Ca, Li)(Si- OAl), (Na, K, Ca, Li) - Silicate unit (Davidovits 1976). While the symbol ˗ indicate the presence of bond. The figure below shows the amorphous to semi- crystalline three dimensional Silico Aluminate structures which are called Geopolymer (Davidovits 2002) .They involve at least four elementary units classified according to the SI : Al atomic ratio. Si:Al=1, Sialate Si:Al=2, Sialate - siloxo Si:Al=3, Sialate - disiloxo Si:Al˃3, Sialate link 3.5.1.1 Sialate, Poly(sialate) Si:Al=1 (-Si-O-Al-O-)

(-Si-O-Al-O-) are chain and ring polymers that are the result of the polycondensation of the monomer, ortho- sialate (OH)3-Si-O-Al-(OH)3 3.5.1.2 Sialate-siloxo, Poly(sialate-siloxo) Si:Al=2 (Si- O-Al-O-Si-O-)

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(Si- 0-Al-O-Si-O-) may be considered as the condensation result of ortho-sialate with orthosilicic acid Si(OH)4. They are three isomorphs, a linear (Si-O-Si-O-AL-O-), mono-siloxosialate and three cycles. 3.5.1.3 Sialate-disiloxo, Poly(sialate-disiloxo) Si:Al=3 (-Si-O-Al-O-Si-O-Si-O-)

(-Si-O-Al-O-Si-O-Si-O-) may be considered as the condensation result of ortho-sialate with two ortho-silicic Si(OH)4. The sialate unit may be atbthe beginning, in the middle or at theend of the sequence. There six isomorphs ; 2 linear, 2 branched and 2 cycles. 3.5.1.4 Sialate Link, Poly(Sialate-multisiloxo) Si:Al˃3

It designates the bridge Si-O-Al between two poly(siloxonate), poly(silanol) or poly(sialate) chains. 3.6 Geopolymerisation Geopolymerisation involves the chemical reaction of aluminosilicate oxides(Si2O5, Al2O5) in alkali precursors yielding polymeric Si-O- Al bonds. The alkalis involved are Polysilicates of

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Soddium or Potassium silicates which are commercially available in chemical industries or micro silica (silica fume) which is a by-product of ferro-silica metallurgy. The schematic formation of geopolymer material is shown below as described by (Davidovits 1999, van Jaarsveld et. al 1997).

(Equ.......1) Geopolymerisation is an exothermic and the chemical reaction may comprise the following steps(Davidovits 1999, Xu and Deventer 2000):   

Dissolution of Si and Al atom FA, S.F or BFS in the presence of OH-. Transportation or orientation or condensation of precursor ions into monomers. Setting or polycondensation/ polymerisation of monomers into polymeric structures.

Moreover, the processes involved in this reaction can be interwoven and happens at the same time, which may not be easy to separate so as to study them individually (Palomo et. al 1999). From equation 1, it is noted that the chemical reaction alumino-silicate with alkali in the formation of geopolymer liberated water which did not take part in the chemical reaction unlike in OPC which used water for hydration process 3.7 Reaction mechanisms The process of reaction of geopolymers which are also referred to as "alkali activated binders" (AAB) is not yet understood, especially as it relates to their setting and hardening. Although it is assumed to depend mainly on source materials such as alumino-silicate materials as well as alkali-activation. Moreover, Glukhorsky et. al (1980), postulated that the activation mechanism of geopolymeric materials is composed of three main reactions of destruction- condensation, that include the destruction of the source materials into smaller structural units, coagulation condensation and condensation crystallization. In the first reaction, the silicon oxide and aluminium silicate bonds were broken down, followed by the accumulation of products from the destruction and formation of crystallized structure. Most of the proposed mechanisms are shown in two phases, which are the first phase of silica-dissolution followed by the second phases of transportation and polycondensation (Davidovits 1988, van Jaarsveld et. al 1998). Similarly, the occurrence of those phases is simultaneous, preventing their analyses in their respective mode (Palomo P. et. al 1999). 83

Moreover, Duxson P. et. al (2007), proposed a flow chart model for geopolymerisation mechanisms as described in Fig 3.3, which consist of the following (1) Dissolution, (2) Speciation equilibrum, (3) Gelation, (4) Reorganisation,(5) Polymerisation and hardening.

Fig 3.3. Flow chart model of polymerisation mechanism (Duxson et al 2007)

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3.7.1 Activation of Fly ash and kaolin Davidovits, (1979), discovered a new type of binder which he called "Geopolymer" produced from the activation of kaolinite with alkali. Hence, other reasearch scholars developed interest on the activation of alumino-silicate materials. However, Criado et. al (2005) noted the distinction in the activation of FA and hydration of OPC but observed the semblance as far as chemical principles in the formation of alkali activated zeolites are concerned. Reporting on the alkali-activation of FA, Fernandez-Jimenez et. al (2004), noted the dissolution of aluminium and silicon with condensation of higher molecules into gel and subsequent attack by the alkali which exposes the internal spherical structure of the fly ash. At this point, the reaction products both inside and outside the spheres are formed after the small internal spherical spheres are completely and partially dissolved as shown in fig 3.4 and3. 5).

Fig. 3.4. Microstructure of FA activated by alkali: (a) original FA spheres; (b) broken spheres after activated with NaOH (Fernandez-Jimenez et.al 2004).

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Fig. 3.5. Illustrative model of alkali activation of FA (Fernandez-Jimenez et. al 2004) Palomo et al (2004) described FA activated by alkali as a zeolitisation process in which the last phase does not occur, mainly because of rapid dissolution and condensation reaction under experimental conditions which later slows down when hardened. SEM results of FA based GPC activated with sodium hydroxide and water glass showed large amount of crystals which were formed due to the unreacted water glass which were crystallised (Xie et al 2001). Reporting on the SEM analysis of the same geopolymer concrete samples, Krevenko et al (2002) noted zeolitic formation when the ratio of SiO2/Al2O3 was increased. The reaction mechanism of metakaolin paste was studied by other authors who agreed that the initial phase formed during the geopolymerisation was later transformed into a second which are more ordered phase than the first. They also observed a reduction in the initial rate of reaction as the ratio of alumino-silicate (SiO2/AlO3) increased(Provis et. al 2007, Provis et. al 2005). Fig 3.6 shows the flow chart model of the reaction processes involved in the geopolymerisation of metakaolin (Provis et.al 2007). This process can also be applied to every material rich in aluminosilicate compounds (Deventer et. al (2007). Moreover, it important to note that other chemical elements like iron and calcium play a significant part in geopolymerisation reaction when fly ash geopolymer is activated which provide extra nucleation sites (Deventer et al 2007).

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Fig. 3.6. Reaction processes involved in geopolymerisation (Provis et. al 2007) However, Granizo (1998), noted that a different reactions were obtained when metakaolin was activated with sodium hydroxide and water glass (Na2SO3). According to him, the first phase involve dissolution and induction in which the accumulation of destroyed product begins, while second phase starts after a fast dissolution phase, then a subsequent rapid polycondensation reaction. Palomo et. al (1999), proposed two models of alkali-activation in which blast furnace could be activated with diluted alkali solution where the reaction products are CSH, and activation of metakaolin with high concentration of alkali in which high strength and polymeric model are major characteristics. Fig 3.7 shows heat evolution test on the activation of metakaolin conducted by Alonso and Palomo (2001), in which several peak phases were identified. The phases include dissolution of aluminium silicate material seconded by induction of period marked by low heat evolution and final peak with exorthermic reaction. Heat evolution test conducted by Krizan and Zivanovic (2002) showed that silica modulus and sodium content play a major role in hydration process of alkali activation of blast furnace slag. According to them, as the ratio of Na2O and silica modulus increased, hydration also increased. Jaarsveld et. al (2002), confirmed the three stage reaction mechanisms of dissolution, orientation and condensation which characterised geopolymer formation. Moreover, Lee and Deventer(2002) made a comparative study on the hydration of OPC binders and activated binders and shows that the former utilises water with initial pH at neutral level which gradually turns to alkali as the hydration progressed (Langton et. al 1984, Roy et. al 1989).

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Fig 3.7 Heat evolution in samples activated with Sodium hydroxide(Alanso et. al 2001). The activation of FA is a process that may be considered as a zeolitisation in which the last phase does not occur, since the experimental conditions lead to very fast dissolution and condensation reactions but a lower one when the hardening take place,(Palomo et. al 2004). 3.7.2 Characterisation of alkali-aluminosilicate materials The reaction products and microstructural behaviour of activated aluminiumsilicate materials have been studied by many research scholars. Ariffin et al (2013) studied the micro structure of alkali activation of palm oil fuel ash(POFA) with X-ray difraction and detected albite and gmelinite, with sodalite and natrolite identified as reaction products of geopolymerisation when exposed to sulphuric solution. Fig 3.8 depicts the XRD analysis of POFA geopolymer concrete and the reaction products (Ariffin 2013)

Fig 3.8. XRD analysis of POFA geopolymer concrete (Ariffin 2013)

The activation of fly ash was studied by Criado et. al (2005). XRD analysis showed presence of hydrosodalite and herschilite as the hydration products. Other scholars (Shi et. al 1991, Wang and Scrivener 1995) , studied BFS activated by alkali, and identified the formation of CHS gel and Xonolite as the reaction products, while Fernando P.T. et. al 2008 and Bough et. al 1996, detected hydrotalcite and gismondito. However, the presence of zeolites were not detected by other research scholars but noted that hydrated compounds of sodium may likely be in CHS gel due to low c/s ratio(Jonh Vanderley 1995, Silva Maristela 1998), which was 88

confirmed by Gilfford et al (1996). Moreover, as the c/s ratio decreased, the amount of sodium in the CHS phase increased (Stade H, 1989, Hong et. al, 1999, 2002). The high amount of sodium in the gel prompted the name sodium calcium silicate hydrates((Stade H. 1989, Macphee D.E. 1989 and Palomo A. et.al 1999). However, blending fly ash and slag yielded CHS gel as the products of reaction with some hydrotalcite and calcite(Puertas et al 2000). The microstructural analysis of alkali-activated fly ash prepared with sodium silicate and NaOH was studied by Xi and Xi (2001), Krivenko and Kovalchuk (2002). SEM report detected high amount of crystals which resulted from the unreacted sodium silicate crystals, while XRD characterisation showed peaks of initial fly ash as reaction products. Moreover, other researchers (Zhihua et. al 2002, 2003, Bough and Atkinson 2002, Escalante G. J, et. al 2003) investigated alkali- activation of slag blended with red mud, slag mortars using SEM/XRD analysis. The result shows CHS as the reaction products. SEM analysis detected formation of hydrotalcite while XRD did not detect any crystalline products. Song et al (2004, 1999) confirmed that the reaction products of hydrated activated slag as analysed by XRD was CHS gel with small hydrotalcite. Comparative study of microstructural behavior of activated fly ash and metakaolin was investigated by Duxson(2006). The result of activation of fly ash showed deposition of some crystalline impurities resulting from unreacted fly ash and glassy phases which are not soluble in alkali as shown in fig3.9b, while activation of metakaolin are composed mainly gel phase which are more reactive than fly ash as in fig. 3.9a.

a

b

Fig 3.9. SEM pictures of activated metakaolin and fly ash (Duxton 2006) 3.8 Properties of geopolymer Various researchers have reported on the physical and chemical properties of geopolymers. Geopolymer was reported to have an excellent mechanical and durability properties especially in aggressive environment (Davidovits 1994, Sofi M, et., al 2007, Wallah S.E, et. al 2003). Some of these properties are stated below; 89

3.8.1 High early compressive strength One of the major properties of Portland cement concrete is its high compressive strength which increases with age. However, the present construction requires a high early strength concrete so as to complete the project on schedule. This demand for early strength made them to use early strength cement, use low w/c ratio through the use of increase cement content and reduced water content. This has resulted to high thermal shrinkage, drying shrinkage, modulus of elasticity and lower creep coefficients. In contrary, the chemical reaction which take place in the reaction of GPC increase their rheological properties, resistance to heat and other chemical attack. Another difference is that the reaction of geopolymer concrete is faster with increased temperature in alkaline solution than reaction of cement with water. Moreover, various studied conducted on geopolymer concrete showed high early strength. Aleem et. al 2012, reported that high and early strength was obtained in geopolymer concrete mix. They obtained average of 38 Mpa and 52 Mpa at 7 days and 28 days respectively. The strength of geopolymer concrete increases with increasing temperatures and morality of the mixtures. 3.8.2 Resistance to acid attack Portland cement concrete is not resistant to acids. Most acid solutions will gradually or rapidly disintegrate Portland cement concrete depending on the concentration. Comparatively, geopolymer concrete performed better in resistance to acid attack (Davidovits 1994, Gourley and Johnson 2005). Rangan 2010 observed that the better performance of geopolymer concrete to acid attack is as a result of low calcium content of the component materials. Geopolymer cement do not depend on lime for strength and are insoluble in acidic solutions. Wallah et. al 2006, performed acid test experiment by immersing the samples in 5%sulphuric and hydrochloric acids and reported that geopolymer cement performed better with approximately 5-8% weight loss against OPC which lost approx. 30-60% weight and suffered destruction in acid. Other authors (Bakharev T. 2005, Gourley et. al 2005, Song X.J. et. al 2005) noted that geopolymer concrete performed better than OPC concrete when exposed to acid environment. They compared the weight loss of the two samples and discovered that the weight loss of GPC was much lower. Moreover, they also reported a reduction of compressive strength in acid exposure which decreased with age. U.S Army corp of engineers also reported similar trend with more reveletion of superiority og GPC to chemical attack, freezing and thawing, coupled with very little expansion (Comrie D.C. et. al 1988, Malone P.G. et. al 1985). 3.8.3 Fire resistance Geopolymer concrete show high resistance to fire even at very high temperature. Similarly, Portland cement concrete is non- combustible and high resistance to fire. The ability of concrete to resist fire is governed by its ability to absorb heat and retain its strength with failure under heat. However, it was reported that Portland cement concrete experience rapid deterioration in compressive strength atb300oC while geopolymer concretes are stable at a high temperature of 600oC(Davidovits 1988). 90

3.8.4 Resistance to alkali- aggregate reaction Some aggregates which are used in Portland cement concrete contain reactive silica, which reacts with alkali present in cement. Alkali- aggregate reaction(AAR) is a chemical reaction between the hydroxyl ions in the pore water within the aggregate(Shetty2005). However, the geopolymer concrete, even in high alkali content does not show any dangerous alkaliaggregate reaction (Davidovits 1994a, 1994b). Literature shows that alkali found in OPC or normal concrete induce reaction of aggregate and alkali with its harmful consequences. Hence, care should be taken so that alkali should not be allowed in OPC. However, some natural pozzolans such as alumino-silicates of sodium or potassium can greatly reduce alkaliaggregate reaction of cements with high alkalis(Mehta 1981, Davis et.al 1935, Mindess and Young 1981, and Roy 1986). Sersale and Frigione(1987) tried addition of zeolite (alkalialuminosilicates) in order to weaken the alkali-aggregate reaction. Other authors reported geopolymer binders are highly resistance to alkali-aggregate reaction (Haekkinen1986, Metso1982, Talling and Brandstertr1989). 3.9 Application of geopolymer 3.9.1Fire resistant wood panels Geopolymer is successfully applied in building products such as fire-resistant chip-board panels and other light weight partition panels which are coated with nano composite materials. 3.9.2 Ceramic application Geopolymer materials are also used in Ceramics for the manufacture of electrical fuses. In this case, the natural blend of kaolionite and quartz are used, in which case, the quartz crystals are coated kaoliniteo micelles to obtain a new nano composite known as SILIFACE COR 70 used as electrical appliances, which has low thermal expansion and exceptional temperature stability. 3.9.3 Geopolymer Cement Geopolymer cement is used as binder for concrete works and as other cementitious products which can be available commercially (22).. It is an innovative material and an alternative to conventional Portland cement for use in transportation, infrastructure, construction and offshore applications. The production of gopolymer cement from industrial waste materials gave it greater advantage over Portland cement binder because of it reduction of carbon footprint.. Addition of BFS increases the setting time and improves the rheological properties. Creating geopolymer cement requires an aluminium silicate material, a user friendly alkaline reagent,(Sodium or potassium soluble silicates with molar ratio SiO2:M2O>1;6.5, M being Na or K) and water. Geopolymer cements cure more rapidly than portland-based cement. They gain most of their strength within 24 hours(Davidovits J. 2013). They set slowly enough that they can be mixed at a batch plant and delivered in a concrete 91

mixer. Geopolymer cement has the ability to form a strong chemical bond with all kinds of rock-based aggregates. 3.9.4 Aviation applications Geopolymer materials can also be used in air craft cabin materials such as; ceiling, linings, floor panels, partition and side walls, storage bin and wire insulation. Carbon fibre reinforced potassium aluminate resin (geopolymer) composite are non-combustible materials which are suited for construction, transportation and infrastructure applications where a combination of fire endurance, non-combustibility and specific flexural strength is needed. At the present, affordable, low temperature process able matrix materials for fire resistant composites are unavailable since most organic polymers melt ant ignite at temperature of 400-600oC, characteristic of fuel fire exposure conditions(Lyon R.O. et. al 1997). 3.9.5 Civil and military ships/submarines The major problem of civil and military hardwares is there ability to cash fire easily because of type of the composite materials used for their construction, which results in conflagration and subsequent emission of carbon and other dangerous gases. Fire safety regulations require that" unprotected composite systems cannot meet the stringent fire requirements specified for interior spaces". Duty demands military vessels to carry out their mission even when damaged, and must ensure fire resistance for sufficient period of time to carry out rescue operations. The effects of fires aboard vessels have been demonstrated as a result of collision between ships and ferries both in peace time and war time, eg. American Navy in the Persian Gulf, British Navy in the Falkland Island. Moreover, because of the ability of these materials to catch fire easily, they cannot be used in military works (Demarco R.A, 1991), ground water transportation (Hathaway W.T 1991), and commercial aircraft (Hill R.G et. al 1985). 3.9.6 Radioactive and toxic waste encapsulation Radioactive and toxic wastes are dangerous to health and as such, in treating or discarding them, adequate care should be taken so that preservation of life must be paramount. The reason why encapsulation of these dangerous wastes is done is to shield individuals involved in it so that they will not be exposed beyond limit to its emissions. This is done by using waste encapsulation container or barrier which can be used to encase the waste. Their function can be in many folds; (a) It is capable of protecting individuals expose to it from dangerous gases emanating from the waste. (b) They reduce and retard their transport, so that it will not come near persons with higher concentration. The most effective way of containing nuclear wastes is by concrete and natural barriers. However, geopolymer concrete helps to remove water faster maintains the stability of wastes at temperature approx. 1000oC (Davidovits 1994).

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3.9.7 Paint sludge disposal The manufacturing of paints generally involves the production of hazardous wastes. The wastes, in the form of sludge are usually dewatered but still contain high concentration of heavy metals such as chromium and zinc. In general, waste water treatment sludge from the production of chrome and zinc pigments is classified as 'hazardous industrial waste' and must be disposed of accordingly. The high cost of disposing of these wastes has led paint manufacturers to find alternative methods. One of such methods that show promising result is stabilising the waste with geopolymer (Comrie et. al 1988). Dewatered steel paint sludge was treated in various concentrations with geopolymer (Geopolymite50TM). In some cases, sand is added to provide greater strength to solidified waste. After curing for one to three days, the samples were crushed and leached in acid for 24 hours according to Environmental Protection Act procedures. However, result show that without sand, it could still be soft to touch. It was leached nonetheless, and the results show that the contaminants were stabilised. The elements most dramatically stabilised in the experiments were zinc, manganese, cobalt and vanadium. In each of these cases, more than 90% of the soluble containment was locked into the geopolymeric matrix. The physical strength of the samples not only increased with sand content, but also the leachability of some of the toxins did as well, The most dramatic increases were found in zinc and cobalt. However, the loss to leachate even with 30% sand was only a fraction of the raw waste leachability. The amount of geopolymer required to effectively stabilise any particular paint sludge depends on the unique chemistry of the waste and local regulations regarding leachate contaminant concentrations. The government regulations stipulate maximum allowable concentrations of contaminants in the leachate be expressed as milligrams per litre of solution. The treatment with geopolymer 50 at a concentration of 25% shows that the chromium leachate levels were reduced below the regulated limit. The increase in leachability with the addition of sand indicate that the geopolymer was preferentially complexing with the sand rather than the waste. This is likely due to the fact that the geopolymer has high affinity with silica.

3.9.8 Waste management Geopolymer can also be applied in the management of wastes by a process known as solidification. Sludges from nuclear treatment plants containing radioactive wastes, toxic metals and hydrocarbon can be treated and solidified by geopolymer. This can be done through the technology of mixing the sludges with geopolymer then moulding the mixture and allowed to solidify to a state that it can be handled, stored and monitored(Hermann E. et. al 1999).

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3.9 Geopolymer brick Geopolymer brick is an ideal construction materials for technologically emerging countries, because it offers characteristic that meet the demand. The brick uses a very cheap material available in great quantity; lateritic clay earth which is mixed with geopolymer binder and compressed to give the shape of a brick and then heated in a furnace. At 85oC the brick is water stable and has enough compressive strength to build a wall. At 200oC, it resist freezing. At 450oC, the strength increases more, so that it is possible to manufacture structural elements like beams for doors and windows(Bourtherm C. et. al 2003). It can be used as a sub base for soils with poor bearing capacity like landfills and water log areas or made up soils where rigid structures with heavy loads are required. This will prevent the penetration of underground water into the building and also act as a cover against chemical attack resulting leaching of chemical substances into the building. In other words, the base liners of land fill sites are made less permeable against contaminants and ground water seepage. It can also be used as a lining for reservoir walls to prevent water seepage.

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