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Geopolymer Concrete

Literature Survey on Geopolymer Concretes and a Research Plan in Indian Context

A comprehensive literature survey on various aspects of Geopolymer Concretes (GPCs) has been provided in this paper to under-stand the nature of GPCs from engineering applications point of view so that a rational technical plan for development of GPCs with given aluminosilicate sources (such as fly ash, blast furnace slag powder etc) can be formulated. The literature survey indicates that ‘geopolymer’ (GP) is only one of the many names used for describing the binder formed with alumino-silicate gel structure. Com-paratively, more papers are published on science of geopolymerisation where often paste is utilised. Concretes and mortars based on GPs are also reported, but, lesser in numbers. The science of GP has not yet reached the stage where GPC mix can be made by user by just adding water as it has happened in case of Portland cement technology. This requires the actual engineer on site to be aware of chemical nature of the GP binding action involved. However, enough qualitative information is available on the mechanical strength so that GPC mixes can be developed to achieve the desired level of strength for use in structures. The second part of this paper would concentrate on the typical research plan to develop engineering properties of GPCs based on the information available in the literature.

Portland Cement Based Concretes

Cement concrete is often considered as an artificial stone which is made by mixing Portland cement (P-C), water, sand, and crushed stone aggregate to produce a mouldable mixture. This concrete, during the last century, has developed into the most important building material in the world; the beginning was made by August Perret, in 1902, by designing and building an apartment building in Paris employing “a system for reinforced concrete” (columns, beams, and slabs, but with no load-bearing walls) [URLa]. Concrete is, now, an essential product used in a variety of constructions including infrastructure and industrial sectors. This is partly due to the fact that concrete is produced from natural materials available in all parts of the globe, and partly due to the fact that concrete is a versatile material, giving architectural freedom. Concrete is used more than any other man-made material in the world [Bjorn Lomborg, 2001]. More than a ton of concrete is produced every year for each human on the earth planet, making the concrete as the second most widely consumed substance on the earth after water [Sara Hart, 2008]. But,

the environmental aspects of concrete are now being discussed with a view to develop an eco-friendly material of construction. In this regard, it would be interesting to note that the ‘embodied carbon dioxide’ (ECO2) of a tonne of concrete was reported to be in the range of 75–176 kg CO2/tonne, depending upon the type and method of mix design [URLb]. The ‘embodied energy’ (EE) content of concrete is also very high which could vary from 400 to 600 kWH/m3 of concrete. Therefore, there is an urgent need for making the concretes more eco-friendly so that both ECO2 and EE of concrete are reduced. It has been well established that any developmental activities aimed towards improvement of quality of life of human beings involves always a large amount of construction activities which in turn require production of varieties of concretes. Therefore, development of concretes with more eco-friendly characteristics has become tasks of many scientists all over the world.

Need for Alternate Concretes

Continuous technological upgrading and assimilation of

Rajamane N. P.1, Nataraja M. C.2, Lakshmanan N 3, and Ambily P S4

1Head, CACR, SRM University, 2Professor, Dept. of Civil Engg, SJCE,3Former Director, CSIR-SERC, 4Scientist, CSIR-SERC

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latest technology has been going on in the cement industry. Presently, 93% of the total capacity in the industry in India is based on modern and environment-friendly dry process technology and only 7% of the capacity is based on old wet and semi-dry process technology. There is a scope for waste heat recovery in cement plants and thereby reduction in emission level.

The cement production is highly energy intensive next only to steel and aluminium (also consumes significant amount of non-renewable natural resources such as lime stone deposits, coal, etc.). The ‘EE’ of P-C being about 1.3 kWh / kg, is a very high quantity. A tonne of P-C production involves emission of about a tonne of CO2, which is a greenhouse gas causing global warming. More than 7% of world CO2 production is attributed towards production of P-C. Moreover, among the greenhouse gases, CO2 contributes about 65% of global warming [McCaffery, 2002]. Therefore, the Portland cement industry does not fit the contemporary desirable picture of a sustainable industry. There is an urgent need to find an alternate to P-C in order to make the construction industry eco-friendly. However, the new binder material should also possess satisfactory strength and durability characteristics which are comparable, preferably superior to those ‘conventional concretes’ (CCs) based on P-C.

Geopolymer as Alternate to Portland Cement

A new binder material, known as ‘geopolymer’ was first introduced by Davidovits in 1978 to describe a family of mineral binders with chemical composition similar to zeolites but with an amorphous microstructure [Davidovits, 1994]. He utilised silica (SiO2) and alumina (Al2O3) available in the specially processed clay (metakaolin) to get inorganic polymeric system of alumino-silicates. Unlike ordinary Portlandcement (P-C), geopolymers do not need calcium-silicate-hydrate(C-S-H) gel for matrix formation and strength, but utilise the polycondensation of silica and alumina precursors to achieve required strength level. Two main constituents of geopolymers are: geopolymer source materials (GSMs) and alkaline activator liquids. The GSMs should be alumino-silicate based and rich in both silicon (Si) and aluminium (Al) and thus, by-product materials such as fly ash, silica fume, slag, rice-husk ash, red mud, etc. can form GSMs.

Recently, Rangan and Hardijto, [2005] exploited silica and alumina of fly ash to produce three-dimensional polymeric chain and ring structure consisting of Si-O-. Geopolymers are unique in comparison to other aluminosilicate materials (e.g. aluminosilicate gels, glasses, and zeolites). The concentration of solids during geopolymerisation reactions is higher than that in aluminosilicate gel or zeolite synthesis

[Rangan, 2005; Rajamane, 2011a and 2011b, Sindhunata, 2006]. Al-O bonds of geopolymeric binder are useful to prepare structural grade concretes.

From above, it is clear that any of the minerals containing reactive oxides of silicon and aluminium can be activated by suitably formulated highly alkaline liquid to obtain inorganic polymeric binding material [Sindhunata, 2006]. Preliminary studies in this regard, were carried out at SERC in early 2000s; both fly ash and Ground Granulated Blast Furnace Slag (GGBS), (either individually or combined in certain proportions) from indigenous sources were found to be suitable to produce geopolymeric systems to achieve sufficient strength levels in geopolymer concretes (GPCs) [Rajamane and Sabitha, 2005]. It was observed that the activation of FA and GGBS involved use of hydroxides and silicates of alkali (such as sodium, potassium) which are commonly available in India; the processing conditions for GPCs were almost similar to Conventional Concretes (CCs) except that during mixing operations of GPCs, instead of water, a premixed alkaline solution, known as ‘Alkaline Activator Solution’ (AAS), was added. Following materials were used to produce GPCs [Rajamane, 2009a]:

- Fly ash,

- Ground Granulated Blast Furnace Slag(GGBS),

- Fine aggregates (in the form of river sand),

- Coarse aggregates (in the form of crushed granite stone),

- Alkaline Activator Solution (AAS):

(It is a combination of solutions of alkali silicates and hydroxides, besides distilled water. The role of AAS is to activate the GSMs, containing Si and Al, such as FA and GGBS).

Besides above mentioned materials, synthetically produced alumina and silica, metakaolin, rice husk ash, silica fume, etc. can also be used appropriately keeping considering that both aluminium and silicon elements are required beside small amount of alkali elements (such as Sodium, Potassium, etc.) to form alumino-silicate geopolymers. GPCs, being a new class of materials (with complete absence of Portland cement), conventional concrete mix design approaches are not generally directly applicable. The formulation of GPC mixtures requires systematic numerous investigations on the materials available [Rajamane, 2005]. However, basic concepts related to particle packing, rheology of fresh mixes, etc, can be judiciously utilised in developing GPC mixes which require AAS consisting of hydroxides and silicates of sodium whose concentration plays a major role in determining the geo polymerization ratio of alumina and silica of ‘geo polymeric source material’

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(GSM) such as fly ash, metakaolin, GGBS etc., of GPC.

Desirable Properties of GPCs

It was recognised that any new binder material to be developed for use in concretes should be eco-friendly and it would be acceptable if it has following characteristics:

- It should be preferably produced from widely available waste by-products from industries

- ‘Internal Energy Content’ (Embodied Energy) should be less

- Chemical activators for generating binding system should be commonly available

- The new binder based concretes should be similar or superior to that of P-C based concretes in respect of :–

- processing conditions for production of fresh mixes

- time required for demoulding or formwork removal

- curing regimes and periods

- rate of strength developments with age

- mechanical properties such as

- compressive strength

- tensile strength

- flexural strength

- modulus of elasticity

- durability related properties such as

- protection to embedded steel reinforcement

- diffusion of

- chloride ions

- moisture/water, etc

- resistance against attack by

- sulphates

- acidic solutions, etc

- cost per unit volume

- long term chemical stability of the binding system formed

- capable of accepting common filler aggregate systems such as sand, crushed natural stones, etc

Literature Review

Origin of Term ‘Geopolymer’

The term ‘‘geopolymers’’ was first introduced to the world by Davidovits of France resulting in a new field of research and technology. Davidovits explained that geosynthesis is the science of manufacturing artificial rock at a temperature below 100°C in order to obtain natural characteristics (hardness, longevity and heat stability) of

rock. Geopolymers can be thus viewed as mineral polymers resulting from geochemistry or geosynthesis. However for the purpose of this literature survey, geopolymer (GP) means any aluminosilicate based binder.

History of Geopolymers

Davidovits coined the term geopolymer in 1978 to represent a broad range of materials characterised by chains or networks of inorganic molecules [Davidovits, 1979, 1993, 2008], and explained in many of his publications about the possibility of GPs being used by Egyptians construction of pyramids, based on microscopy, IR and NMR spectroscopy of sparse specimens from ancient Egyptian constructions [Davidovits and Morris, 1988; Davidovits, 1999]. Demortier observed the noticeable differences in porosities in the top and bottom sections of pyramid blocks which were also subjected to X-ray and NMR analyses to conclude that pyramids could be made from ‘concreting’ operations [Demortier, 2004]. Use of slurry to form bearing courses of horizontal joints and vertical joints between the blocks including presence of hair in the joints of pyramids did indicate the possibility of ‘concrete’ like technology for pyramid constructions [Škvára et al, 2008].

But, the actual modern alumino-silicate based work could be traced to 1930s when alkali oxides were used for reaction with slags to test their suitability for use in Portland cement. A rapid hardening binder by slag activation was reported in 1940 by Belgian scientist [Purdon, 1940]. US Army used, in 1950s, NaCl and NaOH to activate slag to produce binder for use in Military applications [Malone et al, 1986]. Glukhovsky in 1965 observed that alumino-silicate hydrates as solid binder products are formed during alkali activation of slag and these are also noticed during alkali treatment of rock and clay minerals, prompting him to call the binder as ‘soil cements’ and concrete as ‘soil silicate concretes’ [Glukhovsky, 1965]. In 1974, Davidovits and Legrand filed a patent on ‘Siliface process’ which involved use of NaOH, Quartz, kaolinite, and water. It is interesting to note here that alkali activated slag (AAS) would basically consist of silicon element in mainly one dimensional chains whereas, GPs would have a 3-dimensional alkali-alumino-silicate framework [Duxon et al, 2007].

However, after Davidovits (1991) described his new breed of aluminosilicate binders (synthesised by activating calcined kaolinitic clay with sodium silicate solution at low temperature) as ‘geopolymer’ for the first time, the real impetus to the field of GP technology started. The ‘Geopolymer’ was an aluminosilicate gel, where the silicon and aluminium are tetrahedrally-bonded through sharing oxygen atoms forming the basic monomer unit is a sialate (O-Si-O--Al-O) carrying excess negative charge which occurs when the Al3+(of the source material such as clay)

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is substituted by Si4+.The polysialate structure is charge-balanced by alkali metal cations (K+ or Na+).

The field of geopolymers saw major contributions from authors such as Alonso (2001), Bakharev (2005), Sanjayan (1999), Bankowski (2004), Cheng (2003), Duxson (2005), Fernandez-Jimenez (2006a), Iler (1979), Katz (1998), Khalil (1994), Kriven (2003), Krivenko (2002), Lee and Van Deventer (2002a), Li (2006), Palomo (2004), Phair (2001), Provis (2005b), Shi (1996), Sindhunata (2006), Xu (2004), Talling (1989), Van Jaarsveld (2000), Wang (1995), Xu (2000b), Yip (2003), Hardjito, Wallah, Sumajouw, Rangan (2001). Meantime, a few books having comprehensive information on geopolymer based on the vast literature were also published [Davidovits, 2011; Shi, 2006; Provis, 2009].

Though the term, ‘geopolymer’ has become now more common to represent the synthetic alkali aluminosilicate material (produced by reaction of a solid aluminosilicate with a highly concentrated aqueous alkali hydroxide or silicate solution), it is worthwhile to note that the following nomenclatures are also reported to describe similar materials:

- Inorganic polymer [Van Wazer, 1970]

- Low-temperature aluminosilicate glass[Rahier, 1996]

- Alkali-activated cement [Roy, 1999; Palomo, 2003; ]

- Alkali-activated binders [Torgal, Gomes, and Jalali, 2008]

- Geocement [Krivenko, 1994]

- Alkali-bonded ceramic [Mallicoat, 2005]

- Inorganic polymer concrete [Sofi, 2006]

- Hydroceramic [Bao, 2005]

- Mineral Polymers[Davidovits, 1980]

- Inorganic polymer glasses[Rahier, 2003]

- Alkali ash material[Rostami, 2003]

- Soil cements [Glukhovsky, 1965]

- Alkali Activated Binder [Provis and Deventer, 2009],

It is seen that GP is a versatile binder being studied by scientists of various backgrounds and expertise, but, having good potential to become eco-friendly alternate to P-C for use in civil engineering applications. But, to understand various aspects of this new material from, it is necessary to consider the above nomenclatures also so that the information available in various forums is readily utilised. In the present paper, more widely used term, ‘geopolymer’ is adopted for presentation of data and discussions.

Basics of Typical Geopolymer Concretes

Major ingredients of geopolymer concretes (GPCs) having

geopolymer (GP) as the binder, are:

- Geopolymeric source materials (GSMs) such as fly ash, GGBS, etc

- Aggregate system consisting of fine and coarse aggregates

- Alkaline Activator Solution (AAS)

It is seen that GPCs are almost similar to conventional concretes (CCs) (which are P-C based), consisting of binder made from fine powdery materials, bulk volume filling granular particles made of aggregates, and liquid component of the mix made of alkaline chemicals. Thus, the powdery P-C of CCs is replaced by mineral materials (usually referred as mineral or pozzolanic admixtures in CC technology), and liquid component of water of CCs is replaced by viscous, alkaline activator solution made of hydroxides and silicates of alkali metals such as sodium and potassium. The aggregate filler component of CCs is retained in GPCs.

Besides above mentioned materials, synthetically produced alumina and silica, metakaolin, rice husk ash, silica fume, etc. can also be used appropriately keeping in view that both aluminium (Al) and silicon (Si) elements are both required beside small amount of alkali elements such as Sodium (Na), Potassium (K), etc to form alumino-silicate geopolymers.

GPCs being a new class of materials (with complete absence of Portland cement), traditional CC mix design approaches cannot generally be directly applied. The formulation of the GPC mixtures requires systematic numerous investigations on the materials available [Rajamane, 2005]. However, basic concepts related to particle packing, rheology of fresh mixes, etc can be judiciously utilised in developing GPC mixes.

To prepare a typical AAS consisting of hydroxide and silicate of sodium, Sodium Hydroxide flakes (SHf), a highly hygroscopic granular material are first dissolved carefully in Distilled Water (DW), to get sodium hydroxide solution (SHS). After allowing the SHS to cool to room temperature, Sodium Silicate Solution (SSS) is added to it, the resulting liquid is termed as AAS which is used to prepare the GPC mix. Mixing of GSMs, aggregates, and AAS is done using the conventional tools (such as mixer machine) adopted for producing of CCs, however, with due considerations for viscosity and chemical nature of the AAS. The rates of hardening and chemical reactions in GPCs are quite different from the concretes based on Portland cement. Handling of some ingredients, especially, the constituents of AAS would require specific precautions. It is to be noted here the exactly required liquid component of GPCs is

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not readily available in the market, unlike water in case of conventional concrete (CC), and it has to be prepared carefully much before the actual mixing of GPCs is started. It may be worth noting here that specifically formulated chemical additives to function reliably as setting time changing admixtures such as retarders, accelerators, etc are not yet readily available for geopolymers. Hence, field adjustments for changed ambient temperature conditions and changes in properties of ingredients would not be easy, even though conventional methods of mixing, compaction, moulding, and demoulding can be still adopted for GPCs also. However, for some mixes, ambient conditions may not be adequate for demoulding within 24 hours of casting and some higher temperature exposure may have to be created for effecting the setting of the GPC mixes.

Many of the geopolymeric systems reported in the literature are involving use of high temperature curing in the form of storing the moulds containing GPC mixes in hot air oven or steam chambers [Rangan, 2005]. However, the works at CSIR-SERC has shown that it is now possible to formulate the GPC mixes for self-curing so that ambient conditions would be sufficient for setting as well as for gaining mechanical strengths [Rajamane, 2009b].Curing of GPCs may involve application of steam and hot air, in contrast to water curing of CCs. With special formulations, GPCs can get cured at ambient conditions after demoulding, thereby they can be considered as self-curing. For visual inspection, GPCs and CCs would look similar, but, chemical natures of microstructures are quite different.

Literature on Geopolymer Science

Ingredients of GP and Geopolymeric Source Materials

Geopolymer concretes (GPCs) have geopolymer (GP) as the binder to bind the aggregate system consisting of fine and coarse aggregates. Two main ingredients required for creation of geopolymer binders are:

- Geopolymeric source materials (GSMs) rich in silica and alumina, which could be natural minerals (such as kaolinite, clays, etc) or industrial by-products (such as fly ash, silica fume, slag, rice-husk ash etc).

- Alkaline Activator Solution (AAS) based on alkali metals (commonly Sodium or Potassium) based. The most common AAS is a combination of alkali hydroxide (NaOH, KOH) and alkali silicate (Sodium or potassium silicate).

Geopolymers made from calcined source materials, such as metakaolin (calcined kaolin), fly ash, slag etc., yield higher compressive strength when compared to those synthesised from non-calcined materials, such as kaolin clay. The source material used for geopolymerisation can

be a single material or a combination of several types of materials (Xu & van Deventer 2002).

Geopolymerisation Reactions

The mechanism of geopolymerisation may be considered to occur in three stages (Xu & van Deventer 2000) :

- dissolution,- transportation or orientation, and - polycondensation

The reactions of geopolymerisation take place through a series of exothermic processes (Palomo, Grutzeck & Blanco 1999; Davidovits 1999).

Cheng and Chiu (2003) had observed that unlike conventional organic polymers, glass, ceramic, or cement, the geopolymers are formed at low temperatures and they are non-combustible, heat-resistant, and fire/acid resistant. It was recognised that three sources essentially are needed for synthesis of geopolymer: (i) raw materials (such as fly ash, GGBS, MK, etc), (ii) inactive filler (such as sand and crushed granite aggregate), and (iii) geopolymer liquor (Alkali Activator Solution (AAS). Raw materials (or geopolymer source materials) can be industrial wastes, such as fly ash, blast furnace slag, red mud, waste glasses, or some natural minerals and rocks. The active powdery fine material, containing mainly geo-synthesis supporting Al+3 ions, can be kaolinite or metakaolinite. Geopolymer liquor (AAS) consists of sodium silicate solution acting as binder, and alkali hydroxide solution for the dissolution of raw materials. The authors noted that the chemical process to form geopolymers involves two steps: (i) dissolution of raw materials in alkaline solution to form Si and Al gel on the materials’ surface, (ii) polycondensation to form networked polymeric oxide structures. However, the exact mechanism of geopolymer setting and how hardening occurs was felt to be still not fully understood

Xu and Van Deventer (2000) investigated the geopolymerisation of 15 natural Al Si minerals. It was found that the minerals with a higher extent of dissolution demonstrated better compressive strength after polymerisation. The percentage of calcium oxide (CaO), potassium oxide (K2O), the molar ratio of Si-Al in the source material, the type of alkali and the molar ratio of Si/Al in the solution during dissolution had significant effect on the compressive strength.

In the synthesis of geopolymers, there are essentially two types of raw materials, the aluminosilicate-containing solids and alkali-silicate solutions. The aluminosilicate solids function as sols in the alkali-silicate liquid medium. The sol-liquid combination will turn into a sol-gel matrix, as is usually done in the sol-gel methodology. The aluminosilicate

Geopolymer Concrete

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sources include the commonly used kaolinite, especially, calcined kaolinite, or metakaolinite (Barbosa et al., 2000; Davidovits, 1991; 1999; Davidovits and Davidovics, 1998; Palomo and Glasser, 1992; Rahier et al., 1996a; b; 1997) and other natural aluminosilicate minerals (Xu and Van Deventer, 2000b; 2002a) and industrial waste-based materials, such as GGBS (Cheng and Chiu, 2003; Yip and Van Deventer, 2003) and FA(Lee and Van Deventer, 2002a; b; Palomo et al., 1999b; Phair and Van Deventer, 2001).

Van Jaarsveld, Van Deventer and Lukey (2002) studied the interrelationship of parameters that affected the properties of FA-based geopolymer and reported that the properties of geopolymer were influenced by the incomplete dissolution of the materials involved in geopolymerisation. The water content, curing time and curing temperature affected the properties of geopolymer; specifically the curing condition and calcining temperature influenced the compressive strength. When the samples were cured at 700C for 24 hours a substantial increase in the compressive strength was observed. Curing for a longer period of time reduced the compressive strength.

Wang Bao-min and Wang Li-jiu (2005) studied the applications of geopolymeric activation techniques of FA in conventional cement concretes. The research showed that when weight of FA reaches 20%-80% of 32.5 grade cement, M40 concrete with satisfactory properties can be prepared through using activating techniques such as adding some high-efficiency FA activating admixture.

[3] Fresh Geopolymer concrete mixes

Hardjito et al, (2002) observed that fresh geopolymer concrete is highly viscous, and cohesive with low workability when the calcined kaolin was the source material.

Structural Usages

Davidovits and Sawyer (1985) used ground blast furnace slag to produce geopolymer binders. This type of binders patented in the USA under the title ‘Early High-Strength Mineral Polymer’, was used as a supplementary cementing material in the production of precast concrete products.

Activating Medium

A combination of sodium or potassium silicate and sodium or potassium hydroxide has been widely used as the alkaline activator (Palomo et al, 1999; van Jaarsveld, van Deventer & Lukey 2002; Xu & van Deventer, 2000; Swanepoel & Strydom, 2002), with the activator liquid-to-source material ratio by mass in the range of 0.25-0.30 (Palomo, Grutzeck & Blanco 1999; Swanepoel & Strydom 2002).

Anurag Mishra (2008, 2009) conducted experiments on

FA based GPC by varying the concentration of NaOH and curing time. Total nine mixes were prepared with NaOH concentration as 8M, 12M, 16M and curing time as 24hrs, 48hrs, and 72hrs. The investigation indicated: an increase in compressive strength with increase in NaOH concentration and curing time, increase in compressive strength after 48hrs curing time not significant. Compressive strength up to 46 MPa was obtained with curing at 60ºC. Water absorption decreased with increase in NaOH concentration and curing time.

Swanepoel and Strydom (2002) conducted a study on geopolymers produced by mixing FA, kaolinite, sodium silica solution, NaOH and water. Both the curing time and the curing temperature affected the compressive strength, and the optimum strength occurred when specimens were cured at 600C for a period of 48 hours

Palomo, Grutzeck, and Blanco (1999) studied the influence of curing temperature, curing time and alkaline solution-to-FA ratio on the compressive strength. It was reported that both the curing temperature and the curing time influenced the compressive strength. The utilization of sodium hydroxide (NaOH) combined with sodium silicate (Na2SiO3) solution produced the highest strength. Compressive strength up to 60 MPa was obtained when cured at 85ºC for 5 hours. The type of alkaline liquid plays an important role in the polymerisation process. Reactions occur at a high rate when the alkaline liquid contains soluble silicate, either sodium or potassium silicate, compared to the use of only alkaline hydroxides confirmed that the addition of sodium silicate solution to the sodium hydroxide solution as the alkaline liquid enhanced the reaction between the source material and the solution. Furthermore, after a study of the geopolymerisation of sixteen natural Al-Si minerals, they found that generally the NaOH solution caused a higher extent of dissolution of minerals than the KOH solution.

Curing

Because heat is a reaction accelerator, curing of fresh geopolymer is carried out mostly at an elevated temperature (Palomo et al,1999). When curing at elevated temperatures, care must be taken to minimize the loss of water.

Swanepoel and Strydom,(2002), described the effects of curing at 40, 50, 60 and 70ºC for different durations (6, 24, 48 and 72 h) and the optimum condition was noted to be 60ºC for a period of 48 hours.

Curing at room temperature has successfully been carried out by using calcined source material of pure geological origin, such as metakaolin (Davidovits 1999; Barbosa, MacKenzie & Thaumaturgo 2000).

Cheng and Chiu (2003) found that the setting

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time of geopolymer paste made with GGBS as the source material along with metakaolinite, was affected by the curing temperature, type of alkaline activator, and the actual composition of the source material. The setting time of geopolymer paste was observed to range from 15 to 45 minutes at 60o C.

Van Jaarsveld et al. (2002) observed that curing at elevated temperature for long periods of time may weaken the structure of hardened material. The test data showed that curing temperature and its duration significantly influences the compressive strength. Longer curing time and higher curing temperature increased the compressive strength, although the increase in strength may not be significant for curing at more than 60oC and curing for periods longer than 48 hours. The compressive strength of fly ash-based geopolymer concrete cured at 60oC for 24 hours did not vary with the age and remained constant at approximately 60 MPa.

Chindaprasirt (2006) conducted the compression strength test on GPC specimens prepared by using the NaOH of 10M concentration. He concluded that the GPC specimens prepared by using above concentration can achieve high strength (70MPa) when cured in oven at a temperature of 75°C for two days.

GPC Mix Design

Djwantoro Hardjito, et al (2004), showed that the geopolymer paste binds the coarse aggregates, fine aggregates and other un-reacted materials together to form the GPC, and usual concrete technology methods to produce GPC mixes can be often employed. As in the Portland cement concrete, the aggregates occupy the largest volume, (about 75-80 % by mass) in GPCs. The silicon and the aluminium in the fly ash are activated by a combination of sodium hydroxide and sodium silicate.

Rangan and Hardjito (2005) have noted that unlike conventional cement concretes GPCs are a new class of construction materials and therefore no standard mix design approaches are yet available for GPCs. While GPC involves more constituents in its binder (viz., FA, GGBS, sodium silicate, sodium hydroxide and water), whose interactions and final structure and chemical composition are under intense research whereas the chemistry of Portland cement and its structure and chemical composition (before and after hydration) are well established due to extensive research carried out over more than century. While the strength of cement concrete is known to be well related to its water-cement ratio, such a simplistic formulation may not hold good for GPCs. Therefore, the formulation of the GPC has to be done by trial and error basis.

Rajmane (2006) studied the effect of geopolymeric binders such as GGBS and FA by activating silicon dioxide and aluminium oxide present in the binders, to form inorganic polymer binder system. This binder system can be used to produce concretes containing river sand as fine aggregate and coarse aggregate in the form of either sintered FA aggregates (SFFA) or crushed granite aggregates (CGA). It was concluded that the lightweight aggregate based geopolymer concrete have one day compressive strength of about 35 MPa and a 28 days strength of more than 50 MPa. CGA based geopolymer concretes produced marginally higher compressive strength of about 45 MPa at one day and 65 MPa at 28 days

František Škvára et al (2005) showed that the structure of the geopolymers prepared on the basis of fly ashes (cured at 60-80ºC 6-12 hours ) is predominantly of the AlQ4(4Si) type and SiQ4 (4Al), SiQ4 (2-3Al). The strength range was 15 to 70 MPa and is affected substantially by macro-pores (103 nm and more) formed in result of the air entrained into the geopolymers. There are fly ash particles that underwent only partial reaction. The presence of Ca-containing additives (slag, gypsum) reduces considerably the porosity. There was no transition phase of different composition between the geopolymers and the aggregate.

Geopolymers and Zeolites

Geopolymers are unique in comparison to any other aluminosilicate materials (e.g. aluminosilicate gels, glasses, and zeolites). The concentration of solids in geopolymerisation is higher than in aluminosilicate gel or zeolite synthesis. Geopolymers are believed to be an amorphous metastable phase of zeolites (i.e., zeolitic precursors) that can be converted to a more well-defined crystalline phase (zeolites) provided that the right conditions and reactant concentrations are used (Xu and Van Deventer, 2002b).

A recent review by Provis et al. (2005c) suggested that that geopolymer is constituted from agglomerates of zeolitic nanocrystals bound by an amorphous gel phase. The degree of Crystallinity is affected by reaction conditions and starting reactant concentration, particularly silicate and alkali concentrations.

Geopolymerisation Modelling

Sindhunata (2006) studied the conceptual model of geopolymerisation. He conducted studies under controlled conditions typically used for geopolymerisation, thus leading to findings, which improved the understanding of reaction steps. Various influencing parameters investigated, were the concentration of reactants (silicate concentration, alkalinity, and water content) and the curing conditions (temperature,

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time, humidity). A conceptual model of geopolymerisation was developed by incorporating the above mentioned factors. Three key aspects of GPC studied were: firstly, an investigation on the development of pore structure in geopolymers; secondly, an investigation on the competition between dissolution, polymerization, and crystallization of aluminosilicate gels during geopolymerisation and finally, an investigation on the ageing of geopolymers in alkali and carbonate solutions. The occurrence of different reaction mechanisms is influenced by the alkali-silicate concentration and the type of alkali metal cation used. The investigation of ageing provides insight into the reaction mechanisms of late geopolymerisation (i.e. post- set processes).

Reinforced GPCs

Past studies on reinforced fly ash-based geopolymer concrete members are extremely limited. Palomo et.al (2004) investigated the mechanical characteristics of fly ash based geopolymer concrete. It was found that the characteristics of the material were mostly determined by curing methods especially the curing time and curing temperature. Their study also reported some limited number of tests carried out on reinforced geopolymer concrete sleeper specimens. Another study related to the application of geopolymer concrete to structural members was conducted by Brookeet al. al (2005). It was reported that the behaviour of geopolymer concrete beam column joints was similar to that of members made of Portland cement concrete.

Shuguang Hu and Hongxi Wang (2008) investigated the mechanical properties of geopolymeric materials (steel slag based) and other conventional materials. The compressive strength, bond strength and abrasion resistances were experimentally studied. It was found that the bond strength of geopolymeric material with steel slag was 2.6% higher than those of other materials. It was also concluded that the steel slag was almost fully absorbed to take part in the alkali activated reaction and incorporated into the amorphous aluminosilicate geopolymer matrix.

Palomo et.al (2004) investigated the mechanical characteristics of FA based GPC concrete. It was found that the characteristics of the material were mostly determined by curing methods especially the curing time and curing temperature. Their study also reported some limited number of tests carried out on possible use of GPC concrete for the production of prestressed sleeper specimens.

Brooke et al. al (2005) studied the application of GPC concrete to structural members. It was reported that the behaviour of GPC concrete beam column joints was similar to that of members made of Portland cement concrete.

Sumajouw and Rangan (2006) conducted extensive studies

on low-calcium FA based reinforced GPC concrete beams and columns. The behavior and failure modes of reinforced GPC concrete columns and beams were similar to those observed in the case of reinforced Portland cement concrete columns. The results demonstrated that the methods of calculations used in the case of reinforced Portland cement concrete beams and columns are applicable for reinforced GPC concrete beams and columns. The results demonstrated that reinforced low-calcium (ASTM Class F) FA based GPC concrete structural members can be designed using the design provisions currently used in the case of reinforced Portland cement concrete members.Excellent correlation between experimental and analytical results is found.

Prabir Kumar Sarker (2008) reports study on analysis on GPC columns. It is found that the equation of Popovics (proposed for OPC concrete) can be used for geopolymer concrete with minor modification to the expression for the curve fitting factor, to better fit with the post peak parts of the experimental stress–strain curves. A good correlation is achieved between the predicted and measured ultimate loads, load–deflection curves and deflected shapes for 12 slender test columns

Durability

(i) Corrosion of Embedded Steel

Miranda et al (2005) gave details of corrosion potential and polarisation resistances for steel electrodes embedded in Portland cement mortar and two fly ash mortars (respectively activated with NaOH and waterglass+NaOH solutions). Chloride-free activated fly ash mortars were found to passivate steel reinforcement as speedily and effectively as Portland cement mortars. The polarization curves and the response to short-term anodic current pulses (galvanostatic pulse technique) corroborated the full and stable passivation of the steel. They concluded that the icorr value for both OPC and GPC mortar are similar (0.1 µA/cm2).

Yodmuneeand Yodsudjai (2006) studied the corrosion of steel bar located inside in fly ash-based geopolymer concrete in an accelerated corrosion test. All the GPC mixes had higher compressive strength than conventional concrete (10 to 16 MPa). The test results included the half-cell potential and cross sectional loss of steel bar and in both the respects GPCs performed better. He conclude that at 72 hrs, the GPC specimens gives the Half cell potential value of -175mV which is mostly equal to the OPC value (-200 mV).

Holloway and Sykes (2005) studied the Corrosion of mild steel reinforcement in an alkali-activated slag (AAS) cement

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mortar containing NaCl admixtures using an improved galvanostatic pulse technique. He concluded that, at initial stage highest corrosion rates are seen with the lowest chloride levels(0% and 2%), but over time(107days) the corrosion rate decrease to 1 µAcm-2 even when the NaCl level increase to 8%.

Shi 2003a reported that the Alkali activated slag showed much less corroded depth (<4mm) then OPC specimens (>14mm) which are immersed in nitric acid even after 90days continuous exposure.

Davidovits (1994) noted that unlike conventional Portland cement, geopolymeric cements do not rely on lime and are not dissolved by acidic solutions. Geopolymeric cements, Potassium-Poly (sialatesiloxo) type, remain stable with a loss in the 5-8 % range.

(ii) Acid Resistance

Bakharev et al (2003) investigated the durability of alkali-activated slag (AAS) concrete exposed to acetic acid solution of pH = 4. It was found that AAS concrete of Grade 40 had a high resistance in acid environment, superior to the durability of OPC concrete of similar grade.

Songa (2005) investigated the durability property of geopolymer concrete exposed to sulphuric acid corrosion. It was concluded that GPC is highly resistant to sulphuric acid; in terms of a very low mass loss, less than 3%. Moreover, Geopolymer cubes were structurally intact and still had substantial load capacity even though the entire section had been neutralized by sulphuric acid.

(iii) Fire Resistance

Van Jaarsveld, Van Deventer, and Schwartzman (1999) carried out experiments on geopolymers using FA and found them to be fire resistant with compressive strengths of 5 to 51 MPa. The factors affecting the compressive strength were the mixing process and the chemical composition of the FA. A higher CaO content decreased the microstructure porosity and, in turn, increased the compressive strength. Besides, the water-to-FA ratio also influenced the strength. It was found that as the water-to-FA ratio decreased, the compressive strength of the binder increased.

Lyon et al (1996) discussed the fire response of a potassium aluminosilicate (Geopolymer) matrix carbon fiber composite. At irradiance levels of 50 kW/m2(typical of the heat flux in a well developed fire), glass- or carbon-reinforced polyester, vinyl ester, epoxy, bismaleimde, cyanate ester, polyimide, phenolic, and engineering thermoplastic laminates ignited readily and released appreciable heat and smoke, while carbon-fiber reinforced Geopolymer composites did not ignite, burn, or release any smoke even after extended

heat flux exposure. The Geopolymer matrix carbon fiber composite retained 67% of its original flexural strength after a simulated large fire exposure

(iv) Sulphate Attack

Hardjito and Rangan (2005) studied the development and properties of low-calcium FA based geopolymer concrete. The research report described the development, the mixture proportions, and the short-term properties of low-calcium FA based GPC concrete. It was concluded that low-calcium FA-based geopolymer concrete had excellent compressive strength, suffer very little drying shrinkage and low creep, had excellent resistance to sulfate attack, and good acid resistance.

Test results showed that heat-cured low-calcium fly ash-based geopolymer concrete has an excellent resistance to sulphate attack. Research data shows that geopolymeric materials performed significantly better in acid resistance compared to Portland cement (Davidovits, 1994; Gourley and Johnson, 2005).

Bakharev (2003) investigated the durability of alkali-activated slag (AAS) concrete exposed to sulphate attack. AAS concrete was immersed in 5% sodium, 5%magnesium and5% sodium + magnesium sulphate solution. The main parameters studied were the compressive strength, products of degradation, and micro structural changes. It was found that in AAS concrete the material prepared using sodium hydroxide had the best performance due to its stable cross-linked aluminosilicate polymer structure.

Douglas (1992) reported that the changes in dynamic modulus of elasticity, pulse velocity, weight and length of sodium silicate-activated slag cement concrete after 120 days of immersion in 5% sodium sulphate solutions. They noticed that the changes are even smaller than those in the controlled specimens immersed in lime-saturated water.

(v) Salt Environment

Nguyen Van Chanh et al [2008] found that compressive strength of heat-cured fly ash-based geopolymer concrete does not depend on age. Longer curing time (24 to 72 hours) produces higher strength, but, increase in strength beyond 48 hours is not significant. Geopolymer concrete has excellent properties within both acid and salt environments. Comparing to Portland cement, the geopolymers have a relative higher strength, excellent volume stability, better durability.

Gailius and Kazberuk (1998) monitored the long-term behaviour of concretes in a chloride exposure regime under influence of cyclic wetting and drying as well as freezing and thawing with chlorides. They concluded that the

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resistivity of concrete was closely connected with cement type and mineral addition content, cement mass as well as the time of storage. The resistance to chloride penetration was found to increase with time but the value of diffusion coefficient from migration test depended on cement type.

Yang and Cho (2001) Huang stated that the accelerated chloride migration test indicated a good correlation between the charge passed and the steady-state chloride flux.

2.4.3 Applications

(a) Hazardous Waste Encapsulation

Davidovits [2002] informs about zeolitic materials’ abilities to adsorb toxic chemical wastes. Geopolymers behave similarly to zeolites and feldspathoids. They immobilise hazardous elemental wastes within the geopolymeric matrix, as well as act as a binder to convert semi-solid waste into an adhesive solid. Hazardous elements of waste materials mixed with geopolymer get locked into the three dimensional framework of the geopolymeric/zeolitic matrix.

Davidovits (1999) suggested that the atomic ratio of Si-to-Al of should be about 2 for making geopolymeric binder based pastes, mortars and concretes. Geopolymer can also be used for waste encapsulation to immobilise toxic metals (van Jaarsveld, van Deventer & Lorenzen 1997).

Palomo and Palacios (2003), described the stabilisation/solidification capacity of a matrix made using alkali activation of fly ash, in the presence of toxic elements chromium and lead. Leaching tests proved that the matrix is able to stabilise and solidify lead efficiently (analysed lead concentrations from leaching were in parts per billion). However, geopolymer was not efficient for chromium fixation since this element strongly disturbed the alkali-activation mechanism of the ash

(b) Precast Products

Gourley and Johnson (2005) have reported commercial production of geopolymer precast concrete products. Reinforced GPC sewer pipes outperformed comparable Portland cement concrete pipes. Good performance of reinforced GPC railway sleepers on mainline tracks and excellent fire resistance of GP mortar wall panels were also reported.

Siddiqui (2007) demonstrated the successful commercial scale manufacture of reinforced geopolymer concrete culverts.

Davidovits and Sawyer (1985) used ground blast furnace slag to produce geopolymer binders. This type of binders was patented in the USA under the title Early High-Strength Mineral Polymer was used as a supplementary cementing

material in the production of precast concrete products.

(c) Structural Concretes

Zongjin Li et al (2004) terming the geopolymers as sustainable composites and found that they are a type of amorphous alumino-silicate product and can be synthesized by polycondensation reaction of geopolymeric precursor and alkali polysilicates. Geopolymers are energy efficient and environment friendly sustainable cementitious materials with superior properties compared to the Portland cement, such as high early strength, excellent volume stability, better durability, good fire resistance, and easy manufacturing process.

.Djwantoro Hardjito et al (2004) investigated geopolymer as the binder (in place of Portland cement) where binding action is achieved in fly ash by hydroxide-silicate based chemicals (as an initiators or catalysts for polymeric reaction) to produce concrete using the usual concrete technology methods.

Davidovits and Sawyer (1985) had used ground blast furnace slag to produce geopolymer binders. This type of binders patented in the USA under the title Early High-Strength Mineral Polymer for used as a supplementary cementing material in the production of precast concrete products. In addition, a ready-made mortar package that required only the addition of mixing water to produce a durable and very rapid strength gaining material was produced and utilised in restoration of concrete airport runways, aprons and taxiways, highway and bridge decks, and for several new constructions when high early strength was needed.

Concluding Remarks

The literature survey indicates that geopolymer word is one of the many names used for describing the binder formed with alumino-silicate gel structure which according to some researchers need not be in polymeric form. However, Davidovits has data to show that polymer is indeed formed. Commonly, Metakaolin (MK) is often used by some authors to produce so called pure ‘geopolymers’ since MK, mostly consist of alumina and silica. However, much literature exists on activation of MK in combination with FA, GGBS, etc. works on only FA based GPs were also reported, and notable among them is Prof. Rangan, of Curtin University. Davidovits advocates use of slag in combination with other GSMs such as MK and fly ash and he emphasizes on development with lower activation temperatures and lower alkali levels in AAS.

Comparatively, more papers are available in science of geopolymer where often paste is prepared for making test specimens. Concretes and mortars formulation are also

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reported, but, lesser in numbers. GP science has not yet came up with a unique way of describing the matrix of GP and the AAS has to be developed for each set GSMs used in any particular experiment.

About the mechanical strengths, only qualitative information is available which can be used to decide about any particular combination of GP mixes to achieve the desired level of strength.

Works on reinforced GPC are not many and however, the existing test results shows that structural behaviour of GPCs and CCs are essential and similar in nature, except that sometime at the same strength level, GPCs may tend to have lower modulus of elasticity.

Contrastingly, GP composites have performed better than P-C composites in durability related tests such as Sulphate, acid and corrosion resistance. This is mainly due to polymeric nature of GP matrix without presence of free lime.

Numerous studies on GPs indicated that though exact nature of GP microstructure is still to be decided, it is still possible to formulate the GP composites to achieve consistently the desired level of strengths for structural usages by suitable selection of GSMs, AAS, besides curing regimes.

Abbreviations/Notations

AAS = Alkaline Activator Solution Alumina = Al2O3 CCs = Conventional concretes CGA = Crushed granite aggregates C-S-H = Calcium-silicate-hydrate DW = Distilled WaterECO2 = Embodied carbon dioxideEE = Embodied energyFA = Fly ashFAA = Fly Ash Aggregates GGBS = Ground Granulated Blast Furnace Slag GP = Geopolymer GPC = Geopolymer concreteHVFA = High volume fly ash IR = Infrared MK = MetakaolinMR = Molar ratios NMR = Nuclear Magnetic ResonanceOPC = Ordinary Portland CementP-C = Portland CementSHf = Sodium Hydroxide flakes SHS = Sodium hydroxide solution

SiO2 = Silica SSD = Saturated surface drySSS = Sodium Silicate Solution W/C= Water-cement ratio

References

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Synthesis and characterisation of materials based on inorganic polymers of alumina and silica: sodium polysialate polymers, International Journal of Inorganic Materials, 2, 309- 317

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- Chang, E. H., Sarker, P., Lloyd, N., & Rangan, B. V. (2007). Shear behaviour of reinforced fly ash-based geopolymer concrete beams. Paper presented at the The 23rd Biennial Conference of the Concrete Institute of Australia, Adelaide, Australia.

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For a complete list of the references please visit: www.masterbuilder.co.inPublishers Note: Part - 2 to be features in May 2012 edition.

Geopolymer Concrete