50 The Masterbuilder - May 2012 • www.masterbuilder.co.in

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

The literature survey on Geopolymer Concretes (GPCs) presented in the first part of this paper indicated that qualitative information on available on the mechanical properties of GPC mixes is sufficient to develop GPCs for use in civil engineering structures. However, it is seen that with understanding of the similarities and difference between Portland cement and Geopolymer technologies, a rational research plan giving various steps involved can be formulated to achieve desired level of structural and durability related characteristics in structural grade GPC mixes. This aspect is discussed in this paper.

An Overview on Geopolymer Concretes

The general literature study on GPs, as presented earlier, indicate that often GPs are studied by many scientists at paste level, using processed materials such as Metakaolin and industrial waste materials such as fly ash and slag. GPCs are studied for structural applications by few scientists. It would be worthwhile to have an overview on GPC technology so that further studies on actual implementation of GPC technology for civil engineering applications as a rational alternate to P-C can be planned.

The geopolymers (GPs) initiated by Davidovits (1988) has great potential for adoption by concrete construction industry as an alternative binder to the Portland cement (Duxson et al, 2007). GPs could significantly reduce the CO2 emission to the atmosphere caused by the cement industries Gartner (2004). The alkaline liquid was proposed by Davidovits (1988; 1994) for reacting with the silicon (Si) and the Aluminium (Al) present in an alumino-silicate source material which may be either of geological origin or by-product materials such as fly ash and rice husk ash. Since, the chemical reaction involved is an inorganic polymerization process (but under alkaline condition), the word polymer in ‘Geopolymer’ to represent the new binders seems to be logical. The chemical composition of geopolymer material is similar to zeolitic materials, but the microstructure is amorphous. The fast chemical reaction

under alkaline condition of Si- Al minerals results in a three-dimensional polymeric chain and ring structure consisting of Si-O-Al- O bonds (Davidovits, 1994). The formation of geopolymer material can be described by following two Equations (1) and (2) (Davidovits, 1994; van Jaarsveld et al., 1997):

Rajamane N. P.1, Nataraja M. C.2, Lakshmanan N3, and Ambily P S 4

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

The Equation 2 indicates that water is released during formation of geopolymers and forms discontinuous nano-pores in the matrix.

The atomic ratio Si: Al in the polysialate of geopolymer is selected based on the particular application and low Si: Al ratios are suitable for most of the civil engineering applications. Low-calcium (ASTM Class F) fly ash is more common and can be adopted to manufacture geopolymer concrete (GPC). Usually, 80% of the fly ash particles were smaller than 50 μm (Gourley, 2003; Gourley and Johnson, 2005; Hardjito and Rangan, 2005; Wallah and Rangan, 2006; Sumajouw and Rangan, 2006; Fernandez-Jimenez

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et al, 2006a; Sofi et al, 2006a; Siddiqui, 2007). The reactivity of low-calcium fly ash in geopolymer matrix is found to be adequate (Fernandez-Jimenez et al, 2006b). Coarse and fine aggregates of the conventional concretes are suitable to produce GPCs and grading curves are usually applicable to GPC mixes also (Hardjito and Rangan, 2005; Wallah and Rangan, 2006; Sumajouw and Rangan, 2006; Gourey, 2003; Gourley and Johnson, 2005; Siddiqui, 2007). It is recommended that the AAS is prepared at least 24 hours prior mixing of GPCs. Alkali silicate solutions are commercially available with different solid contents and molar ratios (MR). The ratio of [SiO2]/[M2O] is defined as MR where [SiO2] and [M2O] are contents of SiO¬2 (silica) and M2O (alkali oxide) in the alkali silicate. The sodium hydroxide is commercially available in the form of in flake or pellet.

The primary difference between geopolymer concrete and Portland cement concrete is the binder. The silicon and aluminum oxides in the low-calcium fly ash reacts with the alkaline liquid to form the geopolymer paste that binds the loose coarse aggregates, fine aggregates, and other un-reacted materials together to form the geopolymer concrete. The compressive strength and the workability of geopolymer concrete are influenced by the proportions and properties of the constituent materials that make the geopolymer paste. Experimental works of (Hardjito and Rangan, 2005) indicate:

- Higher concentration of sodium hydroxide results in higher strength of GPC.

- Higher the ratio of sodium silicate solution-to-sodium hydroxide solution ratio by mass, higher is the compressive strength of geopolymer concrete.

- The addition of naphthalene sulphonate-based super plasticizer can improves the workability of the fresh geopolymer concrete; however, there is degradation in the compressive strength of hardened concrete.

- The slump value of the fresh geopolymer concrete increases when the water content of the mixture increases.

Geopolymer concrete can be manufactured by adopting the conventional techniques used in the manufacture of Portland cement concrete. The GSM and the aggregates (SSD condition) are first mixed together dry a concrete mixer. The AAS, liquid component of GPC mixture, is then added and the mixing continued. The fresh GPC is cohesive and can be handled up to about 2hours (depending upon the formulation) without any sign of setting and without much effect on the compressive strength. Moulding of specimens, compaction, including workability measurement are similar

to Portland cement concrete (Hardjito and Rangan, 2005; Wallah and Rangan, 2006).

Rangan and his team at Curtin University had steam-cured the fly ash based GPC test specimens at 60ºC for 24 hours and then storing in ambient conditions till testing. Heat-curing substantially assists the chemical reaction that occurs in the geopolymer paste. Both curing time and curing temperature influence the compressive strength of geopolymer concrete and can be manipulated to fit the needs of practical applications. (Hardjito and Rangan, 2005).

A two-stage steam-curing regime was adopted by Siddiqui (2007) in the manufacture of prototype reinforced geopolymer concrete box culverts. It was found that steam curing at 80 °C for a period of 4 hours provided enough strength for de-moulding of the culverts; this was then followed by steam curing further for another 20 hours at 80 °C to attain the required design compressive strength. Also, the start of heat-curing of geopolymer concrete can be delayed for several days (say up to 5 days) without any degradation in the compressive strength. A delay in the start of heat-curing substantially increases the compressive strength of geopolymer concrete (Hardjito and Rangan, 2005). The role and the influence of aggregates in GPCs are considered to be the same as in the case of Portland cement concrete. Hardjito and Rangan (2005) suggests that the ratio of sodium silicate solution-to-sodium hydroxide solution by mass may be taken approximately as 2.5

Test data show that the strain at peak stress for fly ash based GPCs is in the range of 0.0024 to 0.0026 (Hardjito and Rangan, 2005). Collins et al (1993) have proposed that the stress-strain relation of Portland cement concrete in compression can be predicted using the parameters; peak stress and strain at peak stress.

The tensile splitting strength of geopolymer concrete is only a fraction of the compressive strength, as in the case of Portland cement concrete, but, larger than the values recommends by most of the Standards (such as AS3600, IS:456-2000), and Neville (2000) for Portland cement concrete (Sofi et al, 2007a; Hardjito and Rangan, 2005).

The unit-weight of concrete primarily depends on the unit mass of aggregates used in the mixture. Tests show that the unit-weight of the low-calcium fly ash-based geopolymer concrete is similar to that of Portland cement concrete (Hardjito and Rangan, 2005).

The drying shrinkage of heat-cured fly ash-based GPCs over a period of one year is significantly smaller than that experienced by Portland cement concrete. (Wallah and Rangan, 2006).

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The behaviour and failure modes of reinforced geopolymer concrete columns are similar to those observed in the case of reinforced Portland cement concrete columns and hence, reinforced low-calcium (ASTM Class F) fly ash-based geopolymer concrete structural members can be designed using the design provisions currently used in the case of reinforced Portland cement concrete members. (Sumajouw and Rangan, 2006). The studies carried out by Chang, et al (2007), Sarker, et al (2007a, 2007b), and Sofi, et al (2007b) demonstrate the application of fly ash-based geopolymer concrete.

Geopolymer has also been used to replace organic polymer as an adhesive in strengthening structural members. Geopolymers were found to be fire resistant and durable under UV light (Balaguru et al 1997)

Scientists, van Jaarsveld, van Deventer, and Schwartzman (1999) carried out experiments on geopolymers using two types of fly ash. They found that the compressive strength after 14 days was in the range of 5 – 51 MPa. The factors affecting the compressive strength were the mixing process and the chemical composition of the fly ash. A higher CaO content decreased the microstructure porosity and, in turn, increased the compressive strength. Besides, the water-to-fly ash ratio also influenced the strength. It was found that as the water-to-fly ash ratio decreased the compressive strength of the binder increased.

Palomo, Grutzeck, and Blanco (1999) studied the influence of curing temperature, curing time and alkaline solution-to-fly ash 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.

Xu and van Deventer (2000) investigated the geopolymerisation of 15 natural Al-Siminerals. 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.

Swanepoel and Strydom (2002) conducted a study on geopolymers produced by mixing fly ash, 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 60°C for a period of 48 hours.

Van Jaarsveld, van Deventer and Lukey (2002) studied the interrelationship of certain parameters that affected the properties of fly ash-based geopolymer. They 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 70ºC for 24 hours a substantial increase in the compressive13strength was observed. Curing for a longer period of time reduced the compressive strength.

Fly ash had been used in the past to partially replace Portland cement to produce concretes. An important achievement in this regard is the development of high volume fly ash (HVFA) concrete that utilizes up to 60 percent of fly ash, and yet possesses excellent mechanical properties with enhanced durability performance. The test results show that HVFA concrete is more durable than Portland cement concrete (Amphora 2002). Recently, a research group at Montana State University in the USA has demonstrated through field trials of using 100% high-calcium (ASTM Class C) fly ash to replace Portland cement to make concrete. Ready mix concrete equipment was used to produce the fly ash concrete on a large scale. The field trials showed that the fresh concrete can be easily mixed, transported, discharge, placed, and finished (Cross et al, 2005).

Pioneering research on fly ash-based geopolymer concrete was conducted at Curtin University and the results is described great details in widely referred Research Reports GC1 to GC4 (Hardjito and Rangan 2005, Wallah and Rangan 2006).

Directives from Literature for Research

The literature survey related to the use of aluminosilicate based binder, nomenclated now as geopolymer, is actually involves the activation of alumina and silica of any source material (of mineral origin such as fly ash, GGBS, MK, etc) or synthetically produced alumina, silica and their compounds in various forms. Since zeolites now have many commercial applications, the technology of production of these materials [which are also aluminosilicate in nature] is very advanced. Many attempts were made to use the test results of zeolites to geopolymers. But, still the field of geopolymers needs significant amount of research from engineers usage point of view as the standardized simpler models of chemical reactions are not yet available as it

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had happened in case of Portland cement where simple concepts as given below have helped engineers to develop the wide variety of uses of P-C based concrete, without need for deeper understanding of chemistry of hydration of P-C in the field:

- P-C is basically made of 4 Bogue’s compounds – C3A, C4AF, C3S and C2S.

- Bogue’s composition of any P-C can be computed arithmetically based on its oxide contents.

- Properties of P-C concretes can be accounted by relative amount of Bogue’s compounds in the cement. For e.g. for high early strength, C3S should be more and for high Sulphate Resistance, C3A should be less, etc.

- Water for complete chemical hydration reaction of any P-C is about 0.22 to 0.25 by weight of cement. This means, any hydrated cement portion of any matrix contains 22% to 25% by weight of unhydrated cement.

- Lower the W/C ratio, the strength and the denseness (hence degree of impermeability, thereby durability) increase.

- Setting and rheology (ie, behaviour of cement paste in freshly mixed state) can be controlled fairly reliably by well formulated and commercially available chemical admixtures.

- The microstructure of hydrated P-C consists of gel pores and capillary pores whose magnitude can be estimated by simple computations and the microstructure itself can be manipulated easily by processing conditions, addition of admixtures, etc.

- Degrees of hydration of cement can be measured approximately by many techniques, the simplest being the determination of non-evaporable water content of hydrated paste.

Apart from above, many practically useful tips are available to engineers, as a result of R&D on P-Cs spread over more than 2 centuries. Such a stage has not reached yet in case of geopolymers and hence, there is a need for more intense research. Towards this objective, the present study was taken up. Though the chemical nature of geopolymers is not fully understood, since geopolymers can be made from a variety of source materials and the activation of these source materials can be also carried out by many activating chemicals besides the numerous process conditions themselves. Keeping this in view, using the information available in the literature, following methodology can be utilised to develop various experimental programmes in the investigation taken up.

Stage 1: Selection of GSMs

The literature mentions Metakaolin as almost pure source for production of geopolymers, since it is mainly consists of alumina and silica which are basic building blocks of any geopolymer. Since MK is a processed material hence costlier both cost considerations and ‘Embodied Energy’ point of view, it was decided to adopt fly ash and blast furnace slag as the geopolymeric source materials (GSMs) since these are actually industrial waste products. Disposal problems of these wastes can be efficiently solved by using them in GPCs which can even become a good replacement material for conventional Portland cement based concrete.

To avoid the possibility of different types of unexpected/undesirable chemical reaction during GPs, due to variation in chemical and physical characteristics of FA and GGBS, one producer for each of them was identified and the same source materials were used throughout the experimental programme.

Stage 2: Selection of AAS

Alkali Activator Solution (AAS) required for initiating chemical reactions in GSMs consist of sodium hydroxide (NaOH) and sodium silicate solution (SSS). These are actually industrial chemicals used by many industries. For cost consideration and from practical point of view, it is necessary to use as much as possible the commercially available chemicals (instead of laboratory or reagent grades). Preliminary trials, with these chemicals, on production of GPCs indicated the adequacy of the industrial quality of chemicals for use in GPC mixes which themselves consist of fly ash, GGBS, sand and coarse aggregates, which are not themselves of any particular chemical purity, though for quality control purposes for their use in concretes, general practical guidelines for characterizing them are available in Standard Codes.

Sodium hydroxide, in flake from, (SHf) is readily available in market and hence adopted to prepare sodium hydroxide solution (SHS).

Sodium Silicate Solution (SSS) are actually available in different Molar Ratios whose chemical natures vary much from geopolymerisation consideration. Hence, a commercial producer was directly contacted and the SSS was obtained; the physical and chemical characteristics of this material were supplied by the producer himself.

Since, potable water may contain ions which can be taken as acceptable for drinking purpose, but may not be desirable from geopolymerisation, it was decided to use

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distilled water (DW) only in the experiments. However, it was noted that DW is very common commodity available commercially.

Since, a large number of combinations of alkali hydroxide and alkali silicate solution are reported in the literature to form AAS, a series of preliminary experiments was conducted on the typical GPC mixes to arrive at the acceptable proportion of SHf:SSS:DW to prepare AAS and the GPS/AAS ratio (i.e., solids/liquid ratio) for achieving desired workability of freshly mixed GPC mix.

Stage 3: Curing Type

By varying the proportion of AAS, it was possible to achieve desired level of setting / hardening rate in GPC mix such that the test specimens could be demandable after 24 hours of casting, without any need for applying heat or other external treatment for accelerating the geopolymerisation reaction. The main aim of the present work was to avoid steam or hot air exposure to the moulds containing GPC mix, before demoulding. However, information available in the published literature on the contribution of each component of AAS to setting was utilized in preparing AAS such that the moulding and demoulding operations remain essential, same as in the case of CCs.

Stage 4: Selection of typical GPC mixes for structural usage

The literature showed that slag can be activated at room temperature with AAS containing SHS and SSS. Hence, GGBS was basically utilized to formulate AAS and GPC mixes. It is widely reported in literature that generally fly ash alone when forms the GSM, it requires high temperature for activation and this was also the experience in the present work. However, the GPC mix made with GGBS (which was setting at room temperatures for the purpose of demoulding next day after casting) was modified by replacing GGBS partially with FA and still the modified GPC mixes (containing both GGBS and FA) enabled demoulding within 24 hours of casting. However, when the GGBS replacement level was high, there was need for changing the proportions of the AAS.

Stage 5: Mechanical And Durability Properties

Selected GPC mixes were evaluated for various mechanical strengths [fc, ft, fb, etc] including stress strain behaviour. This information is needed to decide whether the GPC mixes developed are different from CCs from structural behaviour considerations. Civil engineering structures are usually subjected to durability problems due to exposure to chloride ions, sulphates and acidic environment.

Hence, currently developed GPC mixes were also studied for durability against the above mentioned aggressive conditions.

Stage 6: Studies of Steel Reinforced GPCs

For any structural usage of concrete, steel reinforcement is a necessity. Hence, the GPCs developed were used to prepare typical beam and column specimens. For confirming the satisfactory structural behaviour of GPCs, the bond between the GPC mix and steel reinforcement should be also adequate. Therefore, this aspect is also to be studied in any planned investigation.

Stage 7: Ecological and economic benefits and practical application

Since the GPCs are required to be assessed for ecological benefits so that they can be accepted for structural application by engineers, the Embodied Energy (EE) and the CO2 Emission (ECO2e) per unit volume of GPCs were estimated using the information available in the literature on the EE of ECO2e of the individual ingredients of GPCs.

For practical application the utility of self curing nature of GPCs developed can be used to join precast slab panels and the joined whole slab could be tested for structural behaviour just after 24 hours of casting. Selected GPC mixes can be employed to produce building blocks of various types in actual block making factory.

A Research Plan in Indian Context

In order to achieve the above objectives and ensure desirable properties listed under Para 1.4, in concretes made with the new binder in the form of geopolymers, a comprehensive research work can aim towards:

- Selection of sources of FA and GGBS suitable to produce geopolymeric binders

- Formulation of ‘Alkaline Activator Solution’ (AAS), to activate FA and GGBS

- Development of ‘geopolymer concretes’ (GPCs) using different combinations of FA, GGBS and appropriate AAS, besides the aggregate system consisting of river sand and crushed granite stone aggregate

- Identification of proper curing regime

- Determination of demoulding time

- Evaluation of rates of strength development in concrete mixes

- Selection of GPC mixes suitable for use as structural grade concretes

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- Determination of stress-strain characteristics of GPCs- Investigation of the bond behaviour of GPC with steel

bars.- Evaluation of behaviour of reinforced GPC flexural

specimens (with different percentages of reinforcement) under flexural and shear

- Structural behaviour of steel reinforced GP concrete columns under uni-axial loading

- Durability aspects of concretes by testing for :- Permeability to water - Permeability to chloride ions- Corrosion of embedded rebar- Resistance to sulphate attack - Resistance to sulphuric acid attack - Statistical variability of characteristic strengths of GPCs - Effect of addition of steel fibres to GPCs- Effect of addition of replacement of ‘normal weight

coarse aggregates’ by fly ash based ‘lightweight coarse aggregates’ in GPCs

- Thermal properties of GPCs- Non-destructive evaluation by Ultrasonic Pulse Velocity,

electrical resistivity, etc.- Techno-economic feasibility of GPCs Vis a Vis Portland

Cement Concretes- Eco-friendliness of GPCs- Practical utilities of GPC such as self curing high early

strength jointing material for precast concrete, building/paver blocks

Concluding Remarks

Based on the literature study using the often mentioned qualitative guidelines and the much quantitative information available, it is possible to practically formulate the AAS for different combination of GGBS and class F fly ash to achieve strength levels useful in most common civil engineering application, in the present investigation. It is therefore possible to prepare typical structural members such as beams and columns using the GPCs [capable of being cured under ambient conditions only] for studying the possibility of applicability in existing structural design

guidelines to new composites. Durability related studies and a few practical applications should be carried out to understand comprehensively the utility of newly developed GPC mixes which are eco-friendly because of their lower carbon foot prints, compared to conventional Portland cement based concretes based on the computation for “Embodied Energy” and “Embodied CO2 emission” of the concretes.

The research plan proposed can generate enough data for engineers to start considering GPCs as desirable material of construction.

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

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