This paper by SOURADEEP GUPTA (National University of Singapore) is aimed at discussing properties of geopolymer concrete, how these differ from properties of ordinary Portland cement, durability properties of fly ash based geopolymer concrete and its advantage when used as a construction material as well. Also some focus has been made on relevant issues that need to be solved and research needs to make it a better construction material.



INTRODUCTION:

Concrete is till now the most popular material for construction on earth. To act as binder ordinary Portland cement (OPC) is most widely used with other materials like water and aggregates. The fact is testified in B.Vijaya Rangan?s article “Fly ash based geopolymer concrete” through the lines – “the global use of concrete is second only to water. As the demand for concrete as construction material increases, so also does the demand for Portland cement.” But still extensive research works are being conducted around the globe to search for an alternative binding agent for concrete keeping in mind the effect of cement on environment as well as the economy involved. Facts exported from the paper “Analysis of geopolymer concrete columns” reveals that the amount of carbon dioxide released during the manufacturing process of OPC is of the order of every ton for one ton of OPC produced. Globally the OPC production contributed to around seven per cent of carbon dioxide. Moreover cement is a costly binder material. Geopolymer is such an alternative construction material which can act as a binder replacing cement. It mainly makes use of waste or byproduct substances like fly ash which is cheap and will reduce environmental pollution to a large extent. Fly ash based geopolymer concrete is a new product in which no cement is used and geopolymer paste acts as only binder [1].Fly ash is available in plenty. In 1998 worldwide production of fly ash was estimated to be 390 millions tons of which only 14% was used. Now it is estimated to be around 800 million tons. Recent research works have focused on the properties of fly ash based geopolymer concrete. The results of these studies have shown potential use of geopolymer concrete as a construction material [1]. It possesses good qualities such as high strength, very little drying shrinkage , low creep, durable nature etc to be used as an alternative of OPC.



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Mineral admixture like fly ash or metakaolin when mixed with geopolymer binder can impart several good qualities to the concrete. But we have to remember here that there are some critical differences between the two mainly in respect of their source of derivation and manufacture. Metakaolin is synthesized by dehydroxylation of phase pure Kaolin [8]. Some commercial metakaolin may contain certain impurities like muscovite and titanium oxide but the basic structure of it is that of disrupted phyllosilicate structure containing aluminum and silicon. Properties can be predicted in case of Metakaolin based geopolymers. Particle size is generally less than five micrometer. Dispersion of particles during mixing affects flowability and other properties like degree of reaction. It has been shown that there is little difference in reaction of metakaolin based geopolymer with variation of raw material surface area.

On the contrary fly ash is completely an industrial by product and obtained mainly from burning of coal. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape and range in size from 0.5 µm to 100 µm. They consist mostly of silicon dioxide (SiO2), which is present in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and hazardous; aluminum oxide (Al2O3) and iron oxide (Fe2O3). Fly ashes are generally highly heterogeneous, consisting of a mixture of glassy particles with various identifiable crystalline phases such as quartz, mullite, and various iron oxides. Based upon composition there are two types of fly ashes- Class C and Class F. Class C fly ashes contain cementitious particles and set once they come in contact with water. Generally Class F fly ashes are used as construction material along with geopolymer binder. Since there is inhomogeneity in fly ash it implies that care must be taken while preparing a mix design to utilize it in an optimal manner for construction. In this paper we will mainly focus on fly ash based geopolymer- its durability and scope of applications.

2. PROPERTIES OF FLY ASH- GEOPOLYMER THAT CONTRIBUTES TOWARDS DURABILITY:

Geopolymer is a combined product of aluminum and silicon. It is produced by geochemistry process [2]. Fly ash based geopolymers use fly ash as the material which acts as source of silicon and aluminum for reaction by an alkali to make silicon and aluminum atoms to form geopolymer paste. When fly ash based geopolymer acts as a binder material, fly ash takes part in the reaction to form alumino-silicate binder [1]. It binds components such as aggregate together to form geopolymer concrete. From various works it has been found that in any geopolymer concrete aggregates occupy 75-80% by mass. The schematic formation of geopolymer material has vividly been shown by Davidovits ( 1994), Van Jarsveld et al ( 1997).



Experimental study reveals that water exists as free water inside the structure. So it justifies that crystalline hydrate form will be absent in the material. Compared to OPC, porosity is higher in fly ash based geopolymer concrete owing to the dissolution of original fly ash particles. This is a negative aspect as foreign substance may enter the concrete through pores and harm durability of the material. Presence of gypsum reduces porosity but excess is not advisable since it may cause volume instability.

Positive aspect that arises when we use fly ash based geopolymer concrete is the absence of transition zone which is present in Portland cement and ingress of foreign substances may occur through transition zone. Research by Frantsek Skvara et al states that there happens almost no significant change in geopolymer composition as it happens in case of ordinary Portland cement concrete. Another advantage that fly ash based geopolymer concrete has, to be used as a durable material, is the effect of age on its compressive strength. Strength remains almost constant or unaltered over age of concrete whereas strength of OPC increases with time but there is a little difference between the ultimate strength of the two. The reason is perhaps the fast process of geopolymerisation [3]. Fly ash concrete gains high early strength compared to OPC.

Unlike OPC shrinkage is not observed in case of fly ash based geopolymer concrete. This greatly contributed to durability. According to Frantsek Skvara et al ratio of compressive strength and tensile strength is around 10:5.5 unlike OPC where the ratio is around 10:1. It shows that geopolymer concrete possesses considerable resistance to tensile forces which are responsible for cracking and other issues in concrete structures. Water- geopolymer fly ash ratio is analogous to water-cement ratio. Higher this ratio is, lower is the strength of fly ash based geopolymer concrete. Another factor that contributes towards durability is the splitting tensile strength of fly ash geopolymer concrete. This parameter is quite higher than what it is in case of OPC. It has been shown from morphological study that use of soluble silicates in geopolymer results in a denser interfacial transition zone ( ITZ) between aggregates and geopolymer matrix

as compared to cement matrix [4,5].



FIG 1: Variation of splitting tensile strength with compressive strength [4].

The formation mechanism of geopolymer concrete also imparts strength to the material. The geopolymerization process (alkaline activation of fly ashes in aqueous environment at pH > 12) accompanied by hardening of the material is different from hydration process of inorganic binders (e.g. OPC) [6]. The whole geopolymerization process occurs entirely through solution.The first step starts when fly ash dissolves. The second step is the formation of geopolymer structure from the solution. Calcium atoms present in the Si-O-Al-O skeleton structure and compensating the charge on aluminum atoms play an important role according to the research done by Frantsek Skvara et al. Calcium atoms also help in connecting Ai-O-Al-O chains. The result is the formation of a structure of high strength that results from alkaline activation of fly ashes in presence of Ca containing material [6].



FIG 2: Beginning of Geopolymer phase development on the surface of fly ash particle [6].



FIG 3 : Conceptual model of geopolymerization [ 3]

3. DURABILITY OF FLY ASH GEOPOLYMER CONCRETE :

From basic concept durability is defined as the capability of concrete to resist weathering action, chemical attack and abrasion while maintaining its desired engineering properties. It normally refers to its lifespan of trouble free performance. Durability is not always an absolute property since different concrete requires different degrees of durability depending upon its use. To discuss about durability of fly ash based geopolymer concrete we here discuss in a bit detail about its resistance to sulphate and chloride attack, high temperature resistance and resistance against freezing and thawing action. We also discuss in brief about creep and drying shrinkage and alkali aggregate reaction of fly ash geopolymer concrete.



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Extensive research conducted by B.Vijaya Rangan, Davidovits J., X.J. Song et al has revealed many facts about resistance of geopolymer concrete to sulphate and chloride attack. It has been found that after being exposed to sulphuric acid solution, fly ash based geopolymer concrete remains structurally inert except development of some fine cracks on the surface whereas OPC concrete shows sign of severe damage. This is noteworthy that geopolymer acts as a better binder than cement as the bonding between aggregates and paste is strong enough so that aggregates do not get exposed on the surface as it happens in case of OPC binder. Even the mass change of fly ash based geopolymer concrete is pretty less compared to OPC concrete. Two graphs exported from the work “Durability of fly ash based Geopolymer concrete against sulphuric acid attack” by X.J. Song et al are shown below to illustrate the comparison between fly ash geopolymer concrete and OPC in respect of mass change. When OPC is used in concrete exposed to sulphurc acid solution surface of concrete gets damaged resulting in exposure of aggregates. Therefor resdua compress strength st can not be done. But since fly ash geopolymer concrete shows very little roughness test results have shown that compressive strength of around 40 MPa has been found even after two months of exposure.



FIG: 4 Comparison between fly ash geopolymer and OPC w.r.t mass change in sulphate solution

So it testifies strong bonding and thus improved resistance of fly ash geopolymer to sulphate attack. Inspite of this good quality care should be taken to improve permeability and use of aggregates. Use of latite aggregates must be avoided since gypsum formation and thus cracking due to expansion occurs when it comes in contact with suphuric acid.

Geopolymer concrete also possesses excellent resistance against chloride attack. In normal OPC concrete, chloride ingress causes corrosion of reinforcement and disintegration of concrete accompanied by loss of strength. On the contrary in case of geopolymer concrete increase in compressive strength has been noticed. The increase in compressive strength also indicated strength formation reactions Experimental study has shown that rate of chloride penetration shows a decreasing trend over time and also concentration in concrete is very low. Since there is no loss of strength it implies that there should not be any formation of corrosion product on the surface. Due to crystalline stress that occurs due to formation of crystalline ettringite or Friedel salt , OPC concrete disintegrates but in fly ash based geopolymer concrete there is only one crystalline phase even after sodium chloride or magnesium chloride exposure resulting in absence of any expansive product like ettringite.

3.2 ALKALI- AGGREGATE REACTION:

Research by Stanton in early 1940 s showed that failure of concrete due to alkali aggregate reaction was due to expansion caused by chemical reaction between alkali in cement and silica contained in aggregate. So if we replace cement to some extent by by mineral admixture like fly ash alkali aggregate reaction can be controlled substantially. If we use class F fly ash ( low lime content) in geopolymer concrete only 15-20% cement replacement will give enough protection.

While geopolymer ensures improved bonding by acting as a binder prohibiting disastrous result by alkali aggregate reaction, fly ash acts as a spice to it by controlling alkali silica reaction ( ASR). Also it is recommended that water cement ratio should be low to control alkali aggregate reaction since water helps alkali-silica gel to swell. Use of fly ash geopolymer concrete utilizes low water-cement ratio maintaining desired workability and hence can make concrete more impermeable and less vulnerable to such reaction. One of the important reasons for using fly ash in highway construction is to inhibit the expansion resulting from ASR. It has been found that 1) the alkalis released by the cement preferentially combine with the reactive silica in the fly ash rather than in the aggregate, and 2) the alkalis are tied up in non-expansive calcium-alkali-silica gel. Thus hydroxyl ions remaining in the solution are insufficient to react with the material in the interior of the larger reactive aggregate particles and disruptive osmotic forces are not generated (Halstead 1986; Olek, Tikalsky, and Carrasquillo 1986; Farbiarz and Carrasquillo 1986).

3.3 EXPOSURE TO HIGH TEMPERATURE:

Fly ash based geopolymer concrete can sustain when exposed to exposed to considerably high temperature. While OPC concrete degrades and degenerates at high temperature, it has been found from different study that fly ash geopolymer concrete can maintain its desired compressive strength even at 400 degree centigrade. Strength starts dropping once temperature crosses 400 centigrade and remains almost constant at higher temperatures. It is noteworthy here that geopolymerization process continues even at high temperature and it is the strength of the binder or bonding that prevents the concrete from disintegration. However lowest residual strength has been marked at 600-700 degree Celsius [6]. The below graph shows compressive strength of fly ash geopolymer concrete ( plotted along Y-axis) at different temperatures (plotted along X-axis).



FIG 5: Compressive strength of flyash geopolymer concrete at different temperatures.

3.4 CREEP AND DRYING SHRINKAGE OF FLY ASH BASED GEOPOLYMERS:

Drying shrinkage is the reduction in volume which is primarily caused by loss of water during the drying process. Factors that affect drying shrinkage of concrete also influences rate of drying shrinkage and intensity. The aggregate plays an important role in affecting shrinkage of concrete. (de Larrard et. al., 1994; Neville, 2000). Davidovits suggested that the smaller drying shrinkage strain of fly ash-based geopolymer concrete may be explained by the block- polymerization? concept. According to this concept, the silicon and aluminum atoms in the fly ash are not entirely dissolved by the alkaline liquid. The polymerization that takes place only on the surface of the atoms is sufficient to form the blocks necessary to produce the geopolymer binder. Therefore, the insides of the atoms are not destroyed and remain stable, so that they can act as micro-aggregates? in the system and this could increase the aggregate content in concrete. As for OPC concrete, aggregate content will influence the magnitude of shrinkage as the shrinkage of concrete will decrease with the increase in the quantity of aggregates. The proportion of aggregates in the mixtures of fly ash-based geopolymer concrete used in this work is approximately similar to that used in OPC concrete. However, the presence of the “micro-aggregates” due to the “block-polymerization” gives the effect of increasing the aggregate content in the concrete. In other words, the presence of the “micro- aggregates” increases the restraining effect of the aggregates on drying shrinkage. Drying shrinkage is even less in fly ash geopolymer concrete that are heat cured. In heat-cured fly ash- based geopolymer concrete, most of the water released during the chemical reaction evaporates during the curing process (Davidovits, 1999; Hardjito & Rangan,2005). Because the remaining water contained in the micro-pores of the hardened concrete is small, the induced drying shrinkage is also very low. As stated by Djwantoro Hardjito et al creep of fly ash geopolymer concrete is also very low. Creep strain remains constant at a very low value upto 6-7 weeks and then decreases to a small extent. Creep factor increases marginally after 6 weeks.



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Non air entrained concrete with low water to binder ratio is durable against freezing and thawing. So it implies that if water-binder ratio is low all water can mix up with binder and other components resulting in low permeability of concrete. It can so hinder the saturation of paste during freezing. If saturation of paste does not take place concrete won’t crack even in absence of any air entraining admixture. Fly ash plays the important role by lowering water-cement ratio and reducing concrete permeability. Experiments conducted by Frantisek Skvara et al has stated that mass od fly ash geopolymer concrete slightly changes after 150 freezing and defrosting cycles and also there was no deformation and cracking [4]. However concrete with upto 50 % fly ash and 10% silicafume with geopolymer binder has proved most efficient. This hints at the disadvantage of having too much fly ash particles in the mixture. It means that excess or unreacted fly ash will not bind well with geopolymer matrix and will weaken the bonding only.

4. SCOPE OF APPLICATION OF FLY ASH BASED GEOPOLYMER CONCRETE:

Fly ash based geopolymer concrete possesses the improved qualities to be used widely for any construction purpose. Geopolymer mixed concrete develops a glossy surface that can give a good appearance if used in constructing floors and walls. Act ivation by alkali gives rise to material with varied properties from that of OPC and make fly ash based geopolymer concrete more fire resistant and resistant against abrasion and cracking. Since fly ash is only a byproduct material found from industrial wastes cost of such geopolymer concrete is less than or at most equal to OPC concrete which uses expensive cement as binder material. According to Djwantoro Hardjito et al “ If one considers the impact of the possible carbon di oxide tax on the price of cement and environmental advantage of utilization of fly ash , the geopolymer concrete may prove to be economically advantageous”. It has been stated earlier that ratio of compressive and tensile strength of fly ash based geopolymer concrete is 10:5.5 unlike OPC where the ratio is 10:1. It means that brittleness of concrete is reduced to a considerable extent and can withstand more tensile stress without much reinforcing steel. Use of fly ash geopolymer also helps in rapid development of strength and so it can be used in underwater structures where early strength and rapid setting is necessary. Geopolymer concrete based on fly ash gain a very high compressive strength in the first few hours of alkali activation (60-70 MPa after 24 hours).Its resistance against sulphate and chloride attack makes it suitable to be used for construction in abrasive soils where ground water contains considerable amount of chloride and sulphate salts. Also fly ash based geopolymer concrete possesses improved rheological properties. Both static and dynamic viscosity is high when fly ash is used. So it also helps in easy transportation and handling of geopolymer concrete with the use of low water-cement ratio and less vibration. But it should also be remembered at the same time that fly ash is an industrial waste and mostly inhomogeneous. The inhomogeneity means that proper care should be taken in respect of quantity of mixing and methods while working with fly ash geopolymer concrete.

5.RESEARCH NEEDS AND OTHER ISSUES:

Concentrating on a number of research papers and books related to geopolymer technology several facts still remain a bit obscure. Much research work has been done over past two or more decades yet much work remains to be done. For example we need to know what will be the effects of impurities on geopolymer reactions and how will it affect the strength of concrete.

Since fly ash is just an industrial waste many types of chemical substances might get mixed with it and may cause significant changes in reactions. For example calcium reacts very fast with silicon and aluminum to form hydrates and it gets stronger in presence of soluble substances produced by dissolution during geopolymerisation. So it?s important to know effect of such similar chemical substances. No data is available regarding the mechanism of reaction, setting and hardening of fly ash geopolymer concrete. That also remains an unsolved issue. Though Fly ash geopolymer concrete is resistant against sulphate and chloride attack ordinary steel reinforcement is not expected to work effectively in low pH environment. So researchers do need to think about stability of fly ash based binder as well as its capacity to safeguard steel reinforcement. This will allow to enhance the design of fly ash geopolymer concrete for controlled cracking and shrinkage cuts. Improving permeability of geopolymer concrete is also a big issue. Concrete based on fly ash geopolymer is a porous material. Closed spherical pores have been identified by scientists through XRD analysis. Mixing slag reduces porosity to a small extent but alternative effective and economic ways must be thought of in this context. It is hoped that concentrating over such more issues and needs and more research in this field will help to emerge fly ash based geopolymer concrete as a commercial and environment-friendly material and solve some material selection problem faced by construction industry.

BIBLIOGRAPHY:

[1]Sarker, P. K. (2009). Analysis of Geopolymer concrete column. Materials and structures 42:715-724 .

[2] J, D. (1994). High Alkali cement for 21 st century Concrete. Concrete Technology past present and future ACI special publication SP 144. Farmington Hills,Michigan,pp-383-398 .

[3]Djwantoro Hardjito, Steenie E. Wallah, Dody M.J. Sumajaow and B.vijaya Rangan. “On the development of flyash based geopolymer concrete.” ACI Material Journal.

[4]Frantisek Skvara, Josef Dolezal, Pavel SVoboda. “geopolymers, Concrete Based on Flyash.”

[5]Lee WKW, Van Deventer. “The interface between natural seliceous aggregates and geopolymers.” Cement Concr Res 34:195-206.

[6]Sarker, Prabir Kumar. “Bond Strength of reinforcing steel embedded in flyash based geopolymer concrete.” Materials and structures DOI: 10.1617.

[7]X.J. Song, M. Marosszeky, M.Brungs, R.Munn. “Durability of fly ash based geopolymer concrete against sulphuric acid attack.” 10DBMC International Conference on Durability of Building Materials and Components. LYON, France, 17-20 April, 2005.

[8] P.Duxson, G. L.-J. (19 December 2006). Geopolymer Technology: the current state of art. Advances in Geopolymer Science and Technology.

[9] K.W. Nasser, Ghosh S, “Durability properties of High strength concrete containing Silica fume and Lignite Fly ash”.

We are thankful to Sir Souradeep Gupta, National University of Singapore for publishing his research work here on engineeringcivil.com. I am sure, this research paper will help many civil engineers around the world in understanding the concept of Geopolymer Concrete.