Scholarly article on topic 'Parametric Studies on Compressive Strength of Geopolymer Concrete'

Parametric Studies on Compressive Strength of Geopolymer Concrete Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Prakash R. Vora, Urmil V. Dave

Abstract The demand of concrete is increasing day by day for satisfying the need of development of infrastructure facilities. It is well established fact that the production of OPC not only consumes significant amount of natural resources and energy but also releases substantial quantity of carbon dioxide to the atmosphere. Therefore, it is essential to find alternatives to make the concrete environment friendly. Geopolymer is an inorganic alumino-silicate compound, synthesized from fly ash. The fly ash, one of the source materials for geopolymer binders, is available abundantly in India, but to date its utilization is limited. Hence it is essential to make the efforts to utilize this by-product in concrete manufacturing in order to make the concrete more environmental friendly. This paper describes the experimental work conducted by casting 20 geopolymer concrete mixes to evaluate the effect of various parameters affecting its the compressive strength in order to enhance its overall performance. Various parameters i.e. ratio of alkaline liquid to fly ash, concentration of sodium hydroxide, ratio of sodium silicate to sodium hydroxide, curing time, curing temperature, dosage of superplasticiser, rest period and additional water content in the mix have been investigated. The test results show that compressive strength increases with increase in the curing time, curing temperature, rest period, concentration of sodium hydroxide solution and decreases with increase in the ratio of water to geopolymer solids by mass & admixture dosage, respectively. The addition of naphthalene based superplasticiser improves the workability of fresh geopolymer concrete. It was further observed that the water content in the geopolymer concrete mix plays significant role in achieving the desired compressive strength.

Academic research paper on topic "Parametric Studies on Compressive Strength of Geopolymer Concrete"

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SciVerse ScíenceDirect Procedía

Engineering

Procedía Engineering 51 (2013) 210 - 219 -

www.elsevier.com/locate/procedia

Chemical, Civil and Mechanical Engineering Tracks of 3rd Nirma University International conference on

Engineering (NUiCONE-2012)

Parametric Studies on Compressive Strength of Geopolymer Concrete

Prakash R. Voraa , Urmil V. Daveb*

aDesign Engineer, Bhagwati Associates Private Ltd., Mumbai, India bProfessor, Department of Civil Engineering, Institute of Technology, Nirma University, Ahmedabad 382481, India

Abstract

The demand of concrete is increasing day by day for satisfying the need of development of infrastructure facilities. It is well established fact that the production of OPC not only consumes significant amount of natural resources and energy but also releases substantial quantity of carbon dioxide to the atmosphere. Therefore, it is essential to find alternatives to make the concrete environment friendly. Geopolymer is an inorganic alumino-silicate compound, synthesized from fly ash. The fly ash, one of the source materials for geopolymer binders, is available abundantly in India, but to date its utilization is limited. Hence it is essential to make the efforts to utilize this by-product in concrete manufacturing in order to make the concrete more environmental friendly. This paper describes the experimental work conducted by casting 20 geopolymer concrete mixes to evaluate the effect of various parameters affecting its the compressive strength in order to enhance its overall performance. Various parameters i.e. ratio of alkaline liquid to fly ash, concentration of sodium hydroxide, ratio of sodium silicate to sodium hydroxide, curing time, curing temperature, dosage of superplasticiser, rest period and additional water content in the mix have been investigated. The test results show that compressive strength increases with increase in the curing time, curing temperature, rest period, concentration of sodium hydroxide solution and decreases with increase in the ratio of water to geopolymer solids by mass & admixture dosage, respectively. The addition of naphthalene based superplasticiser improves the workability of fresh geopolymer concrete. It was further observed that the water content in the geopolymer concrete mix plays significant role in achieving the desired compressive strength.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of Institute of Technology, Nirma University, Ahmedabad. Keywords: Geopolymer, Concrete, Fly ash, Compressive strength, Parametric study

1. Introduction

Concrete is one of the widely used materials all over the world. Ordinary Portland cement (OPC) is used as the primary binder to produce the concrete. The demand of concrete is increasing day by day for the need of development of infrastructure facilities [1]. However, it is well known that the production of OPC not only consumes significant amount of natural resources and energy but also releases substantial quantity of carbon dioxide to the atmosphere [2]. The global cement industry contributes around 2.8 billion tons of the greenhouse gas emissions annually, or about 7% of the total man-

* Corresponding author. Tel.: +91-2717-241911-15; fax: +91-2717-241917. E-mail address: prakashvora1989@gmail.com., urmil.dave@nirmauni.ac.in

ELSEVIER

1877-7058 © 2013 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of Institute of Technology, Nirma University, Ahmedabad. doi:10.1016/j.proeng.2013.01.030

made greenhouse gas emissions to the earth's atmosphere. It is essential to find alternatives to make environment friendly concrete [1].

One of the alternatives to produce more environmentally friendly concrete is to replace the amount of Portland cement in concrete with by-product materials such as fly ash. An important investigation in this regard is the development of high volume fly ash concrete that utilizes up to 60 percent of fly ash, and yet possesses excellent mechanical properties with enhanced durability performance [3].

Another alternative to make environmentally friendly concrete is the development of inorganic alumina-silicate polymer, called Geopolymer, synthesized from materials of geological origin or by-product materials such as fly ash that is rich in silicon and aluminum [4]. Fly ash, one of the source materials for geopolymer binders, is available abundantly worldwide, but to date its utilization is limited. Currently, 90 million tones of fly ash is being generated annually in India. By exploring use of the fly ash based geopolymer concrete two environment related issues are tackled simultaneously i.e. the high amount of CO2 released to the atmosphere during production of OPC and Utilization of this fly ash.

The production of geopolymer concrete is carried out using the conventional concrete technology methods. The fly ash based geopolymer concrete consists 75% to 80% by mass of aggregate, which is bound by a geopolymer paste formed by the reaction of the silicon and aluminum within the fly ash and the alkaline liquid made up of sodium hydroxide and sodium silicate solution with addition of superplasticiser. In this paper, various factors which influencing the strength of concrete are studied and results are discussed by various graphs corresponding to the strength of the test cubes.

It can be observed from the above discussion that the geopolymer concrete has not been studied much in detail in India. India also is facing the problem of depletion on natural resources such as limestone which is the most important ingredient to produce cement and in turn the concrete in India. In this situation, detailed study of geopolymer concrete which is the concrete with zero cement in concrete naturally becomes very important. Therefore, an attempt has been made in the present investigation by casting 20 geopolymer concrete mixes with 100% replacement of OPC with processed fly ash in each concrete mix [5]. Indian standards have always emphasized on the importance of compressive strength amongst various mechanical properties of concrete. Hence, the effect of various parameters affecting the compressive strength i.e. ratio of alkaline liquid to fly ash, concentration of sodium hydroxide, ratio of sodium silicate to sodium hydroxide, curing time, curing temperature, dosage of superplasticiser, rest period and additional water content in the Geopolymer concrete mixes has been investigated in order to enhance its overall performance.

2. Experimental Programme

2.1. Materials

Fly ash used in this study was low calcium class F processed fly ash from Dirk India private limited under the name of the product POZZOCRETE 60. Chemical compositions of the fly ash used along with the specifications are given in Table 1.

Table 1. Composition of class F fly ash (POZZOCRETE )

SiO2 A2O3 Fe2O3 MgO SO3 Na2O CaO LOI

57.30 % 27.13 % 8.06 % 2.13 % 1.06 % 0.73 % 0.03 % 1.60 %

The alkaline liquid used was a combination of sodium hydroxide and sodium silicate solution. Sodium hydroxide (NaOH) in flakes form with 98% purity purchased from local chemical supplier was used and Sodium silicate solution (Na2O = 16.84%, SiO2 = 35.01% and water = 46.37% by mass) was used as alkaline liquid. Sodium hydroxide solution is prepared by dissolving the flakes in water. Tap water available in laboratory was used to prepare NaOH solution. The activator solution was prepared at least one day prior to its use. Locally available 10 mm and 20 mm crushed aggregates have been used as coarse aggregates. Locally available river sand is used as fine aggregate in the concrete mixes. To improve the workability of the fresh geopolymer concrete, naphthalene sulphonate based superplasticiser was used in all of the geopolymer mixes.

2.2. Casting of concrete mixes

The mixing procedure used for geopolymer concrete is similar to that of conventional OPC concrete. Mixing of all the materials have been done in the laboratory at room temperature. The fly ash and the aggregate were mixed together in concrete pan mixture. The mixing is allowed to continue for about 3 to 4 minutes. The alkaline solution which was prepared

one day before is added with additional water in the mix. The liquid component and superplasticiser was added to the dry material and mixing is continued for another 3 to 4 minutes.

The fresh concrete is cast into the moulds immediately after mixing in two layers for prismatic specimens. For compaction of the concrete specimens, each layer was given 25 to 35 manual strokes using 20 mm rod. Concrete specimens were vibrated using vibration table for another 10 to 15 seconds. After the casting, the concrete specimen was kept at room temperature as per the decided rest period.

The details of mix design for geopolymer concrete Mix-1 to Mix-6 is given in Table 2. The coarse and fine aggregate proportions used in different geopolymer concrete mixes are about 75-77 % percent by mass of the concrete.

Table 2. Ingredients for geopolymer concrete Mix-1 to Mix-6

Ingredient Unit Mix-1 Mix-2 Mix-3 Mix-4A Mix-4B Mix-5A Mix-5B Mix-5C Mix-6A Mix-6B

Fly ash kg/m3 428 444 428 428 428 428 428 428 428 428

C.A kg/m3 1170 1170 1170 1170 1170 1170 1170 1170 1170 1170

F.A kg/m3 630 630 630 630 630 630 630 630 630 630

Na2SiO3 kg/m3 122 111 114 122 122 122 122 122 122 122

NaOH kg/m3 49 44 57 49 49 49 49 49 49 49

Extra water kg/m3 43 43 43 43 43 43 43 43 43 43

Molarity 14 14 14 14 14 14 14 14 14 14

Superplasticiser kg/m3 8.5 9 8.5 8.5 8.5 8.5 13 17 8.5 8.5

Curing temp 0C 75 75 75 75 75 75 75 75 75 75

Curing period Hours 24 24 24 24 48 48 48 48 48 48

Rest period Days 1 1 1 0 0 0 0 0 0 1

No. of cube 3 3 3 3 3 3 3 3 3 3

2.3. Curing of concrete specimen

The concrete specimens were heat cured in oven at required temperature up to the completion of curing time. After the curing period, the test specimens left in the moulds for at least 4-6 hours in order to avoid a major change in the environmental conditions. After demoulding, the concrete specimens are allowed to become air-dry in the laboratory until the day of the testing.

3. Results and discussion

3.1. Ratio of alkaline liquid to fly ash

Two concrete mixes Mix-1 and Mix-2 with the alkaline liquid to fly ash ratio 0.35 and 0.4 have been cast using the ingredients as given in Table 2. The effect of alkaline liquid to fly ash ratio by mass on compressive strength of concrete at age 3 days has been evaluated by comparing results of both mixes. The results are presented in Fig. 1. It has been observed that the ratio of alkaline liquid to fly ash, by mass, is not much effective in varying the compressive strength of the geopolymer concrete.

Mix-1 ( 0.40} Mix-2 (0.35)

Alkaline liquid to fly ash ratio

Fig. 1. Effect of alkaline liquid to fly ash ratio on compressive strength

3.2. Ratio of sodium silicate solution to sodium hydroxide solution

The effect of sodium silicate solution to sodium hydroxide solution by mass on compressive strength of concrete has been observed by comparing results of Mix-1 and Mix-3 having ingredients as given in Table 2. The results are presented in Fig. 2. Higher Compressive strength has been observed in concrete Mix-3 as compared to that of concrete Mix-1.

Fig. 2. Effect of sodium silicate solution to sodium hydroxide solution ratio on compressive strength

3.3. Curing time

In order to investigate the effect of curing time, test specimens have been cured for curing periods of 24 hours and 48 hours, respectively. Concrete Mix-4 with ingredients used as per Table 2 has been cast for investigating of the influence of curing time on the compressive strength.

Fig. 3. Effect of curing time on compressive strength

Fig. 3 shows that Longer curing time has improved the polymerisation process resulting in development of higher compressive strength. Rapid rate of increase in the strength has been observed up to the curing time of 24 hours.

3.4. Additional of superplasticiser

Geopolymer concrete has a stiff consistency in the fresh state. Although adequate compaction was achievable, an improvement in the workability was considered as desirable [6]. Superplasticiser has been added in proportion to the fly ash in the concrete mix by mass as given in Table 2. Tests have been performed to study the effect of adding conventional commercially available Naphthalene Sulphonate based super plasticiser.

The test specimens have been cured for 48 hours at 750C in an oven. The effect of addition of superplasticiser on compressive strength of concrete has been observed by comparing results of Mix-5 as presented in Table 2. Concrete mixes 5 have been cast using 2%, 3% and 4% dosage of admixture, respectively.

% of superplasticiser (by mass of fly ash)

Fig. 4. Effect of addition of superplasticiser on workability

It has been observed that the addition of superplasticiser improved the workability of the fresh concrete as shown in Fig. 4. Increase in dosage of the superplasticiser also resulted in improved workability for the geopolymer concrete.

32 30 23

Mix-5A(2%) Mix-5B{3%) Mix-5C(4%) % of superplasticiser (by mass of fly ash)

Fig. 5. Effect of addition of superplasticer on compressive strength

Concrete mix with 2% dosage of superplasticiser has archived higher compressive strength as compared to the 3% and 4% dosage of the superplasticiser. Thus, the addition of naphthalene based superplasticiser has been able to improve the workability of fresh geopolymer concrete. On the other hand, higher dosage of the admixture up to 4% has resulted into reduction of the compressive strength of the geopolymer concrete.

3.5. Rest period

The term 'Rest Period' is defined by the time taken from the completion of casting of concrete specimens to the start of curing at an elevated temperature. This is very important in context to many practical applications. For instance, when the fly ash based geopolymer concrete is used in precast concrete industry, there must be sufficient time available between casting of products and sending them to the curing chamber [7].

50 45 40 35 30 25 20 15 10 5 0

Mix-6A (0 day) Mix-6B (1 day)

Rest period

Fig. 6. Effect of rest period on compressive strength

For studying the effect of rest period concrete mixes 6 using ingredients as given in Table 2 has been cast. The test results for 0 day and 1 day rest period are presented as shown in Fig. 6. It has been observed that one day rest period has resulted into higher gain in compressive strength as compared to that for 0 day rest period.

Amount of concreting ingredients used for casting the geopolymer concrete Mix-7 to Mix-9 is given in Table 3.

Table 3. Ingredients for geopolymer concrete Mix-7 to Mix-9

Ingredient Unit Mix-7A Mix-7B Mix-7C Mix-8A Mix-8B Mix-8C Mix-8D Mix-9A Mix-9B Mix-9C

Fly ash kg/m3 428 428 428 428 428 428 428 428 428 428

C.A kg/m3 1170 1170 1170 1170 1170 1170 1170 1170 1170 1170

F.A kg/m3 630 630 630 630 630 630 630 630 630 630

Na2SiO3 kg/m3 114 114 114 114 114 114 114 114 114 114

NaOH kg/m3 57 57 57 57 57 57 57 57 57 57

Extra water kg/m3 43 64 86 43 43 43 43 43 43 43

Molarity 14 14 14 8 10 12 14 14 14 14

Superplasticiser kg/m3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3 4.3

Curing temp 0C 75 75 75 75 75 75 75 60 75 90

Curing period Hours 24 24 24 24 24 24 24 24 24 24

Rest period Days 1 1 1 1 1 1 1 1 1 1

No. of cube 3 3 3 3 3 3 3 3 3 3

3.6. Water content of mix

The geopolymer concrete mix does not take any part in the chemical reaction. In ordinary Portland cement (OPC) concrete, water in the mix chemically reacts with the cement to produce a paste that binds the aggregates. The chemical reaction that occurs in geopolymers produces water that is eventually expelled from the binder. It has been observed that additional water content in the geopolymer concrete mix affected the properties of concrete in the fresh state as well as in the hardened state [7].

Water to geopolymer solids ratio by mass is very important in the design of geopolymer concrete mix. The total mass of water is the sum of the mass of water contained in the sodium silicate solution, the mass of water used in the making sodium hydroxide solution, and the mass of extra water, if any, present in the mix. The mass of geopolymer solids is the sum of the mass of fly ash, the mass of solids in the sodium silicate solution i.e. the mass of Na2O and SiO2.

100 50 -0 -25 35 AS 55 65 75 85 95 Added Water lKg/m5(

Fig. 7. Effect of water to geopolymer solids ratio on workability

The effect of water content of mix on compressive strength of geopolymer concrete can be observed by comparing results of Mixes 7. The concrete Mixes 7 have been cast using additional water of 10%, 15% and 20% by mass of fly ash as given in Table 3. Fig. 7 shows that increase in water to geopolymer solids ratio results into increase of the workability of mixes.

Fig. 8. Effect of water to geopolymer solids ratio on compressive strength

Fig. 8 shows the reduction in compressive strength of 33% for concrete Mix-7C as compared to that of concrete Mix-7A. Compressive strength of geopolymer concrete reduces with increase in the ratio of water to geopolymer solids. This test results are similar to the effect of water to cement ratio on the compressive strength of Portland cement concrete.

3.7. Concentration of sodium hydroxide solution

Mixes 8 have been cast using ingredients as shown in Table 3 to study the effect of concentration of sodium hydroxide solution on the compressive strength of geopolymer concrete. The measured compressive strength of test specimens after 7 days is given in Fig. 9.

Mix-SA(SM) Mix-SB (10M) Mix-8C(12M) Mix-8D|14M) Concentration of sodium hydroxide solution

Fig. 9. Effect of concentration of sodium hydroxide solution on compressive strength

The difference between cases of Mixes 8 in terms of variation in compressive strength is the molar concentration of sodium hydroxide solution. Mix-8D with 14 M concentration of sodium hydroxide solution has been able to gain much higher compressive strength as compared to other cases. It has been observed that, higher concentration of sodium hydroxide solution gives better results in terms of compressive strength of geopolymer concrete.

3.8. Curing temperature

The effect of curing temperature on compressive strength of concrete has been observed by comparing results of concrete Mixes 9. Details of the mixes are given in Table 3. Specimens of all mixes have been cured after heat curing the test specimens in an oven for 24 hours. The measured compressive strength of test specimens after 7 days is given in Fig. 10.

Fig. 10. Effect of curing temperature on compressive strength

The curing temperature has resulted in achieving higher compressive strength for geopolymer concrete. Increase in curing temperature beyond 750C has yielded into minor increment in the compressive strength for the concrete.

4. Conclusions

Following concluding remarks have been made on basis of the work conducted:

• The ratio of alkaline liquid to fly ash, by mass does not affect the compressive strength of the geopolymer concrete.

• The sodium silicate to sodium hydroxide ratio by mass equal to 2 has resulted into the higher compressive strength as compared to the ratio of 2.5 for the geopolymer concrete.

• The compressive strength of the geopolymer concrete increases with increase of concentration in terms of molarities of sodium hydroxide.

• The compressive strength of the geopolymer concrete increases with increase in the curing time. However, the increase in strength beyond 24 hours is not much significant.

• Workability of the geopolymer concrete mix increases with the addition of superplasticiser up to 4% of fly ash by mass. Minor reduction of compressive strength of the geopolymer concrete is observed when the superplasticiser dosage used is greater than 2%.

• 1 day rest period increases the compressive strength of the geopolymer concrete as compared to that for the concrete without the rest period.

• Compressive strength of the geopolymer concrete decreases with increase in the ratio of water to geopolymer solids by mass.

• The workability of the geopolymer concrete in fresh state increases with the increase of extra water added to the mix.

• With increase in the curing temperature in the range of 600C to 900C, the compressive strength of the geopolymer concrete also increases.

It has been observed from the above discussion that wide variety of parameters affect the compressive strength of the geopolymer concrete. Therefore, parametric study of various factors affecting the compressive strength of the geopolymer concrete is strongly recommended first before conducting any further investigations related to mechanical properties and durability of the geopolymer concrete in order to get the desirable benefits from the further investigations.

Acknowledgement

Authors are thankful to management and Director, Institute of Technology, Nirma University, Ahmedabad for extending permission for conducting the work and publication for this work.

References

[1] Malhotra V. M., 2000. Introduction: Sustainable Development and Concrete technology, ACI Concrete Journal, pp.1147-1165.

[2] Sumajouw D.M., Hardjito D Wallah S.E., Rangan B.V., 2006. Fly ash based Geopolymer Concrete: Study of Slender Reinforced Columns, Advances in geopolymer science and technology, pp. 3124-3130

[3] Mehta P.K., 2001. Reducing the Environmental Impact of Concrete, Concrete International, pp. 61-66.

[4] Davidovits J., 1998. "Chemistry of Geopolymeric Systems, Terminology, Geopolymer" 99 International Conference, France, pp. 3077-3085

[5] Vora Prakash R., 2012. "Eco-friendly and Sustainable Zero Cement Concrete*, M. Tech. Dissertation, Nirma University, Ahmedabad, India.

[6] Hardjito D., Wallah E., Sumajouw D.M.J, Rangan B.V., 2004. Factors Influencing the Compressive Strength of Fly ash-based Geopolymer Concrete, Civil engineering dimension, pp. 88-93. .

[7] Hardjito D., Rangan В. V., 2005. "Development and Properties of Low Calcium Fly ash based Geopolymer Concrete', Research report GC1, Curtain University, Australia.