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30th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 23-24 August 2005, Singapore

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INTRODUCING FLY ASH-BASED GEOPOLYMER CONCRETE: MANUFACTURE AND ENGINEERING PROPERTIES

D. Hardjito 1, 2, S.E. Wallah 2, D.M.J. Sumajouw 2, B.V. Rangan 2 1 Curtin University of Technology, Miri, Sarawak, Malaysia

2 Curtin University of Technology, Perth, Australia

ABSTRACT: This paper presents the results of a study on fly ash-based geopolymer concrete. The test parameters covered certain aspects of manufacture of geopolymer concrete. The paper also reports the stress-strain behavior of the concrete with compressive strength in the range of 40 to 65 MPa. Tests were carried out on 100mmx200mm cylindrical geopolymer concrete specimens. Test results show that a good agreement exists between the measured stress-strain relations of fly ash-based geopolymer concrete and those predicted by a model developed originally for Portland cement concrete. Keywords: Geopolymer concrete; fly ash; manufacture; properties.

1 INTRODUCTION

The development of fly ash-based geopolymer concrete is in response for the need of a ‘greener’ concrete in order to reduce the carbon dioxide emission from the cement production. Geopolymer concrete is manufactured from predominantly silica and alumina containing source material. It offers a significant opportunity to materialise ‘green’ concrete as it is possible to utilise an industrial by-product such as fly ash, to totally replace the use of ordinary Portland cement in concrete, and hence to reduce the emission of carbon dioxide to the atmosphere.

Several papers on the engineering properties of geopolymer concrete have been published [1, 4-9]. In an earlier paper, the authors [7] have reported some results on the stress-strain behaviour in compression and elastic constants. As these are important material characteristics of concrete, especially in design of structural concrete members, study on this matter need to be carried out.

This paper presents the effect of some parameters on the engineering properties of fly ash-based geopolymer concrete. The measured stress-strain curves of geopolymer concrete in compression are also compared with those calculated using a model available for Portland cement concrete.

2 GEOPOLYMER AND GEOPOLYMER CONCRETE

Geopolymer is an inorganic alumino-silicate polymer synthesized from predominantly silicon (Si) and aluminum (Al) materials of geological origin or by-product materials such as fly ash. The term geopolymer was introduced by Davidovits [3] to represent the mineral polymers resulting from geochemistry. The process involves a chemical reaction under highly alkaline conditions on Si-Al minerals, yielding polymeric Si-O-Al-O bonds in amorphous form. It has been reported that


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geopolymer material does not suffer from alkali-aggregate reaction even in the presence of high alkalinity [3], and possesses excellent fire resistant [1].

In the authors’ experimental work, geopolymer is used as the binder, instead of cement paste, to produce concrete. The geopolymer paste binds the loose coarse aggregates, fine aggregates and other un-reacted materials together to form the geopolymer concrete. The manufacture of geopolymer concrete is carried out using the usual concrete technology methods.

As in the Portland cement concrete, the aggregates occupy the largest volume, i.e. about 75-80 % by mass, in geopolymer concrete. The silicon and the aluminum in the fly ash are activated by a combination of sodium hydroxide and sodium silicate solutions to form the geopolymer paste that binds the aggregates and other un-reacted materials.

3 MANUFACTURE

3.1 Materials

The materials needed to manufacture the fly ash-based geopolymer concrete are the same as those for making Portland cement concrete, except for the Portland cement. Low calcium (class F) dry fly ash obtained from a local power station was used as the source material.

For the alkaline activator, a combination of sodium hydroxide solution and sodium silicate solution was used. The sodium hydroxide solution was prepared by dissolving the sodium hydroxide solids, either in the form of pellets or flakes, in water. Extra water and Naphthalene Sulfonate-based superplasticizer were also added to improve the workability of the fresh fly ash-based geopolymer concrete. The sodium silicate solution used contained Na2O=14.7%, SiO2=29.4%, and 55.9% of water, by mass. All the liquids were mixed together before adding to the solids.

3.2 Mixing and Compacting

The aggregates in saturated surface dry condition and the dry fly ash were mixed in a pan mixer for 3-4 minutes. At the end of this mixing, the liquid component of the geopolymer concrete mixture, i.e. the combination of the alkaline solution, the superplasticiser and the extra water, was added to the solids, and the mixing continued for a specified period of time. In this study, the wet mixing period was designated as the ‘mixing time’. The fresh concrete had a stiff consistency and was glossy in appearance.

The fresh concrete was then cast in moulds. Compaction was performed using the usual practice, either by applying strokes or using vibration or a combination of both. After casting, the concrete samples were cured at an elevated temperature for a specified period of time.

3.3 Curing

In this study, curing was carried out at a specified elevated temperature, either in an oven (dry curing) or in a steam chamber. At the end of the curing period, the test specimens were left in the mold for about six hours. The samples were then removed from the molds, and left to air dry in the room temperature before testing at a specified age.

4 EFFECT OF MANUFACTURING PROCESS

In our previous papers, we have reported some aspects of the manufacturing conditions that affected the properties of the fly ash-based geopolymer concrete. We observed that for specimens cured at 60

oC for 24 hours, the compressive strength did not depend on age, unlike the cement-based

concrete [4-6]. We also reported that geopolymer concrete shows excellent resistance to sulfate attack [11], and undergoes low creep and very little drying shrinkage [12]. The successful application of this material to make reinforced geopolymer concrete columns was also reported [10].

Table 1. Composition of fly ash as determined by XRF (mass %)

*) LOI = Loss on ignition

SiO2 Al2O3 CaO Cr Fe2O3 K2O MgO Na2O P2O5 SO3 TiO2 MnO LOI*)

47.8 24.40 2.42 0.01 17.40 0.55 1.19 0.31 2.00 0.29 1.328 0.12 1.10


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Table 2: Details of Mixtures

Aggre-gates

Fly Ash

Sodium Silicate solution

NaOH solution

Super Plasticiser

***)

Added Water

Mix No

[ kg / m3 ]

Curing

1 1848 408 103 41 (14 M) *) 6 20.7 60

oC (Steam)

2 1848 408 103 41 (14M) *) 6 16.5 90

oC (Steam)

3 1848 408 103 41 (12 M) *) 6 14.3 60

oC (Steam)

4 1848 408 103 41 (14M) *) 6 17.6 60

oC (Steam)

5 1848 408 103 41 (12M) **) 6 14.3 60

oC (Steam)

6 1848 408 103 41 (8M) **) 6 0 60

oC (Oven)

7 1848 408 103 41 (14M) **) 8 0 90

oC (Oven)

8 1848 408 103 41 (8M) **) 6 0 90

oC (Oven)

9 1848 408 103 55.4(8M) **) 6 0 60

oC (Steam)

*) NaOH Commercial Grade (97% purity);

**) NaOH Technical Grade (98% purity) ***) Naphthalene

Sulphonate Superplasticiser The chemical composition of fly ash used in this study is given in Table 1; 80% of the fly ash had

particles of less than 38 µm in size. The geopolymer concrete was manufactured as described earlier and was cast in 100x200 mm cylinder steel moulds in 3 layers. Each layer received 60 manual strokes, and vibrated for ten seconds on a vibrating table. Five cylinders were prepared for each test variable. The compositions of the mixtures are given in Table 2.

Mixtures 1 and 2 were prepared to investigate the effect of mixing time on the engineering properties of fly ash-based geopolymer concrete. The mixing time ranged from two minutes to sixteen minutes. The slump of the fresh concrete was measured immediately after mixing.

In the authors’ work, the term ‘rest period’ is used to indicate the period between the end of casting and the start of curing of specimens at an elevated temperature. Previously, we have reported that a rest period of only 60 minutes before curing in the oven at 60

oC for 24 hours did not show any

variation in compressive strength compared to the specimens with no rest period. To investigate the effect of longer rest periods on the compressive strength, four different mixtures were made i.e. Mixtures No 3 to 6. The mixing time of these Mixtures was four minutes.

After casting, the specimens from Mixtures No 3, 4 and 6 were left at room temperature until the start of curing. On the other hand, the specimens from Mixture No 5 were placed in an oven during the rest period. The oven temperature on the first day of rest period was 32

oC; on other days of the

rest period, the temperature was 40oC. This variation in temperature simulated the ambient variations

during the rest period.

4.1 Effect of Mixing Time

As shown in Fig. 1, the slump value decreased when the mixing time increased. Longer mixing time produced higher compressive strength and higher density (Figs. 2 and 3). This suggests that the extended mixing time resulted in better polymerisation process, and hence enhanced properties of hardened concrete.


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Figure 1: Mixing Time versus Slump

Figure 2: Mixing Time versus Compressive Strength

Figure 3: Mixing Time versus Density

Mixture 1

Mixture 2

0

50

100

150

200

250

0 5 10 15 20

Mixing Time (minutes)

Slump (mm)

Mixture 1

Mixture 2

0

10

20

30

40

50

60

70

0 5 10 15 20


Compressive strength (MPa)

Mixture 2

Mixture 1

2000

2050

2100

2150

2200

2250

2300

2350

2400

2450

0 5 10 15 20


Density (kg/m

3)

Mixture 1 Mixture 2


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0

10

20

30

40

50

60

70

0 0.005 0.01 0.015

Strain

Stress

Figure 4: Rest Period versus Compressive Strength

4.2 Effect of Rest Period

As seen in Fig. 4, the compressive strength of specimens increased with a rest period of one day or more after casting. The extent of strength gain is significant, in the range of 20 to 50 percent compared to the compressive strength of specimens with no rest period. The exact reason for this strength gain during the rest period is unclear. Fundamental research is needed to understand this phenomenon.

5 STRESS-STRAIN RELATION

5.1 Mixture proportions

Mixtures 7, 8, and 9 were made to yield three different compressive strengths ranging from 40 to 65 MPa. The details of the mixtures are given in Table 2. The mixing time of these Mixtures was four minutes. There was no rest period before the start of curing.

5.2 Stress-Strain Curves

Figure 5. Stress-strain Relations of Geopolymer Concrete

In order to obtain the stress-strain curves in compression, tests were performed in a deformation-control testing machine. Figure 5 shows the measured stress-strain relations of fly ash-based

Mixture 7

Mixture 8

Mixture 9

0

10

20

30

40

50

60

70

80

90

0 2 4 6

Rest Period (days)

Compressive Strength (MPa)

Mixture 6

Mixture 5

Mixture 4

Mixture 3


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0

10

20

30

40

50

60

70

0 0.002 0.004 0.006 0.008 0.01

Strain

Stress (MPa)

0

10

20

30

40

50

60

70

0 0.002 0.004 0.006 0.008

Strain

Stress (MPa)

geopolymer concrete for various compressive strengths. The test data show that geopolymer concrete with higher compressive strength tends to have higher modulus of elasticity.

In order to ascertain the fact that the stress-strain relation of geopolymer concrete is similar to that of Portland cement concrete, an analytical model proposed by Collins et al [2] was used. The model, which applies to both normal and high strength Portland cement concrete is given by :

(1)

where:

fcm = peak stress

εcm = strain at peak stress n = 0.8 + (fcm/17)

k = 0.67 + (fcm/62) when εc/εcm>1

= 1.0 when εc/εcm≤1

Figure 6: Predicted and Test Stress-Strain Relations for Concrete made from Mixture 7


nk

cmccm

c

cmcn

nf

)(1 εεε

εσ

+−=

Measured

Equation 1

Measured

Equation 1


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0

5

10

15

20

25

30

35

40

45

0 0.005 0.01 0.015

Strain

Stress (MPa)


In Figures 6 to 8, the stress-strain relations predicted by Equation 1 are compared with the test

curves. The analytical curves were calculated by using the measured values of fcm and εcm in Equation 1. This comparison reveals that the stress-strain relations of geopolymer concrete can be predicted by using Equation 1 developed for Portland cement concretes. Earlier, the authors’ [7] have reported that the modulus of elasticity, the Poisson’s ratio, and the tensile strength of fly ash-based geopolymer concrete are similar to those of Portland cement concrete.

6 CONCLUSIONS

The paper reported the results of a study carried out on fly ash-based geopolymer concrete. The following conclusions are drawn from this study:

a. Longer mixing time yielded lower slump of fresh concrete, and higher compressive strength and higher density of hardened concrete (Figures 1 to 3). This suggests that the extended mixing time resulted in better polymerisation process, and hence enhanced properties of hardened concrete.

b. The term ‘rest period’ is used to indicate the time taken from the end of casting to start of curing at an elevated temperature. The compressive strength of specimens increased with a rest period of one day or more after casting. The extent of strength gain is significant, in the range of 20 to 50 percent compared to the compressive strength of specimens with no rest period (Figure 4).

c. The measured stress-strain relations of fly ash-based geopolymer concrete, both the ascending and the descending parts, agree well with the predictions of Equation 1 developed originally for Portland cement concrete (Figures 6 to 8).

d. Because the modulus of elasticity, the Poisson’s ratio, the tensile strength, and the stress-strain relations of geopolymer concrete are similar to those of Portland cement concrete, the provisions of current codes and standards for concrete structures can be used to design fly ash-based geopolymer concrete structures. The authors’ [10] earlier work on reinforced geopolymer concrete columns also supports this conclusion.

7 ACKNOWLEDGMENTS

The first and second authors are recipient of the Australian Development Scholarship. The third author is supported by the TPSDP Asian Development Bank. The authors express their gratitude to Professor Stephen Foster of the University of New South Wales for his help in obtaining the test data reported in Figures 6 to 8.

Measured

Equation 1


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8 REFERENCES

1. Cheng, T. W. & J. P. Chiu (2003). "Fire-resistant Geopolymer Produced by Granulated Blast Furnace Slag." Minerals Engineering 16(3): 205-210.

2. Collins, M. P., D. Mitchell & J. G. MacGregor (1993). "Structural Design Considerations for High Strength Concrete." ACI Concrete International 15(5): 27-34.

3. Davidovits, J. (1999). Chemistry of Geopolymeric Systems, Terminology. Geopolymer '99 International Conference, France: 9-40.

4. Hardjito, D., S. E. Wallah, D. M. J. Sumajouw & B. V. Rangan (2003). Geopolymer Concrete: Turn Waste Into Environmentally Friendly Concrete. International Conference on Recent Trends in Concrete Technology and Structures (INCONTEST), I, Coimbatore, India, KCT: 129-140.

5. Hardjito, D., S. E. Wallah, D. M. J. Sumajouw & B. V. Rangan (2004). "On The Development of Fly Ash-Based Geopolymer Concrete." ACI Materials Journal, Vol. 101, No. 6, pp 467-472.

6. Hardjito, D., S. E. Wallah, D. M. J. Sumajouw & B. V. Rangan (2004). Properties of Geopolymer Concrete with Fly Ash as Source Material: Effect of Mixture Composition. Seventh CANMET/ACI International Conference on Recent Advances in Concrete Technology, ACI SP 222-8, Las Vegas, USA: 109-118.

7. Hardjito, D., S.E. Wallah, D.M.J. Sumajouw & B.V. Rangan (2004). The Stress-Strain Behaviour of Fly Ash-Based Geopolymer Concrete. Development in Mechanics of Structures & Materials, A.A. Balkema Publishers, pp. 831-834.

8. Hardjito, D., S.E. Wallah, D.M.J. Sumajouw & B.V. Rangan (2005). Fly Ash-Based Geopolymer Concrete. Australian Journal of Structural Engineering. Vol. 6, No. 1, pp. 77-85.

9. Palomo, A., A. Fernandez-Jimenez, C. Lopez-Hombrados & J. L. Lleyda (2004). Precast Elements Made of Alkali-Activated Fly Ash Concrete. Eighth CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, Supplementary Papers, Las Vegas, USA: 545-558.

10. Sumajouw, D. M. J., D. Hardjito, S. E. Wallah & B. V. Rangan (2004). Behaviour and Strength of Geopolymer Concrete Column. ACMSM 18, Perth, Australia.

11. Wallah, S. E., D. Hardjito, D. M. J. Sumajouw & B. V. Rangan (2003). Sulfate Resistance of Fly Ash-Based Geopolymer Concrete. The 21st Biennial Conference of The Concrete Institute of Australia, Brisbane, Queensland, Australia.

12. Wallah, S. E., D. Hardjito, D. M. J. Sumajouw & B. V. Rangan (2004). Creep Behaviour of Fly Ash-Based Geopolymer Concrete. Seventh CANMET/ACI International Conference on Recent Advances in Concrete Technology, Supplementary Papers, Las Vegas, USA: 49-60.

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