CHAPTER 3 FLY ASH BASED GEOPOLYMER …shodhganga.inflibnet.ac.in/bitstream/10603/15807/8/08...29 CHAPTER 3 FLY ASH BASED GEOPOLYMER CONCRETE 3.1 GENERAL The fresh and hardened properties

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CHAPTER 3

FLY ASH BASED GEOPOLYMER CONCRETE

3.1 GENERAL

The fresh and hardened properties such as workability, density,

compressive strength, split tensile strength and flexural strength of fly ash

based Geopolymer Concrete (GPC) is presented in this chapter. This chapter

describes the mix design and manufacturing process of geopolymer concrete.

Fly ash collected from two different sources was used in making of the

geopolymer concrete. The effect of concentration of alkaline liquids on the

strength of geopolymer concrete is presented. The effect of curing conditions

on the mechanical properties of geopolymer concrete is also discussed in this

chapter.

3.2 EXPERIMENTAL PROGRAMME

3.2.1 Parameters of Study

The following parameters were considered in this experimental

investigation:

(a) Source of fly ash: Fly ash collected from Mettur and Tuticorin

thermal power stations of TamilNadu, India

(b) Concentration of sodium hydroxide used for preparation of

geopolymer concrete: 8 Molarity (8 M), 12 Molarity (12 M)

and 16 Molarity (16 M)

30

(c) Curing temperature: Ambient curing at room temperature and

heat curing at 60o C for 24 hours in hot air oven

(d) Age of concrete at the time of testing: 7 days and 28 days

3.2.2 Materials Used

Fly ash : Class F dry fly ash conforming to IS 3812-2003 obtained

from Mettur and Tuticorin thermal power stations of Tamilnadu from

southern part of India was made use of in the casting of the specimens.

Table 3.1 gives the chemical composition of fly ashes used in this

experimental investigation

Table 3.1 Chemical composition of fly ash

Oxides Mettur Fly ash

Tuticorin Fly ash

Requirements as per IS 3812-2003

SiO2 59.93% 63.24%SiO2 >35%

Total - >70% Al2O3 19.66% 17.35%Fe2O3 2.82% 2.63%CaO 3.33% 2.05% -Na2O 0.34% 0.24%

<1.5%K2O 0.22% 0.32%MgO 1.12% 0.96% <5% LOI 1.56% 0.95% <12%

Fine aggregate: Locally available river sand having a fineness

modulus of 2.75, specific gravity of 2.81 and conforming to grading zone-III

as per Indian Standards IS: 383 - 1970 was used. Bulk density of the fine

aggregate is 1693 kg/m3. Details of sieve analysis of sand are given in

Appendix1.

31

Coarse aggregate: Crushed granite coarse aggregates of maximum

size 19 mm , fineness modulus of 6.64 and a specific gravity of 2.73 were

used. Bulk density of the coarse aggregate is 1527 kg/m3. Details of sieve

analysis of coarse aggregate are given in Appendix 1.

Sodium hydroxide: Sodium hydroxide solids in the form of flakes

as shown in Figure 3.1, with 97% purity manufactured by Merck Specialties

Private Limited, Mumbai was used in the preparation of alkaline activator.

Figure 3.1 Sodium hydroxide flakes

Sodium silicate: Sodium silicate in the form of solution as shown

in Figure 3.2, supplied by Salfa Industries, Madurai was used in the

preparation of alkaline activator. The chemical composition of Sodium

silicate solution supplied by the manufacturer is as follows: 14.7% of Na2O,

29.4% of SiO2 and 55.9% of water by mass.

32

Figure 3.2 Sodium silicate solution

Super plasticizer: To achieve workability of fresh geopolymer

Concrete, sulphonated napthalene polymer based super plasticizer Conplast

SP 430 was used in all the mixtures. Conplast SP 430 is available in the form

of a brown liquid that is instantly dispersible in water and is manufactured by

Fosroc Chemicals (India) private limited, Bangalore.

Water: Distilled water was used for the preparation of sodium

hydroxide solution and for extra water added to achieve workability.

3.2.3 Preparation of Alkaline Activator Solution

A combination of sodium hydroxide solution and sodium silicate

solution was used as alkaline activators for geopolymerisation. To prepare

sodium hydroxide solution of 8 molarity (8 M), 320 g (8 x 40 i.e, molarity x

molecular weight) of sodium hydroxide flakes were dissolved in distilled

water and made up to one litre. The mass of NaOH solid mass in a solution

will vary depending on the concentration of the solution expressed in terms of

molarity, M. The mass of solid NaOH was measured as 255 g/kg in the 8 M

NaOH solution, 354.45 g/kg in the 12 M NaOH solution and 444.6 g/kg in the

16 M NaOH solution .This shows that water was the major component in the

33

sodium hydroxide solution and NaOH solids was only a fraction of the mass

of NaOH solution.

3.2.4 Mix Design of Geopolymer Concrete

In the design of geopolymer concrete mix, coarse and fine

aggregates together were taken as 77% of entire mixture by mass. This value

is similar to that used in OPC concrete in which it will be in the range of 75%

to 80% of the entire mixture by mass. Fine aggregate was taken as 30% by

mass of the total aggregates. From the past literatures it is clear that the

average density of fly ash-based geopolymer concrete is similar to that of

OPC concrete (2400 kg/m3). Knowing the density of concrete, the combined

mass of alkaline liquid and fly ash can be derived. By assuming the ratio of

alkaline liquid to fly ash as 0.4, mass of fly ash and mass of alkaline liquid

was found out. To obtain mass of sodium hydroxide and sodium silicate

solutions, the ratio of sodium silicate solution to sodium hydroxide solution

was kept as 2.5. Extra water (other than the water used for the preparation of

alkaline solutions) and super plasticizer Conplast SP430 based on sulphonated

napthalene polymers were added to the mix in a proportion of 10% and 3% by

mass of fly ash respectively to achieve workable concrete. The mix design

calculations are given in Appendix 2. The mix proportion is given in

Table 3.2.

Table 3.2 Details of mix proportion of geopolymer concrete

Flyash

kg/m3

Fine aggregate

kg/m3

Coarse aggregate

kg/m3

NaOHsolution kg/m3

Na2SiO3

solution kg/m3

Extra waterkg/m3

SPkg/m3

394.3 554.4 1293.4 45.1 112.6 39.43 11.83

34

3.2.5 Preparation of Geopolymer Concrete Specimens

The prepared solution of sodium hydroxide was mixed with sodium

silicate solution one day before mixing the concrete to get the desired

alkalinity in the alkaline activator solution. Initially fine aggregates, fly ash

and coarse aggregates were dry mixed in a horizontal pan mixer for three

minutes. After dry mixing, alkaline activator solution was added to the dry

mix and wet mixing was done for 4 minutes. Finally extra water along with

super plasticizer was added to get workable geopolymer concrete.

Totally 72 cubes (150 mm x 150 mm x 150 mm) for compressive

strength, 72 cylinders (150 mm diameter and 300 mm height) for split tensile

strength and 36 prisms (100 mm x 100 mm x 500 mm) for flexural strength

were cast. Standard cast iron moulds were used for casting the test specimens.

Before casting, machine oil was smeared on the inner surfaces of moulds.

Geopolymer concrete was mixed using a horizontal pan mixer machine and

was poured into the moulds in layers. Each layer of concrete was compacted

using a table vibrator.

3.2.6 Curing of Geopolymer Concrete Specimens

After casting the specimens, they were kept in moulds for a rest

period of four days and then they were demoulded, since the geopolymer

concrete did not harden immediately at room temperature as in conventional

concrete. The term rest period indicates the time taken from the completion of

casting of test specimens to the start of curing at an elevated temperature.

Geopolymer concrete specimens took a minimum of 3 days for complete

setting without leaving a nail impression on the hardened surface. All the

specimens were given an uniform rest period of four days and at the end of

the rest period, thirty six cubes, thirty six cylinders and eighteen prisms were

kept under ambient conditions for curing at room temperature. Remaining

35

thirty six cubes, thirty six cylinders and eighteen prisms were heat cured at

60oC in hot air oven for 24 hours as shown in Figure 3.3.

Figure 3.3 Specimens under heat curing

3.2.7 Designation of Specimens

Specimens have been given descriptive names, composed of four

terms. Each of these terms gives information about some aspect of the

specimens which is described as follows: The first term describes the source

of fly ash used for casting the specimens. ‘Fm’ refers to fly ash collected from

Mettur thermal power station and ‘Ft’ refers to fly ash collected from

Tuticorin thermal power station. The second term which has a number in the

suffix refers to the molarity of sodium hydroxide solution used for the

preparation of alkaline activators. ‘M8’ refers to 8 M NaOH solution, ‘M12’

refers to 12 M NaOH solution and ‘M16’ 16 M NaOH solution. The third term

refers to the curing condition of the specimen. ‘Ca’ refers to the specimens

that were cured at ambient conditions at room temperature and ‘Ch’ refers to

the specimens that were cured at 60o C in hot air oven. The fourth term refers

to the age of concrete at the time of testing. ‘A7’ refers to tests conducted at

36

7 days age of concrete and ‘A28’ refers to tests conducted at 28 days age of

concrete.

3.2.8 Instrumentation and Testing Procedure

All the freshly prepared geopolymer concrete mixes were tested for

workability by using the standard slump cone apparatus. The slump cone was

filled with freshly mixed geopolymer concrete and was compacted with a

tamping bar in four layers. The top of the slump cone was leveled off, then

the cone was lifted vertically up and the slump of the sample was immediately

measured. The compressive and flexural strengths were evaluated as per the

test procedure given in Indian Standards IS.516.

For the evaluation of compressive strength, all the cube specimens

were subjected to compressive load in a digital compression testing machine

with a loading capacity of 2000 kN. Before subjected to the test, weight of

each specimen was recorded and density of each specimen was calculated by

dividing the weight of the specimen by its volume. Specimens were placed in

the machine in such a manner that the load shall be applied to opposite sides

of the cubes as cast, that is, not to the top and bottom. Test set up is shown in

Figure 3.4.The load was applied without shock and increased continuously at

a rate of approximately 14 N/mm2/min until the resistance of the specimen to

the increasing load breaks down and no greater load can be sustained. The

maximum load applied to the specimen was recorded. The compressive

strength of the specimen was calculated using Equation (3.1)

APfc (3.1)

where fc is the compressive strength, P is the maximum load applied to the

specimen and A is the cross-sectional area of the specimen.

37

Figure 3.4 Test set-up for compressive strength

Split tensile strength was evaluated as per the test procedure given

in Indian Standards IS.5816. In order to evaluate the splitting tensile strength

of geopolymer concrete, all the cylinder specimens were subjected to split

tensile strength test in a 2000 kN digital compression testing machine.

Specimens were placed in the machine in a horizontal manner in between the

two parallel steel strips one at top and another at the bottom such that the load

shall be applied along the 300 mm length as shown in Figure 3.5. The load

was applied without shock and increased continuously at a nominal rate

within the range of 1.2 N/(mm2/min) to 2.4 N/(mm2/min) until the specimen

failed. The maximum load applied to the specimen was recorded and the split

tensile strength of the specimen was calculated using Equation (3.2)

DLPft

2(3.2)

where ft is the split tensile strength, P is the maximum load applied to the

specimen , D is the diameter of the specimen and L is the length of the

specimen.

38

Figure 3.5 Test set-up for split tensile strength

Flexural strength of geopolymer concrete was determined using

prism specimens by subjecting them to two point bending in Universal

Testing Machine having a capacity of 1000 kN. Specimens were placed in the

machine in such a manner that the load shall be applied to the uppermost

surface as cast in the mould along two lines spaced at 13.3 cm apart as shown

in Figure 3.6. The load was applied without shock and increased continuously

at a rate of 1800 N/min until the specimen failed. The maximum load applied

to the specimen was recorded and the flexural strength of the specimen was

calculated using Equation (3.3)

2bdPlfr (3.3)

where fr is the flexural strength, P is the maximum load applied to the

specimen , l is the supported length of the specimen , b is the width of the

specimen and d is the depth of the specimen.

39

Figure 3.6 Test set-up for flexural strength

3.3 RESULTS AND DISCUSSION

3.3.1 Workability

Workability of freshly prepared geopolymer concrete mixes was

measured in terms of its slump using the conventional slump cone apparatus.

All the mixtures were generally cohesive and shiny in appearance due to the

presence of sodium silicate. Even though the measured slump values are more

than 150mm, all the mixtures were generally stiff and the workability was

poor. Geopolymer concrete prepared by using fly ash from Tuticorin thermal

power station has better workability than the geopolymer concrete prepared

from Mettur fly ash. Workability of geopolymer concrete decreases as the

concentration of NaOH in the alkaline activator solution increases irrespective

of the source of fly ash as shown in Figure 3.7. This may be due to the reason

that increasing the concentration of NaOH increases the total solid content in

the mixture thereby reducing the water content.

40

190

182

173

202

194

180

150

160

170

180

190

200

210

8M 12M 16M

MFATFA

Figure 3.7 Effect of concentration of NaOH on workability

3.3.2 Density

Density of geopolymer concrete for all the mixes is given in

Table 3.3. Average density values of geopolymer concrete range from 2337 to

2405 kg/m3 and 2316 kg/m3 to 2397 kg/m3 for Mettur fly ash and Tuticorin fly

ash, respectively as shown in Figure 3.8. Variation of density is not much

significant with respect to the source of fly ash, the concentration of NaOH

solution, the type of curing and the age of concrete. The density of

geopolymer concrete was found approximately equivalent to that of

conventional concrete.

41

Table 3.3 Density of geopolymer concrete

Spec. Avg. Weight in kg

Avg. Density kg/m3

Fm M8 Ca A7 8.075 2392.59Fm M8 Ch A7 7.980 2364.44Fm M8 Ca A28 8.052 2385.68Fm M8 Ch A28 7.888 2337.28Fm M12 Ca A7 7.922 2347.16Fm M12 Ch A7 8.022 2376.79Fm M12 Ca A28 8.047 2384.20Fm M12 Ch A28 8.027 2378.27Fm M16 Ca A7 8.118 2405.43Fm M16 Ch A7 8.045 2383.70Fm M16 Ca A28 7.943 2353.58Fm M16 Ch A28 8.050 2385.19

Ft M8 Ca A7 8.047 2384.20Ft M8 Ch A7 8.090 2397.04Ft M8 Ca A28 7.978 2363.95Ft M8 Ch A28 7.968 2360.99Ft M12 Ca A7 7.975 2362.96Ft M12 Ch A7 8.050 2385.19Ft M12 Ca A28 7.883 2335.80Ft M12 Ch A28 7.975 2362.96Ft M16 Ca A7 7.952 2356.05Ft M16 Ch A7 8.080 2394.07Ft M16 Ca A28 7.815 2315.56Ft M16 Ch A28 8.068 2390.62

42

2280

2320

2360

2400

2440

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Specimen number

MFA, AC MFA, HC TFA, AC TFA, HC

Figure 3.8 Ranges of density of geopolymer concrete

3.3.3 Compressive Strength

The effect of various factors such as the source of fly ash, the

concentration of NaOH solution in terms of molarity, the curing temperature

namely room temperature curing and heat curing at 60oC and the age of

concrete at the time of testing, on the compressive strength of geopolymer

concrete has been investigated and presented. Test results of compressive

strength are presented in Table 3.4.

The effect of source of fly ash on the compressive strength of

geopolymer concrete is discussed in terms of compressive strength index. The

compressive strength index is the ratio between the compressive strength of

geopolymer concrete prepared by using Mettur fly ash and the compressive

strength of geopolymer concrete prepared from Tuticorin fly ash for the same

concentration of NaOH, identical curing temperature and at the same age of

concrete.

It was observed that, in case of ambient curing at room temperature,

the compressive strength index is greater than one for all the three molarities

of NaOH solution both at 7 days and 28 days age of concrete as shown in

Figure 3.9. This indicates that the compressive strength of geopolymer

43

concrete prepared by using Mettur fly ash is higher than that of geopolymer

concrete prepared from Tuticorin fly ash in ambient curing at room

temperature. But in heat curing, compressive strength indices for most of the

cases is less than one which indicates that the compressive strength of

geopolymer concrete prepared by using Tuticorin flyash is greater than that of

geopolymer concrete prepared by using Mettur fly ash in heat curing as

shown in Figure 3.10.

Table 3.4 Compressive strength of geopolymer concrete

Spec. Avg. Ultimate load in kN

Avg. Compressive Strength

MPaFm M8 Ca A7 124.70 5.54Fm M8 Ch A7 362.37 16.11Fm M8 Ca A28 400.70 17.81Fm M8 Ch A28 434.10 19.29Fm M12 Ca A7 177.73 7.90Fm M12 Ch A7 481.87 21.42Fm M12 Ca A28 498.97 22.18Fm M12 Ch A28 640.93 28.49Fm M16 Ca A7 195.13 8.67Fm M16 Ch A7 489.20 21.74Fm M16 Ca A28 576.60 25.63Fm M16 Ch A28 654.87 29.11


44

1.46

1.781.56

1.021.25 1.30

0.00

0.50

1.00

1.50

2.00

8M 12M 16M

Concentration of NaOH Solution

7 days28 days

Figure 3.9 Compressive strength index - ambient curing

1.120.96

0.870.941.09

0.94

0.00

0.50

1.00

1.50

8M 12M 16M


7 days28 days

Figure 3.10 Compressive strength index - heat curing

The effect of concentration of NaOH solution on the compressive

strength of geopolymer concrete prepared by using Mettur fly ash is presented

in Figure 3.11. From the test results, it was found that for all the cases,

compressive strength of geopolymer concrete increases as the concentration

of NaOH solution increases. Under heat curing conditions, increasing the

concentration of NaOH from 8 M to 12 M resulted in an enhancement of

compressive strength by about 33% and 48% for 7 days and 28 days

respectively. When the concentration of NaOH solution is further increased

45

from 12 M to 16 M, the compressive strength also increases by about 2% for

both 7 days and 28 days. Similarly under ambient curing conditions,

increasing the concentration of NaOH from 8 M to 12 M resulted in an

improvement of compressive strength by about 43% and 25% for 7 days and

28 days respectively. When the concentration of NaOH solution is further

increased from 12M to 16M, the compressive strength also increases by 10%

and 16% for 7 days and 28 days respectively.

0

5

10

15

20

25

30

35

8M 12M 16M


AC, 7daysHC, 7 daysAC, 28 daysHC, 28 days

Figure 3.11 Effect of concentration of NaOH -Mettur fly ash

For geopolymer concrete prepared by using Tuticorin fly ash, the

effect of concentration of NaOH solution on the compressive strength is

presented in Figure 3.12. From the test results, it can be seen that the

compressive strength of geopolymer concrete increases as the concentration

of NaOH solution increases for all the cases. Under heat curing conditions,

increasing the concentration of NaOH solution from 8 M to 12 M resulted in

the compressive strength enhancement of 55% and 26% for 7 days and

28 days respectively. When the concentration of NaOH solution is further

increased from 12 M to 16 M, the compressive strength also increases by

about 12% and 19% for 7 days and 28 days respectively. Similarly under

ambient curing conditions, increasing the concentration of NaOH from 8 M to

46

12 M resulted in an improvement of compressive strength by about 17% and

2% for 7 days and 28 days respectively. When the concentration of NaOH

solution is further increased from 12 M to 16 M, the compressive strength

also increases by 26% and 11% for 7 days and 28 days respectively.

0

5

10

15

20

25

30

35

8M 12M 16M



Figure 3.12 Effect of concentration of NaOH - Tuticorin fly ash

Due to heat curing, the compressive strength is improved for both

the sources of fly ash at all concentrations of NaOH solution in 7 days and 28

days. The gain in compressive strength due to heat curing for geopolymer

concrete prepared by using Mettur fly ash and Tuticorin fly ash is presented in

Figure 3.13 and Figure 3.14 respectively. For geopolymer concrete prepared

by using Mettur fly ash, at 7 days age of concrete, the gain in compressive

strength due to heat curing is about 191%, 171% and 151% for 8 M, 12 M and

16 M concentrations of NaOH solution respectively. Similarly at 28 days, the

gain in compressive strength is about 8%, 28% and 14% for 8 M, 12 M and

16 M concentrations of NaOH solution respectively. In case of geopolymer

concrete prepared by using Tuticorin fly ash, at 7 days age of concrete, the

gain in compressive strength due to heat curing is about 278%, 403% and

348% for 8 M, 12 M and 16 M concentration of NaOH solution respectively.

Similarly at 28 days, the gain in compressive strength is about 18%, 46% and

47

57% for 8 M, 12 M and 16 M concentration of NaOH solution respectively.

From the test results it was observed that heat curing resulted in an

enhancement of compressive strength at early ages only and the increase in

compressive strength is not much significant after 7 days.

190.59171.12

150.70

8.3428.45

13.57

0

25

50

75

100

125

150

175

200

8M 12M 16M


7 days28 days

Figure 3.13 Effect of heat curing - Mettur fly ash

278.40

403.01348.14

17.8646.48 56.91

050

100150200250300350400450

8M 12M 16M


7 days28 days

Figure 3.14 Effect of heat curing - Tuticorin fly ash

From the test results it was observed that, as the age of the concrete

increases from 7 days to 28 days, the compressive strength also increases for

all the specimens. But the rate of increase in compressive strength with age of

48

concrete is more significant in case of ambient curing at room temperature

when compared with heat curing at 60oC.

3.3.4 Split Tensile Strength


concentration of NaOH solution, the curing temperature and the age of

concrete on the split tensile strength of geopolymer concrete has been

investigated and presented. Test results of split tensile strength are presented

in Table 3.5.

The effect of source of fly ash on the split tensile strength of

geopolymer concrete is discussed in terms of split tensile strength index. Split

tensile strength index is the ratio between the split tensile strength of

geopolymer concrete prepared by using Mettur fly ash and the split tensile

strength of geopolymer concrete prepared by using Tuticorin fly ash for the

same concentration of NaOH, identical curing temperature and at the same

age of concrete. It was observed that, in case of ambient curing at

room temperature, the split tensile strength index is greater than one for all the

three molarities of NaOH solution both at 7 days and 28 days as shown in

Figure 3.15. This indicates that the split tensile strength of geopolymer

concrete prepared by using Mettur fly ash is higher than that of geopolymer

concrete prepared by using Tuticorin fly ash in ambient curing at room

temperature. Similarly in heat curing, split tensile strength indices for most of

the cases is greater than one which indicates that the split tensile strength of

geopolymer concrete prepared by using Mettur fly ash is higher than that

of geopolymer concrete prepared by using Tuticorin fly ash as shown in

Figure 3.16.

49

Table 3.5 Split tensile strength of geopolymer concrete

Spec.Avg.

Ultimate load in kN

Avg. Split tensile

Strength MPa

Fm M8 Ca A7 14.33 0.20Fm M8 Ch A7 63.60 0.90Fm M8 Ca A28 68.43 0.97Fm M8 Ch A28 88.10 1.25Fm M12 Ca A7 18.93 0.27Fm M12 Ch A7 76.70 1.09Fm M12 Ca A28 82.47 1.17Fm M12 Ch A28 94.13 1.33Fm M16 Ca A7 24.63 0.35Fm M16 Ch A7 102.73 1.45Fm M16 Ca A28 97.20 1.38Fm M16 Ch A28 107.57 1.52


50

1.67

1.29

1.031.20 1.26

1.17

0.00

0.30

0.60

0.90

1.20

1.50

1.80

8M 12M 16M


7 days28 days

Figure 3.15 Split tensile strength index - ambient curing

1.201.06 1.02

1.34

0.92

0.62

0.00

0.30

0.60

0.90

1.20

1.50

8M 12M 16M

Concentration of NaOH

7 days28 days

Figure 3.16 Split tensile strength index - heat curing

The effect of concentration of NaOH solution on the split tensile

strength of geopolymer concrete prepared by using Mettur fly ash is presented

in Figure 3.17. From the test results, it was found that split tensile strength of

geopolymer concrete increases as the concentration of NaOH solution

increases for all the cases. Under heat curing conditions, increasing the

concentration of NaOH solution from 8 M to 12 M resulted in an

enhancement of split tensile strength by about 21% and 7% for 7 days and 28

days respectively. When the concentration of NaOH solution is further

51

increased from 12 M to 16 M, the split tensile strength also increases by about

34% and 14% for 7 days and 28 days respectively. Similarly under ambient

curing conditions, increasing the concentration of NaOH from 8 M to 12 M

resulted in an enhancement of split tensile strength by about 32% and 21% for

7 days and 28 days respectively. When the concentration of NaOH solution is

further increased from 12 M to 16 M, the split tensile strength also increases

by 30% and 18% for 7 days and 28 days respectively.

0

0.4

0.8

1.2

1.6

8M 12M 16M



Figure 3.17 Effect of concentration of NaOH - Mettur fly ash

The effect of concentration of NaOH solution on the split tensile

strength of geopolymer concrete prepared by using Tuticorin fly ash is

presented in Figure 3.18. From the test results, it was found that split tensile

strength of geopolymer concrete increases as the concentration of NaOH

solution increases for all the cases. Under heat curing conditions, increasing

the concentration of NaOH from 8 M to 12 M resulted in an enhancement of

split tensile strength by about 37% and 54% for 7 days and 28 days

respectively. When the concentration of NaOH solution is further increased

from 12 M to 16 M, the split tensile strength also increases by about 38% and

69% for 7 days and 28 days respectively. Similarly under ambient curing

conditions, increasing the concentration of NaOH solution from 8 M to 12 M

52

resulted in an improvement of split tensile strength by about 78% and 15% for

7 days and 28 days respectively. When the concentration of NaOH solution is

further increased from 12 M to 16 M, the split tensile strength also increases

by 62% and 27% for 7 days and 28 days respectively.

0

0.5

1

1.5

2

2.5

8M 12M 16M



Figure 3.18 Effect of concentration of NaOH - Tuticorin fly ash

Due to heat curing, the split tensile strength is improved for both

the sources of fly ash, at all the concentrations of NaOH solution in 7 days

and 28 days. The gain in split tensile strength due to heat curing for

geopolymer concrete prepared by using Mettur fly ash and Tuticorin fly ash is

presented in Figure 3.19 and Figure 3.20 respectively. For geopolymer

concrete prepared by using Mettur fly ash, at the age of 7 days, the gain in

split tensile strength due to heat curing is about 344%, 305% and 317% for 8

M, 12 M and 16 M concentrations of NaOH solution respectively. Similarly at

28 days, the gain in split tensile strength is about 29%, 14% and 11% for 8 M,

12 M and 16 M concentrations of NaOH solution respectively. In case of

geopolymer concrete prepared by using Tuticorin fly ash, at 7 days, the gain

in split tensile strength due to heat curing is about 536%, 390% and 317% for

8 M, 12 M and 16 M concentrations of NaOH solution respectively. Similarly

at the age of 28 days, the gain in split tensile strength is about 15%, 55% and

53

106% for 8 M, 12 M and 16 M concentrations of NaOH solution respectively.

From the test results it was observed that heat curing resulted in an

enhancement of split tensile strength at early ages only. The effect of heat

curing on the increase in split tensile strength is not much significant after

7 days as evidenced from the test results.

343.72305.11 317.05

28.74 14.15 10.670

50

100

150

200

250

300

350

8M 12M 16M


7 days28 days

Figure 3.19 Effect of heat curing - Mettur fly ash

535.74

390.09317.08

15.2654.82

106.31

0

100

200

300

400

500

600

8M 12M 16M


7 days

28 days

Figure 3.20 Effect of heat curing - Tuticorin fly ash

From the test results it was also noted that, as the age of the

concrete increases from 7 days to 28 days, the split tensile strength also

54

increases for all the specimens. But the rate of increase in split tensile strength

with age of concrete is more significant in case of ambient curing at room

temperature in comparison with heat curing at 60oC.

3.3.5 Flexural Strength


concentration of NaOH solution and the curing temperature on the flexural

strength of geopolymer concrete has been investigated and presented. Test

results of flexural strength are presented in Table 3.6.

Table 3.6 Flexural strength of geopolymer concrete

Spec.Avg. Ultimate

load in kN

Avg.Flexural Strength

MPaFm M8 Ca A28 10.0 4.00Fm M8 Ch A28 11.7 4.67Fm M12 Ca A28 12.5 5.00Fm M12 Ch A28 13.5 5.40Fm M16 Ca A28 15.0 6.00Fm M16 Ch A28 19.2 7.67Ft M8 Ca A28 7.7 3.07Ft M8 Ch A28 9.7 3.87Ft M12 Ca A28 11.0 4.40Ft M12 Ch A28 12.2 4.87Ft M16 Ca A28 14.0 5.60Ft M16 Ch A28 17.0 6.80

The effect of source of fly ash on the flexural strength of

geopolymer concrete is discussed in terms of flexural strength index. Flexural

strength index is the ratio between the flexural strength of geopolymer

55

concrete prepared by using Mettur fly ash and the flexural strength of

geopolymer concrete prepared by using Tuticorin fly ash for the same

concentration of NaOH, identical curing temperature and at 28 days age of

concrete. It was observed that, the flexural strength index is greater than one

for all the three molarities of NaOH solution both in ambient curing and heat

curing as shown in Figure 3.21. This indicates that the flexural strength of

geopolymer concrete prepared by using Mettur fly ash is greater than that of

geopolymer concrete prepared by using Tuticorin fly ash.

1.301.14 1.07

1.211.11 1.13

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

8M 12M 16M


ACHC

Figure 3.21 Flexural strength index

The effect of concentration of NaOH solution on the flexural

strength of geopolymer concrete is presented in Figure 3.22. From the test

results, it was found that flexural strength of geopolymer concrete increases as

the concentration of NaOH solution increases for all the cases. For

geopolymer concrete prepared by using Mettur fly ash, increasing the

concentration of NaOH solution from 8 M to 12 M resulted in an

improvement of flexural strength by about 16% under heat curing conditions.

The flexural strength also increases by about 42% when the concentration of

NaOH solution is further increased from 12 M to 16 M. Similarly under

ambient curing conditions, increasing the concentration of NaOH from 8 M to

56

12 M resulted in an enhancement of flexural strength by 25%. When the

concentration of NaOH solution is further increased from 12 M to 16 M, the

flexural strength gets increased by 20%. In case of geopolymer concrete

prepared by using Tuticorin fly ash, increasing the concentration of NaOH

from 8 M to 12 M resulted in an improvement of flexural strength by about

26% when cured at 60oC. The flexural strength also increases by about 40%

when the concentration of NaOH solution is further increased from 12 M to

16 M. Similarly under ambient curing conditions, increasing the concentration

of NaOH from 8 M to 12 M resulted in an enhancement of flexural strength

by 43%. When the concentration of NaOH solution is further increased from

12 M to 16 M, the flexural strength gets increased by 27%.

0

1

2

3

4

5

6

7

8

8M 12M 16M


MFA, ACMFA, HCTFA, ACTFA, HC

Figure 3.22 Effect of concentration of NaOH on flexural strength

The flexural strength is improved due to heat curing for both

sources of fly ash at all concentrations of NaOH solution. The gain in flexural

strength due to heat curing is presented in Figure 3.23. For Mettur fly ash

geopolymer concrete, the gain in flexural strength due to heat curing is about

17%, 8% and 28% for 8 M, 12 M and 16 M concentrations of NaOH solution

respectively. In case of Tuticorin fly ash geopolymer concrete, the gain in

57

flexural strength due to heat curing is about 26%, 11% and 21% for 8 M, 12

M and 16 M concentrations of NaOH solution respectively.

16.67

8

27.7826.09

10.61

21.43

0

5

10

15

20

25

30

8M 12M 16M


MFATFA

Figure 3.23 Gain in flexural strength due to heat curing

3.4 CONCLUSIONS

Based on the results obtained in this investigation, the following

conclusions are drawn:

Geopolymer concrete prepared by using fly ash obtained from

Tuticorin thermal power station has better workability than the

geopolymer concrete prepared from Mettur based fly ash.

Irrespective of the source of fly ash, workability of geopolymer

concrete decreases as the concentration of sodium hydroxide in

the alkaline activator solution increases.

The average density values of geopolymer concrete ranges from

2316 kg/m3 to 2405 kg/m3 which was found approximately

closer to that of ordinary Portland cement concrete. Variation of

density is not much significant with respect to the source of fly

ash, the concentration of NaOH solution, the type of curing and

the age of concrete.

58

Compressive strength of Mettur fly ash geopolymer concrete is

higher than that of Tuticorin fly ash based geopolymer concrete

in ambient curing at room temperature.

Compressive strength of geopolymer concrete increases as the

concentration of NaOH solution increases. This is applicable

for all the curing temperatures, age of concrete and sources of

fly ash.

Rate of increase in compressive strength and split tensile

strength with respect to the age of concrete is more significant

in case of ambient curing at room temperature in comparison

with heat curing at 60oC.Heat curing resulted in an enhancement

of compressive strength and split tensile strength at early ages

only. The effect of heat curing on the increase in compressive

strength and split tensile strength is not much significant after

7 days.

For the same concentrations of NaOH, identical curing

temperature and the age of concrete, split tensile strength and

flexural strength is higher in case of mettur fly ash based

geopolymer concrete.

Geopolymer concrete did not harden immediately at room

temperature as in conventional concrete. Geopolymer concrete

specimens took a minimum of 3 days for complete setting

without leaving a nail impression on the hardened surface.

These two observations are considered as drawbacks of this

concrete to be used for practical applications.

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CHAPTER 3 FLY ASH BASED GEOPOLYMER …shodhganga.inflibnet.ac.in/bitstream/10603/15807/8/08...29 CHAPTER 3 FLY ASH BASED GEOPOLYMER CONCRETE 3.1 GENERAL The fresh and hardened properties