CHAPTER 3 FLY ASH BASED GEOPOLYMER CONCRETE

Transcription

1 29 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 Parameters of Study investigation: The following parameters were considered in this experimental (a) (b) Source of fly ash: Fly ash collected from Mettur and Tuticorin thermal power stations of TamilNadu, India Concentration of sodium hydroxide used for preparation of geopolymer concrete: 8 Molarity (8 M), 12 Molarity (12 M) and 16 Molarity (16 M)

2 30 (c) (d) Curing temperature: Ambient curing at room temperature and heat curing at 60 o C for 24 hours in hot air oven Age of concrete at the time of testing: 7 days and 28 days Materials Used Fly ash : Class F dry fly ash conforming to IS 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 SiO % 63.24% Al 2 O % 17.35% Fe 2 O % 2.63% Requirements as per IS SiO 2 >35% Total - >70% CaO 3.33% 2.05% - Na 2 O 0.34% 0.24% K 2 O 0.22% 0.32% <1.5% 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: was used. Bulk density of the fine aggregate is 1693 kg/m 3. Details of sieve analysis of sand are given in Appendix1.

3 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/m 3. 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 Na 2 O, 29.4% of SiO 2 and 55.9% of water by mass.

4 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 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, g/kg in the 12 M NaOH solution and g/kg in the 16 M NaOH solution.this shows that water was the major component in the

5 33 sodium hydroxide solution and NaOH solids was only a fraction of the mass of NaOH solution 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/m 3 ). 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 Fly ash kg/m 3 Fine aggregate kg/m 3 Coarse aggregate kg/m 3 NaOH solution kg/m 3 Na 2 SiO 3 solution kg/m 3 Extra water kg/m 3 SP kg/m

6 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 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

7 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 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 F t 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. M 8 refers to 8 M NaOH solution, M 12 refers to 12 M NaOH solution and M16 16 M NaOH solution. The third term refers to the curing condition of the specimen. C a refers to the specimens that were cured at ambient conditions at room temperature and C h refers to the specimens that were cured at 60 o 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

8 36 7 days age of concrete and A 28 refers to tests conducted at 28 days age of concrete 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/mm 2 /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) f c P A (3.1) where f c is the compressive strength, P is the maximum load applied to the specimen and A is the cross-sectional area of the specimen.

9 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 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) ft 2P DL (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.

10 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) fr Pl bd 2 (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.

11 39 Figure 3.6 Test set-up for flexural strength 3.3 RESULTS AND DISCUSSION 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.

12 MFA TFA Figure 3.7 Effect of concentration of NaOH on workability 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/m 3 and 2316 kg/m 3 to 2397 kg/m 3 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.

13 41 Table 3.3 Density of geopolymer concrete Spec. Avg. Weight in kg Avg. Density kg/m 3 F m M 8 C a A F m M 8 C h A F m M 8 C a A F m M 8 C h A F m M 12 C a A F m M 12 C h A F m M 12 C a A F m M 12 C h A F m M 16 C a A F m M 16 C h A F m M 16 C a A F m M 16 C h A F t M 8 C a A F t M 8 C h A F t M 8 C a A F t M 8 C h A F t M 12 C a A F t M 12 C h A F t M 12 C a A F t M 12 C h A F t M 16 C a A F t M 16 C h A F t M 16 C a A F t M 16 C h A

14 42 MFA, AC MFA, HC TFA, AC TFA, HC Specimen number Figure 3.8 Ranges of density of geopolymer concrete 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 60 o C 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

15 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 Table 3.4 Compressive strength of geopolymer concrete Spec. Avg. Ultimate load in kn Avg. Compressive Strength MPa F m M 8 C a A F m M 8 C h A F m M 8 C a A F m M 8 C h A F m M 12 C a A F m M 12 C h A F m M 12 C a A F m M 12 C h A F m M 16 C a A F m M 16 C h A F m M 16 C a A F m M 16 C h A F t M 8 C a A F t M 8 C h A F t M 8 C a A F t M 8 C h A F t M 12 C a A F t M 12 C h A F t M 12 C a A F t M 12 C h A F t M 16 C a A F t M 16 C h A F t M 16 C a A F t M 16 C h A

16 days 28 days Concentration of NaOH Solution Figure 3.9 Compressive strength index - ambient curing days 28 days Concentration of NaOH Solution 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 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

17 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 Concentration of NaOH Solution AC, 7days HC, 7 days AC, 28 days HC, 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 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

18 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 AC, 7days HC, 7 days AC, 28 days HC, 28 days Concentration of NaOH Solution 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

19 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 Concentration of NaOH Solution 7 days 28 days Figure 3.13 Effect of heat curing - Mettur fly ash Concentration of NaOH Solution 7 days 28 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

20 48 concrete is more significant in case of ambient curing at room temperature when compared with heat curing at 60 o C Split Tensile Strength The effect of various factors such as the source of fly ash, the 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 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.

21 49 Table 3.5 Split tensile strength of geopolymer concrete Spec. Avg. Ultimate load in kn Avg. Split tensile Strength MPa F m M 8 C a A F m M 8 C h A F m M 8 C a A F m M 8 C h A F m M 12 C a A F m M 12 C h A F m M 12 C a A F m M 12 C h A F m M 16 C a A F m M 16 C h A F m M 16 C a A F m M 16 C h A F t M 8 C a A F t M 8 C h A F t M 8 C a A F t M 8 C h A F t M 12 C a A F t M 12 C h A F t M 12 C a A F t M 12 C h A F t M 16 C a A F t M 16 C h A F t M 16 C a A F t M 16 C h A

22 days 28 days Concentration of NaOH Solution Figure 3.15 Split tensile strength index - ambient curing days 28 days Concentration of NaOH 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 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

23 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 AC, 7days HC, 7 days AC, 28 days HC, 28 days Concentration of NaOH Solution 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 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

24 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 AC, 7days HC, 7 days AC, 28 days HC, 28 days Concentration of NaOH Solution 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

25 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 days 28 days Concentration of NaOH Solution Figure 3.19 Effect of heat curing - Mettur fly ash Concentration of NaOH Solution 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

26 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 60 o C Flexural Strength The effect of various factors such as the source of fly ash, the 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 MPa F m M 8 C a A F m M 8 C h A F m M 12 C a A F m M 12 C h A F m M 16 C a A F m M 16 C h A F t M 8 C a A F t M 8 C h A F t M 12 C a A F t M 12 C h A F t M 16 C a A F t M 16 C h A 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

27 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 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 Concentration of NaOH Solution AC HC Figure 3.21 Flexural strength index The effect of concentration of NaOH solution on the flexural strength of geopolymer concrete is presented in Figure 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

28 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 60 o C. 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% Concentration of NaOH Solution MFA, AC MFA, HC TFA, AC TFA, 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 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

29 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 MFA TFA Concentration of NaOH Solution 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/m 3 to 2405 kg/m 3 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.

30 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 60 o C.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.