CHAPTER 4 GEOPOLYMER CONCRETE COMPOSITES

Transcription

1 59 CHAPTER 4 GEOPOLYMER CONCRETE COMPOSITES 4.1 GENERAL From the detailed experimental investigations on fly ash based Geopolymer concrete (GPC) given in chapter 3 the following two limitations have been observed namely delay in setting time and necessity of heat curing to gain strength at early ages. These limitations are considered as the drawbacks of this concrete to be used for practical applications. In order to overcome these two limitations of GPC mix, 10% of fly ash in the geopolymer concrete was replaced by Ordinary Portland Cement (OPC) and the mix design was altered accordingly which results in Geopolymer Concrete Composites (GPCC). This chapter describes the mix proportion and preparation of Geopolymer concrete composites. The fresh and hardened properties such as workability, density, compressive strength, split tensile strength and flexural strength of Geopolymer Concrete Composites (GPCC) are presented in this chapter. A comparison on the strength and behaviour between GPC and GPCC is also discussed. 4.2 EXPERIMENTAL PROGRAMME Parameters of Study investigation: The following parameters were considered in this experimental (a) Quantity of fly ash and OPC:100% & 0% and 90% & 10% respectively (by weight)

2 60 (b) Curing temperature: Ambient curing at room temperature and heat curing at 60 o C in hot air oven for 24 hours (c) Age of concrete at time of testing: 1day, 3days, 7 days and 28 days Materials Used Fly ash : Class F dry fly ash conforming to IS obtained from Mettur thermal power station of Tamilnadu from southern part of India was used in the casting of the specimens. Cement: Ordinary Portland Cement (OPC) conforming to IS: , with a specific gravity of 3.15 was used in the casting of the test specimens. Table 4.1 gives the properties of cement used. Table 4.1 Properties of cement Description of test Test results Requirements of IS: Initial setting time 70 minutes Min. 30 minutes Final setting time 295 minutes Max. 600 minutes Compressive strength of cement mortar cubes at: 3 days 7 days 28 days MPa MPa MPa 23 MPa 33 MPa 43 MPa Fine aggregate: Locally available river sand having a bulk density of 1693 kg/m 3, fineness modulus of 2.75, specific gravity of 2.81 and conforming to grading zone-iii as per IS: was used.

3 61 Coarse aggregate: Crushed granite coarse aggregates of 19 mm maximum size having a fineness modulus of 6.64 and specific gravity of 2.73 were used. Bulk Density of the coarse aggregate used is 1527 kg/m 3. Sodium hydroxide: Sodium hydroxide solids in the form of flakes with 97% purity manufactured by Merck Specialties Private Limited, Mumbai was used in the preparation of alkaline activator. Sodium silicate: Sodium silicate in the form of solution 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. Super plasticizer: To achieve workability of fresh geopolymer concrete, Sulphonated napthalene polymer based super plasticizer Conplast SP430 in the form of a brown liquid instantly dispersible in water, manufactured by Fosroc Chemicals (India) private limited, Bangalore, was used in all the mixtures. 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 of 12 molarity and sodium silicate solution was used as alkaline activator solution for geopolymerisation. To prepare sodium hydroxide solution of 12 molarity (12 M), 480 g (12 x 40 i.e, molarity x molecular weight) of sodium hydroxide flakes was dissolved in distilled water and makeup to one liter. The mass of NaOH solids is equal to g per kg of NaOH solution.

4 Mix Proportion of Geopolymer Concrete Composites 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% of the total aggregates. From the past literatures it was 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 arrived. 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, 10% by weight of fly ash (other than the water used for the preparation of alkaline activator solutions) and super plasticizer Conplast SP 430, 3% by weight of fly ash was added to the mix to achieve workable concrete. This resulted in GPC mix. In order to get the mix proportions of GPCC, 10% of fly ash in GPC mix was replaced by OPC. Accordingly the alkaline activator solution content was altered such that alkaline liquid to fly ash ratio is maintained as 0.4. Extra water content was also modified such that the quantity of liquid component (NaOH solution + Na 2 SiO 3 solution + Extra water) in GPC and GPCC is same. In GPCC mix the quantity of super plasticizer Conplast SP 430 is 3% by combined weight of fly ash and OPC. The mix proportions of GPC and GPCC are given in Table 4.2. Table 4.2 Details of mix proportions of GPC and GPCC Mix Fly Ash kg/m 3 OPC kg/m 3 FA kg/m 3 CA kg/m 3 NaOH Solution kg/m 3 Na 2 SiO 3 Solution kg/m 3 Extra Water kg/m 3 SP kg/m 3 GPC GPCC

5 Preparation of GPCC Specimens The prepared solution of sodium hydroxide of 12 M concentration was mixed with sodium silicate solution one day before mixing the concrete to get the desired alkalinity in the alkaline activator solution. Initially fine aggregate, fly ash, OPC and coarse aggregate were dry mixed for three minutes in a horizontal pan mixer. 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 GPCC. Totally eighteen cubes of size 150 mm x 150 mm x 150 mm, eighteen cylinders having a diameter of 150 mm and 300 mm length and twelve prisms of 500 mm x 100 mm x100 mm were cast to study the mechanical properties of GPCC. 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 GPCC Specimens GPCC specimens were removed from the mould immediately after 24 hours since they set in a similar fashion as that of conventional concrete. Six specimens each in cubes, cylinders and prisms were heat cured at 60 o C in hot air oven for 24 hours and remaining specimens were left at room temperature in ambient curing.

6 Designation of Specimens The test specimens were designated with three terms. Each of these terms gives information about some aspect of the specimens which is described as follows: The first term describes the quantity of fly ash and OPC used in geopolymer concrete mix for casting the specimens. F 90 refers to GPCC specimens that contain 90% fly ash and 10% OPC. F 100 refers to GPC specimens containing 100% fly ash and 0% OPC. The second term refers to the curing condition of the specimen. C a refers to the specimens cured at ambient condition at room temperature and C h refers to the specimens cured at 60 o C in hot air oven. The third term refers to the age of concrete at the time of testing. A 1, A 3, A 7 and A 28 refer to tests conducted at respective age of concrete in days Instrumentation and Testing Procedure All the freshly prepared GPCC mixes were tested for workability by using the conventional slump cone apparatus. The slump cone was filled with freshly mixed geopolymer concrete composite mix 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. Split tensile strength was evaluated as per the test procedure given in Indian Standards IS 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. The maximum load applied to the specimen was recorded and the compressive strength was calculated. Before

7 65 subjecting the specimens to compression test, weight of each specimen was recorded and density of each specimen was calculated by dividing the weight of the specimen by its volume. To evaluate the splitting tensile strength of geopolymer concrete composites, all the cylinder specimens were subjected to split tensile test in a 2000 kn digital compression testing machine. The maximum load applied to the specimen was recorded and the split tensile strength was calculated. Flexural strength of geopolymer concrete composites was determined using prism specimens by subjecting them to two point bending in Universal Testing Machine having a capacity of 1000 kn. The maximum load applied to the specimen was recorded and the flexural strength was calculated. 4.3 RESULTS AND DISCUSSION Workability Workability of freshly prepared geopolymer concrete composite mix was measured in terms of its slump using the conventional slump cone apparatus. GPCC mix was generally cohesive and shiny in appearance due to the presence of sodium silicate. Workability of GPCC is better when compared with the workability of GPC as shown in Figure GPCC GPC GPCC GPC Figure 4.1 Workability of GPC and GPCC mixes

8 Density Density of all the mixes is presented in Table 4.3. The density of GPCC ranges from 2336 kg/m 3 to 2424 kg/m 3 and the density of GPC ranges from 2336 kg/m 3 to 2413 kg/m 3 as shown in Figure 4.2. Variation of density is not much significant with respect to type of curing and age of concrete. The density of GPCC and GPC was found close to that of ordinary Portland cement concrete. Table 4.3 Density of GPC and GPCC Spec. Avg. Weight in kg Avg. Density kg/m 3 F 90 C a A F 90 C a A F 90 C a A F 90 C h A F 90 C a A F 90 C h A F 100 C a A F 100 C h A F 100 C a A F 100 C h A GPC GPCC Specimen number Figure 4.2 Ranges of density of GPC and GPCC

9 Compressive Strength The effect of various factors such as replacement of 10% of fly ash by OPC, curing temperature namely ambient curing at room temperature and heat curing at 60 o C and 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 4.4. Table 4.4 Compressive strength of GPCC and GPC specimens Spec. Avg. Ultimate load in kn Avg. Compressive Strength MPa F 90 C a A F 90 C a A F 90 C a A F 90 C h A F 90 C a A F 90 C h A F 100 C a A F 100 C h A F 100 C a A F 100 C h A Unlike GPC, geopolymer concrete composite hardens immediately and starts gaining compressive strength within a day without any necessity of heat curing. In ambient curing at room temperature, within a day itself, GPCC specimens gained 20% of its 28 days heat cured compressive strength. Similarly in ambient curing condition at the age of 3 days, 7 days and 28 days, GPCC specimens gained 32%, 50% and 97% of its 28 days heat cured strength respectively as shown in Figure 4.3. This shows that ambient curing

10 68 at room temperature itself is sufficient for GPCC specimens to gain its compressive strength day 3 days 7 days 28 days Age of concrete Ambient Curing Heat Curing Figure 4.3 Gain in compressive strength with age Replacement of 10% of fly ash by OPC in GPC mix resulted in an enhanced compressive strength. In ambient curing, the compressive strength of GPCC increases by about 151% and 73% at the age of 7 days and 28 days respectively with reference to GPC mix as shown in Figure 4.4. Similarly in heat curing, the compressive strength of GPCC increases by about 64% and 39% with reference to GPC mix at the age of 7 days and 28 days respectively days Age of concrete 28 days HC AC Figure 4.4 Compressive strength gain of GPCC

11 69 From the test results, it is revealed that the gain in compressive strength is more in case of ambient curing as compared to heat curing. This may be due to the reason that in ambient curing in addition to the prevailing room temperature, the heat evolved by hydration of 10% of OPC will also contribute some amount of heat that is required for the polymerization of 90% of fly ash present in the GPCC mixes. Similarly water liberated during the polymerization process of fly ash is utilized for curing of OPC present in the mix. The effect of heat curing on the compressive strength for GPCC and GPC is presented in Figure 4.5. In case of GPCC specimens, the gain in compressive strength due to heat curing was about 77% and 3% for 7 days and 28 days respectively whereas for GPC specimens, the gain in compressive strength due to heat curing was about 171% and 28% for 7 days and 28 days respectively. It was found that the gain in compressive strength due to heat curing is more in case of GPC specimens when compared to GPCC specimens. At the age of 28 days, compressive strength of GPCC specimen is enhanced only by 3% due to heat curing. This shows that heat curing is not necessary for GPCC specimens. GPCC GPC days 28 days Age of concrete Figure 4.5 Effect of heat curing on compressive strength

12 Split Tensile Strength The effect of various parameters such as replacement of 10% of fly ash by OPC, curing temperature namely ambient curing at room temperature and heat curing at 60 o C and age of concrete at the time of testing on the split tensile strength of geopolymer concrete has been studied and presented. Test results of split tensile strength are presented in Table 4.5. Unlike GPC, geopolymer concrete composite hardens immediately and starts gaining split tensile strength within a day without any necessity of heat curing. In ambient curing at room temperature, within a day itself, GPCC specimens gained 7% of its 28 days heat cured split tensile strength. Similarly in ambient curing condition at the age of 3 days, 7 days and 28 days, GPCC specimens gained 14%, 41% and 88% of its 28 days heat cured split tensile strength as shown in Figure 4.6. This shows that ambient curing at room temperature itself is sufficient for GPCC specimens to gain its split tensile strength Table 4.5 Split tensile strength of GPCC and GPC specimens Spec. Avg. Ultimate load in kn Avg. Split tensile Strength MPa F 90 C a A F 90 C a A F 90 C a A F 90 C h A F 90 C a A F 90 C h A F 100 C a A F 100 C h A F 100 C a A F 100 C h A

13 day 3 days 7 days 28 days Age of concrete Ambient Curing Heat Curing Figure 4.6 Gain in split tensile Strength with age When 10% of fly ash is replaced by OPC in GPC mix, there is an enhancement of split tensile strength. In ambient curing, the split tensile strength of GPCC increases by about 357% and 128% at the age of 7 days and 28 days respectively with reference to GPC mix as shown in Figure 4.7. Similarly in heat curing, the split tensile strength of GPCC increases by about 176% and 127% with reference to GPC mix at the age of 7 days and 28 days respectively days 28 days AC HC Age of Concrete Figure 4.7 Split tensile strength gain of GPCC

14 72 From the test results, it is revealed that the gain in split tensile strength is more in case of ambient curing when compared to heat curing. This may be due to the reason that in ambient curing in addition to room temperature, the heat evolved by hydration of 10% of OPC will also contribute some amount of heat that is required for the polymerization of 90% of fly ash present in the GPCC mixes. Similarly water liberated during the polymerization process of fly ash is utilized for curing of OPC present in the mix. The effect of heat curing on the split tensile strength for GPCC and GPC is presented in Figure 4.8. In case of GPCC specimens, the gain in split tensile strength due to heat curing was about 145% and 13% for 7 days and 28 days respectively whereas for GPC specimens, the gain in split tensile strength due to heat curing was about 305% and 14% for 7 days and 28 days respectively. It was found that the gain in split tensile strength due to heat curing is more in case of GPC specimens when compared to GPCC specimens. At the age of 28 days, split tensile strength of GPCC specimen is enhanced only by 13% due to heat curing. This shows that heat curing is not necessary for GPCC specimens days days 14 GPCC GPC Age of Concrete Figure 4.8 Effect of heat curing on split tensile strength

15 Flexural Strength The effect of various factors such as replacement of 10% of fly ash by OPC, curing temperature namely ambient curing at room temperature and heat curing at 60 o C and age of concrete at the time of testing on the flexural strength of geopolymer concrete has been investigated and presented. Test results of flexural strength are presented in Table 4.6. Geopolymer concrete composite specimens harden immediately and start gaining flexural strength without any need of heat curing. In ambient curing at room temperature, within 7days, GPCC specimens gained 66% of its 28 days heat cured flexural strength. Similarly in ambient curing condition at the age of 28 days, GPCC specimens gained 98% of its 28 days heat cured flexural strength as shown in Figure 4.9. This shows that ambient curing at room temperature itself is sufficient for GPCC specimens to gain flexural strength. Table 4.6 Flexural strength of GPCC and GPC specimens Spec. Avg. Ultimate load in kn Avg. Flexural Strength MPa F 90 C a A F 90 C h A F 90 C a A F 90 C h A F 100 C a A F 100 C h A F 100 C a A F 100 C h A

16 Ambient Curing Heat Curing days 28 days Age of Concrete Figure 4.9 Gain in flexural strength of GPCC with age Replacement of 10% of fly ash by OPC in GPC mix resulted in an enhanced flexural strength. In ambient curing, the flexural strength of GPCC increases by about 28% and 17% at the age of 7 days and 28 days respectively with reference to GPC mix as shown in Figure Similarly in heat curing, the flexural strength of GPCC increases by about 20% and 11% with reference to GPC mix at the age of 7 days and 28 days respectively days Age of Concrete 28 days AC HC Figure 4.10 Flexural strength gain of GPCC Vs GPC

17 75 From the test results, it is revealed that the gain in flexural strength is more in case of ambient curing as compared to heat curing. This may be due to the reason that in ambient curing in addition to the prevailing room temperature, the heat evolved by hydration of 10% of OPC will also contribute some amount of heat that is required for the polymerization of 90% of fly ash present in the GPCC mixes. Similarly water liberated during the polymerization process of fly ash is utilized for curing of OPC present in the mix. The effect of heat curing on the flexural strength for GPCC and GPC is presented in Figure In case of GPCC specimens, the gain in flexural strength due to heat curing was about 32% and 2% for 7 days and 28 days respectively whereas for GPC specimens, the gain in flexural strength due to heat curing was about 41% and 8% for 7 days and 28 days respectively. It was found that the gain in flexural strength due to heat curing is more in case of GPC specimens when compared to GPCC specimens. At the age of 28 days, flexural strength of GPCC specimen is enhanced only by 2% due to heat curing. This shows that heat curing is not necessary for GPCC specimens GPCC GPC 7 days 28 days Age of Concrete Figure 4.11 Effect of heat curing on Flexural strength

18 CONCLUSIONS Based on the results obtained in this investigation, the following conclusions are drawn: Geopolymer Concrete has two limitations such as delayed setting and necessity of heat curing to gain strength. These limitations are considered as the drawbacks of this concrete to be used for practical applications. These two limitations of GPC mix were overcome by replacing 10% of fly ash by OPC which resulted in Geopolymer concrete composite. Workability of GPCC is better when compared with the workability of GPC. The average density values of GPCC ranges from 2362 kg/m 3 to 2415 kg/m 3 which is found approximately closer to that of ordinary Portland cement concrete. Variation of density is not much significant with respect to type of curing and age of concrete. Unlike GPC, geopolymer concrete composite hardens immediately and starts gaining its strength within a day without any necessity of heat curing. Replacement of 10% of fly ash by OPC in GPC mix resulted in an enhanced compressive strength, split tensile strength and flexural strength by 73%, 128% and 17% respectively with reference to GPC mix in ambient curing at the age of 28 days. At the age of 28 days, in ambient curing itself GPCC specimens gained 97% of its heat cured compressive strength, 88% of its

19 77 heat cured split tensile strength and 98% of its heat cured flexural strength. This shows that ambient curing at room temperature itself is sufficient for GPCC specimens to gain strength. In GPCC specimens, the gain in strength is more in case of ambient curing as compared to heat curing. This may be due to the reason that in ambient curing in addition to the prevailing room temperature, the heat evolved by hydration of 10% of OPC will also contribute some amount of heat that is required for the polymerization of 90% of fly ash present in the GPCC mixes. Similarly water liberated during the polymerization process of fly ash is utilized for curing of OPC present in the mix. At the age of 28 days, the enhancement of compressive strength, split tensile strength and flexural strength of GPCC specimens due to heat curing is only 3%, 13% and 2% respectively. This shows that heat curing is not necessary for GPCC specimens. At the age of 28 days, the compressive strength, split tensile strength and flexural strength of GPCC in ambient curing itself is more than that of GPC in heat curing. This may be due to the reason that sufficient heat is evolved during the hydration of 10% of cement which is utilized for the polymerization process of 90% fly ash present in the mix.