Nomenclature of GPC specimens.
Geopolymer concrete (GPC) has significant potential as a more sustainable alternative for ordinary Portland cement concrete. GPC had been introduced to reduce carbon footprints and thereby safeguarding environment. This emerging eco friendly construction product finds majority of its application in precast and prefabricated structures due to the special curing conditions required. Sustained research efforts are being taken to make the product suitable for in situ applications. The developed technology will certainly address the issues of huge energy consumption as well reduce water use which is becoming scarce nowadays. Ground Granulated Blast Furnace Slag (GGBS) a by-product of iron industries in combination with fly ash has proved to give enhanced strength, durability as well reduced setting time. This study investigates the effect of GGBS as partial replacement of fly ash in the manufacture of GPC. Cube and cylindrical specimens were cast and subjected to ambient curing as well to alternate wetting-drying cycles. The 28 day compressive strength, split tensile strength, flexural strength and density of GPC specimens were found. The study revealed increase in compressive strength, split tensile strength, density as well flexural strength up to 40 percent replacement of fly ash by GGBS.
- geopolymer concrete
- ambient curing
- Setting time
It is well known that concrete is one of the most widely used reliable and effective construction material all over the World [1, 2]. Rapid urban growth development leads to the usage of concrete in construction industry increase day by day which further increases the demand of ordinary Portland cement. In order to meet the huge demand, the production of ordinary Portland cement (OPC) increases every year. During the production of OPC, an enormous amount of greenhouse gas such as carbon dioxide (CO2) will be emitted giving rise to global warming issues . As such invention of alternative binding materials evolved. Geopolymer concrete proved itself to address the issues by reduced carbon footprints. Further, the enormous energy required in the production of OPC could be avoided leading to energy conservation.
Geopolymer had proved further superior to OPC in terms of acid resistance, sulphate resistance, withstanding heat , fire and possessing corrosion resistance. Geopolymerization involves a heterogeneous chemical reaction between silicon and aluminium in a source of geological origin or industrial by products such as flyash and GGBS (binder) with high alkaline solution of sodium hydroxide and sodium silicate resulting in the form of three-dimensional amorphous to semi crystalline polymeric and ring structure comprising Si-O-Al and Si-O-Si bonds.
There exist some limitations in practical usage of geopolymer concrete in construction industry. Usage of alkaline solution in geopolymer concrete contributes to cost aspect. Further, requirement of specialized curing namely heat curing or steam curing makes it difficult for in situ applications.
Due to the heavy need of electricity, numerous thermal power stations have been installed throughout the country which gives rise to fly ash generation. Flyash is a heterogeneous by-product material produced in the combustion process of coal used in power stations. Fly ash particles are almost spherical in shape which allows them to flow and blend freely in mixtures. This characteristic makes fly ash a desirable binder for concrete. Further, control of high thermal gradients, pore refinement, depletion of cement alkalis, resistance to chloride  and sulphate penetration and continued micro structural development through a long-term hydration and pozzolanic reaction contributes to added durability aspects. Also, the magnitude of reinforcement steel production is also enormous so as to meet the present day needs of multistoreyed structures. The by product of iron manufacturing by heating iron ore, lime stone and coke at very high temperature of 1500 degree celcisus is GGBS.
GGBS posseses good mobility characteristics arising from consistent fineness, unique particle shape and from lower relative density. Also, workability gets improved due to the smoother surface texture and glassy surface of GGBS. Also, efflorescence and staining of concrete shall be prevented by the use of GGBS. It is evident that due to its superior performance, it replaces Sulphate Resisting Portland Cement (SRPC) and useful against severe chloride attack in reinforced concrete in marine environment.
In this work, an attempt was made to study the performance of GGBS admixed geopolymer concrete under ambient curing condition. The optimum percentage replacement of fly ash by GGBS had been determined by conducting compressive strength and split tensile strength tests. The results obtained would help resolve the issues addressed above.
2. Experimental procedure
In this study, Class F fly ash with specific gravity 2.3 and GGBS having specific gravity 2.1 was used. River sand was used as fine aggregate as per standards .
Coarse aggregate of different sizes 7 mm, 14 mm and 20 mm was graded using sieve analyser with specific gravity 2.717, 2.81, 2.76 has determined using pycnometer. The water absorption was found as 0.26%, 0.15%, 0.42% respectively while the aggregates are immersed in water for 24 hours. Sodium hydroxide (NaOH) having 14 M molarity with 97–98% purity in the form of pellets was used . Sodium silicate available in the form of gel was used. Both chemicals have brought from the local supplier (Thirumala Nadar and Sons, Madurai, India-625001).
2.2 Concrete mix proportions and specimen preparation
The geopolymer concrete comprises binder, fine aggregate, coarse aggregate and alkaline solution. The materials were quantified before mixing using standard guidelines as per Rangan and Hardjito method . Sodium hydroxide was mixed with water and then sodium silicate was added to the diluted sodium hydroxide to make alkaline solution. The alkaline solution was prepared one day before concrete mix to facilitate polymerization process.
The percentage replacement of fly ash by GGBS was varied from 0–50%. Including the control mix, six mix proportions were used for both ambient curing and for wetting-drying cycles. Due to the reason that quick setting developed , percentage replacement was not done above 50%.
It is a usual procedure to first mix the binder materials, fine aggregate and coarse aggregate using mixer machine  for about three to four minutes. Then the prepared alkaline liquid should be poured over the dry mix concrete and mixing to be continued for about four to five minutes to initiate geopolymerisation process.
After getting an intimate mix, they were cast using moulds. The cast specimens were cured under ambient curing for 28 days while another set of specimens were subjected to wetting for 2 days in curing tank and drying for 2 days in ambient condition. These wetting and drying cycles were continued upto 28 days. The cast cube and cylinder specimens subjected to ambient curing are shown in Figures 1 and 2 respectively.
Prisms of size 500 mm x 100 mm x100mm were cast to study the flexural behaviour of GPC using fly ash and GGBS combination. Prisms exposed to ambient curing for 28 days are shown in Figure 3.
The nomenclature of GPC specimens is presented in Table 1.
|0,10.20,30, 40,50||Percentage of GGBS in Fly ash|
|A’||Wetting Drying Cycles|
2.3 Test methods
To ascertain mechanical behavior of geopolymer concrete under ambient curing, compressive strength, split tensile strength and flexural strength tests were carried out. SEM analysis was done to study the microstructure, particles and formation of specimen .
2.3.1 Compressive strength test
Cube specimens were subjected to compressive axial load applied at the rate of 1.2 N/mm2 to 2.4 N/mm2 in accordance with IS code 5816:1999  (Figure 4). The load was applied in such a way that the fracture plane crosses the trowel surface.
2.3.2 Split tensile strength test
To measure the tensile strength of concrete, cylinder specimens are placed centrally between the plates and the load is gradually applied in such a way that fracture plane will pass along its vertical diameter of specimen (Figure 5). The test was carried out as per IS code 5816:1999  speficications.
2.3.3 Flexural strength test
The flexural strength test set up is shown in Figure 6. Load is applied to the uppermost surface as cast along two lines 150 mm apart. The load was increased continuously till the specimen failed recording the failure load.
2.3.4 Microstructural analysis
To ascertain the morphological features of geopolymer concrete made with new combination of fine aggregates, micro structural analysis becomes essential. The test results would reveal the factors affecting the mechanical properties. Presently available micro structural analysis include Scanning Electron Microscope (SEM), X Ray Diffraction Analysis (XRD), Energy Dispersive Spectroscopy (EDS) or Energy Dispersive X-Ray Analysis (EDAX) etc. In the present investigation, SEM and EDAX were used for the purpose for all mixes.
220.127.116.11 Energy dispersive X-ray analysis (EDAX)
EDAX shall very well be utilised to obtain the elemental composition or chemical characterisation of an area of interest on a specimen. The EDX or EDAX spectrum displays spectra showing peaks corresponding to the energy levels of elements making up the true composition of samples being analysed as received by X-rays. An excitation source, X-ray detector, pulse processor and the analyser constitute the primary components of EDAX. It is also possible to get elemental mapping of a sample as well image analysis. Samples of all proportions were made ready in powder form for analysis. For better accuracy, quantitative correction procedures are need to be applied.
18.104.22.168 Scanning electron microscopy (SEM)
In SEM, the surface of specimens will be scanned with a focused beam of electrons and images would be produced. The atoms in the sample will be interacted by the electrons and produce signals giving information regarding the topography of surface and composition of the sample. Resolution in the range of even 1 nanometer shall be achieved with the help of SEM.
A scanning electron microscope within built energy dispersive X-ray analysis (SEM- EDX) serves as good supplement to the optical microscope in examining new, old and deteriorated concrete. SEM analysis was done both for the samples having combination of fine aggregates (DMS and M-Sand) as well for samples with DMS alone. Samples for SEM analysis were collected from destructed pieces.
Thus, various tests as discussed above were conducted on geopolymer mortar as well on geopolymer concrete specimens conforming to relevant codal provisions.
3. Results and discussion
3.1 Compressive strength
The obtained compressive strength values for mixes G10A, G20A, G30A, G40A, G50A and G0A are respectively 30.88, 34.22, 37.78, 41.78, 36.22 and 26.5 MPa and the compressive strength values for mixes G10A’, G20A’, G30A’, G40A’, G50A’and G0A’ are 30.56, 33.85, 36.65, 41.52, 38.5 and 27.76 MPa respectively. The values are plotted in Figures 7 and 8.
The test results yield an inference that addition of GGBS in fly ash increases the compressive strength upto 40% replacement [13, 14]. The presence of calcium content in GGBS seems to be a real contributor for increase in strength upto 40% after which microstructure gets altered causing decrease in compressive strength. The compressive strength of control mix was low compared to other mixes using GGBS.
3.2 Split tensile strength
The split tensile strength after 28 days for mixes G10A, G20A, G30A, G40A, G50A and G0A are 3.54, 3.96, 4.24, 4.81, 4.05 and 2.86 MPa respectively and these values are represented in Figure 9. The test results indicate that the maximum split tensile strength of 4.81 MPa is achieved for G40A mix. This may be due to the fact that addition of GGBS upto 40% increases the consistency, mortar phase and bond strength between mortar and aggregate. However, the split tensile strength of concrete decreased after 40% addition of GGBS which may be due to the glossy surface, surface texture and relatively low density reducing the bond strength and consistency (Figure 10).
The measured density for the mixes G10A, G20A, G30A, G40A, G50A and G0A are respectively 2267, 2306, 2378, 2392, 2384 and 2240 kg/m3 as shown in Figure 10. and the density values for mixes G10A’, G20A’, G30A’, G40A’, G50A’ and G0A are 2254, 2258, 2327, 2385, 2346 and 2217 kg/m3 as shown in Figure 11. Here again, upto 40% replacement, increase in density could be noticed.
3.4 Flexural strength
The flexural strength values of mixes G10A, G20A, G30A, G40A, G50A and G0A are 2.35, 3.59, 4.39, 6.5, 5.84 and 5.35 MPa respectively as shown in Figure 12. Flexural strength test also yields the same inference that increase in strength upto 40% replacement of flyash by GGBS and getting decreased after that.
3.5 Microstructural analysis
EDAX and SEM analysis were carried out on specimens as a part of microstructural analysis  to know the factors contributing to the behaviour of new combination of fine aggregates used in geopolymer concrete.
3.5.1 EDAX analysis
The elemental composition of D20, D40, D60, D80 and D100 specimens are shown in Figures 13–17 which show the presence of Ca, Si, Al confirming the formation of geopolymeric gel as well the process of geopolymerisation. The presence of calcium in geopolymer concrete indicates that the setting time fastens during process. The rapid hardening process through solidification of gel frame work is found to be contributed by C-A-S-H gel. This would facilititate ambient curing conditions.
Elemental Weights obtained from EDAX analysis are given in Table 2. It shall be observed that oxides are present in larger extent in all mixes. Binding property could be weekened by the presence of oxides. The decrease in strength initially (after D20) and subsequent increase in strength (from D80) may be due to the above reason. Si/Al ratio confirms the structure of sialate link as (Sialate- Di Silaxo-D20), (Silate Silaxo-D40), (Silate Tri Silaxo-D60), (Silate Silaxo-D80) and (Silate Tri Silaxo-D100). Si/Al ratio of 1 indicates that the structure is Sialate Silaxo, Si/Al ratio of 2 indicates that the structure is Silate Di Silaxo and Si/Al of 3 represents that it is Silate Tri Silaxo.
|S. No.||Elemental Composition||G10A||G20A||G30A||G40A||G50A|
3.5.2 SEM analysis
From Figure 18, it shall be observed that geopolymer matrix is well formed and the ITZ gap between the aggregates and matrix is less leading to greater compressive strength in case of D20 which confirms the inference obtained earlier that strength decreases after D20. Further, micro cracks are observed in case of D100. (Figure 19).
This project mainly focused on arriving at the optimum percentage replacement of flyash by GGBS in geopolymer concrete. The following conclusions are derived from the investigations carried out on compressive strength, split tensile strength, density measurements and flexural strength:
Compression, split tension as well flexure yielded the same inference that increase in strength was upto 40% replacement of fly ash by GGBS beyond which strength got decreased due to the fact that microstructure gets altered as evidenced by EDAX and SEM analysis.
Workability was observed to be stiff upto 40% replacement requiring use of superplasticisers at the time of casting.
Strength values of specimens subjected to alternate wetting drying conditions were slightly lower compared to ambient cured specimens.