Effect of silica fume on the mechanical properties of fly ash based-geopolymer concrete

Author’s Accepted Manuscript Effect of silica fume on the mechanical properties of fly ash based-geopolymer concrete F.N. Okoye, J. Durgaprasad, N.B. Singh

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To appear in: Ceramics International Received date: 22 August 2015 Revised date: 14 October 2015 Accepted date: 15 October 2015 Cite this article as: F.N. Okoye, J. Durgaprasad and N.B. Singh, Effect of silica fume on the mechanical properties of fly ash based-geopolymer concrete, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.10.084 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of silica fume on the mechanical properties of fly ash based-geopolymer concrete F.N.Okoye1, J.Durgaprasad1 and N.B.Singh2 Department of Civil Engineering, Sharda University, Greater Noida, India 2 Research and Technology Development Centre, Sharda University, Greater Noida, India 1.

Abstract In this paper fly ash based geopolymer concretes with different percentages of silica fume were made by using NaOH/sodium silicate and cured in oven at 100oC. Workability, compressive strength, flexural and tensile strengths were determined. Portland cement concrete was used as a reference. Sodium hydroxide (14M) and sodium silicate were used as alkali activators. The results have shown that addition of silica fume improved the compressive strength of the produced geopolymer concretes. Tensile and flexural strengths also increased as the silica fume content increased. The geopolymer concretes were found quite durable in the presence of 2% H2SO4, 5%Na2SO4 and 5% NaCl. Key words: Geopolymer, Silica fume, Fly ash, Compressive strength, Flexural strength, Tensile strength.

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1. Introduction: The global use of concrete is second only to water. As the demand for concrete as construction material increases, so also the demand for Portland Cement.The cement manufacture is highly energy intensive and each tonne emits about a tonne of CO2, which is a greenhouse gas causing global warming [1-3]. Thus, there is an urgent need to produce an alternative to cement material with adequate strength and durability in order to make cement industry more eco-friendly and sustainable. A new binding material known as ‘geopolymer’ was first introduced by Davidovits in 1978 [4]. The reaction of aluminosilicate materials such as fly ash [5-8], metakaoline [9-12], silica fume [13,14], slag [15,16], rice-husk ash [17], red mud [18], etc with highly alkaline solutions (hydroxides, silicates) produces geopolymers. Unlike ordinary Portland cement (OPC), geopolymers do not require calcium-silicate-hydrate(C-S-H) gel for matrix formation and strength, but utilize the polycondensation of silica and alumina precursors to achieve the required strength level. In recent years considerable amount of research work on geopolymer cement and concrete is being carried out to elucidate the mechanism of formation, strength development and durability [5]. In our earlier publication we reported the effect of metakaoline on the properties of fly ash based geopolymer concrete [19]. In this paper we have studied the effect of silica fume on the mechanical properties of fly ash based geopolymer concrete. 2. Experimental 2.1. Materials Low calcium fly ash (Class F) (ASTM C618) was used in this investigation. The fly ash used was obtained from National Power station, Dadri, Uttar Pradesh, India. Silica fume was obtained from Counto microfine products Pvt. Ltd, Pissurlem industrial estate, Pissurlem, Sattari, Goa, India, ( BS EN 13263-1 (2005)). OPC-43 was used for making reference concrete. The chemical compositions of OPC, fly ash and silica fume are given in Table 1. Coarse aggregates of sizes 20mm and 10mm were used. Sieve analyses were performed to determine the particle size distribution as prescribed in BS 812, Part1, 1975 while fine aggregate used was river sand and graded as prescribed in BS 812, Part1, 1975 (Fig.1). The physical properties of coarse and fine aggregates are given in Table 2.

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Sodium hydroxide in the form of pellets with 98% purity and commercially available sodium silicate solution were used. The sodium silicate solution used had a silicon dioxide to sodium oxide ratio (SiO2/Na2O) of 2.0 with 60% water by the total weight. 2.2 Preparation of alkali Solution of sodium hydroxide (14M) was prepared and left for 24 hours before mixing with sodium silicate. The mixture of sodium hydroxide and sodium silicate solutions were left for one day and then used for geopolymerisation process. 2.3 Mix proportion of geopolymer concrete The designs of geopolymer concretes with fly ash were similar to that of OPC concrete. Coarse and fine aggregates were taken as 77% by mass of the entire mixture. The concentration of NaOH solution was 14M as this concentration gave the highest strength [19]. 1% Naphthalene sulfonate based superplasticiser was used to improve the workability of fresh geopolymer mix. Higher W/S ratio improved the workability but the compressive strength was reduced. So a fixed W/S ratio (0.2) was used in order to have higher compressive strength. The detailed mix design of geopolymer concrete mixes are given in Table 3. Geopolymer mixes with different amount of silica fume (5%, 10%, 15% , 20%, 30% and 40% ) were also made. A control mix was cast as M40 with Portland cement concrete to compare the performance of geopolymer concrete. 2.4 Workability test The workability of the fresh concretes was determined by using slump cone test in compliance with BS EN 12350-2:2000 standard. 2.5 Casting of geopolymer concrete Casting of geopolymer concrete was done at room temperature in the laboratory in a similar way as described earlier [19]. After mixing, the concrete mixture was cast in a 100mm x100mm steel mould in three layers, and each layer was given 60 strokes with 20mm compacting rod. Six concrete cubes were cast for each mix beside the trial mixes for compressive strength. The split tensile and flexural tests were performed with cylindrical moulds of 150mm diameter and 300mm height as per ASTM C 496-90 requirement and beam mould of 100mm x 100mm x 500mm in compliance with EN 12390-51997 requirement respectively.

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2.6 Curing and testing of geopolymer specimens The concrete cubes were demoulded after 48 hours and cured in an oven at 100oC for 72 hours since at this temperature, strength was found maximum [19]. The specimens were left at room temperature until the day of testing. The compressive strength of the cubes were determined after 3, 7, 14, 21 and 28 days while the specimens in cylindrical forms for tensile and flexural strengths were left at room temperature for 28 days before testing. 2.7 SEM studies SEM pictures of GPF and GP4 hydrated for 28 days were recorded with Quanta FEG 250 ESEM instrument. 2.8 Durability tests After curing, the concrete cubes (M40 and GP4) were immersed in 2% sulphuric acid, 5% sodium sulphate and 5% sodium chloride solutions separately. The compressive strength of each cube was determined after 3, 28, 56 and 90 days. SEM picture of GP4 exposed to 2% H2SO4 was also recorded. For each test three cubes were immersed in the solutions and the strength measured. The values plotted were the average values which deviated within a limit of 5%. 3. Results and discussions 3.1. Workability Workability of geopolymer concrete was studied using slump cone test. It was observed that the workability of geopolyer was low as compared to that of control (Fig.2). The fresh geopolymer concrete formed pellets when mixed properly in a drum concrete mixer. Although the mixes were highly viscous, even then adequate compaction could be achieved. To improve the workability of fresh geopolymer mix, Naphthalene sulfonate based superplasticizer was used. The slump of M40 concrete with 100% OPC was higher than those of geopolymer concretes with fly ash and silica fume. It was found that the slump of geopolymer concretes decreased with the increase of silica fume content. The poor workability of geopolymer concretes could be attributed due to high viscosity of the mixes. The geopolymer mixes were more viscous than the OPC concrete due to cohesiveness of the system [20, 21]. When sodium silicate solution was added in the

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mixes, the binding of the fine and aggregates occurred and the system became sticky. As a result of this the slump was reduced. 3.2. Compressive strength The variation of compressive strength of different mixes with time is shown in Fig. 3. The compressive strength increased continuously with time in the following sequence and the minimum value was for M40. M40 < GP1 < GP2 < GP3 < GP4 The relative percentage increase in compressive strength with reference to control (M40) is given in Table 4. From the table it is quite clear that the percentage increase is maximum (almost double) in the case of GP4 containing 40% SF for 3days. The variation of compressive strength with silica fume at 28 days is shown in Fig. 4. The compressive strength increased with the increase of silica fume. The parameters that affect compressive strength of geopolymer concretes are: type and concentration of alkaline activator, the curing temperature, the curing time, relative amounts of Si, Al, K, Na and molar ratio of Si to Al present in solution [22]. The presence of silicate ions in the alkaline solution substantially improved the mechanical strength and modulus of elasticity values. It is also reported that the H2O/M2O molar ratio in the mixture significantly affected the compressive strength [22]. Skvara et al., [23] found that the Na2O content and SiO2/Na2O ratio of geopolymer mix significantly affects pore characteristics and compressive strength. Dutta etal [13] studied the effect of 5% silica fume on porosity and compressive strength of geopolymer paste and mortar. They found that the compressive strength of paste decreased with the addition of silica fume because of increase of porosity. On the other hand the compressive strength of mortar increased due to decrease of porosity. Addition of silica fume which has a high percentage of SiO2 could have hindered the process of geopolymerisation. This should be attributed to the fact that with increased silica fume content, Na2O required for complete dissolution is not available in the activator solution. Due to this, a part of silica fume remained unreacted in the produced geopolymer gel. However, in case of mortars , these excess fine silica fume particles may enter in the voids and enhance the compactness. As a result of this, the porosity may decrease. These significant variations in the geopolymer specimens may affect their mechanical properties. Porosity has been reported to be the main microstructural variable limiting the mechanical properties of 5

geopolymers [24]. Adak etal [14] studied the effect of colloidal nanosilica on the compressive strength of geopolymer mortars and found that 6% additions increased the strength but beyond that there was detrimental effect. However, in the present case the effect of silica fume on compressive strength of geopolymer concrete was studied. It was found that the strength increased continuously upto 40 % silica fume addition (maximum silica fume added in the present experiment). Since the concrete has a porous structure, silica fume which consists of spherical fine particles of amorphous silicon dioxide (may be of nanodimension) enters the pores making the structure more compact. Due to additional amount of amorphous silica, larger quantity of aluminosilicate gels might be formed in the geopolymer matrix. To some extent it can be detected by XRD pattern. Thus both the effects may generate dense geopolymer structure with increased compressive strength. SEM pictures of GPF and GP4 hydrated for 28 days were recorded and are given in Fig.5. Morphology of GPF shows fractured surfaces whereas GP4 shows compact and smooth surface. These morphological differences may also be responsible for compressive strength differences. 3.3. Flexural and Tensile strength Flexural test is the most common test conducted on hardened concrete. The flexural strength increased continuously as the percentage replacement of fly ash with silica fume increased (Fig.6). Tensile strength of concrete determines the load at which the concrete cracks. The tensile strength also increased as the percentage replacement of fly ash with silica fume increased, first slowly and then rapidly (Fig.7). In general both the flexural and tensile strength of fly ash based geopolymer concretes containing silica fume followed the same pattern as compressive strength. All the mix proportions containing silica fume performed better than the control mix.

3.4 Durability The variation of compressive strength (average of 3 samples) after immersion in different chemical environments is shown in Fig. 8. From the figure, it is quite obvious that there is a rapid loss in compressive strength in the concrete made from OPC (M40). Compressive strength at different intervals of time in different chemical environment is given in Table 5. M40 sample was severely deteriorated on exposure to sulphuric acid and Na2SO4 for 90 days. Strength losses in M40 are possibly connected to the decomposition of hydration products 6

and migration of alkalis from the specimens into the solution imposed by the acid attack. However, in presence of 5% NaCl, there is some increase in compressive strength in the case of M40. It appears that due to porous nature of concrete, Cl- ions enter and accelerate the hydration upto certain period of time causing an increase in the strength and after that there is detrimental effect. In the case of GP4, there is a very little loss in compressive strength. It is seen that there is slight increase in strength in the case of GP4 in 5% NaCl after 56 days. This may be due to some passivation effect. The small loss occurred may be due to some depolymerization of aluminosilicate. SEM picture (Fig.9) of GP4 kept for 90 days in 2% H2SO4 indicates some morphological change supporting some deterioration. Both physical and chemical interactions occur within concrete when it is exposed to aggressive chemical environment. The movement of deteriorating ions in concrete is a function of several variables. These include the concentration of aggressive chemical at the concrete surface, pore size and spacing, pore volume fraction, changes in pore size with respect to location within the cement paste, and chemical composition of phases present in the hydrated cement paste. During hydration reaction OPC yields C-S-H gels while during polymerization, geopolymer binders yield N-A-S-H, C-A-S-H or C-S-H gels. Due to the total contrast of hydration reaction and the reaction products, geopolymer binders are reported to have superior resistance towards sulfate and acid attacks [25]. It is reported that geopolymer materials have better resistance in the presence of H2SO4 than Portland cement based specimens [26]. Duan etal [10] have reported that geopolymer presents denser microstructure, lower total pore volume and optimized pore structure compared to OPC paste and therefore geopolymer concrete is much more durable in an aggressive environment. It is reported that NO3− or

SO4−2 ions have some deteriorating effect on compressive strength of

geopopolymer concrete [27]. Probably both the anions consume some of the available alkali activators hindering geopolymerisation reactions.

4. Conclusions The effect of different concentration of silica fume on mechanical properties of fly ash based geopolymer concrete was studied and results are discussed. It is found that in the presence of silica fume, the workability of the paste decrease. The geopolymer concrete containing silica fume showed higher compressive, tensile and flexural strengths as compared to that of control and these values increased with the increase of silica fume content. This was due to 7

an increase in compactness and denser microstructure. Geopolymer concretes were found to be more durable in 2% H2SO4, 5% NaCl and 5% Na2SO4 solutions. Still there is a significant gap of knowledge related to the geopolymerization reaction, strength development and durability issues. Thus there is a need to investigate all these problems in detail. Acknowledgement One of us (FNO) wishes to express his profound gratitude to Federal government of Nigeria through the TET FUND for their financial support, without which, it wouldn't have been possible to carry out this research work. We are also thankful to Federal Polytechnic Oko, Nigeria for their moral support.

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References 1. D.N. Huntzinger, T.D.Eatmon, A life-cycle assessment of Portland cement manufacturing: comparing the traditional process with alternative technologies, J.Clean.Product.17(2009)668–675. 2. K.H. Yang, J.K.Song , K.I.Song , Assessment of CO2 reduction of alkali-activated concrete, J.Clean.Product.39(2013)265–272. 3. K. Komnitsas, Potential of geopolymer technology towards green buildings and sustainable cities, Procedia Eng. 21(2011) 1023-1032. 4. J.Davidovits, High-alkali cements for 21st century concretes. In: Kumar Metha, P. (Ed.), Concrete Technology, Past, Present and Future. Proceedings of V. Mohan Malhotra Symposium ACI SP, vol. 144(1994)383–397. 5. B. Singh, G. Ishwarya, M. Gupta, S.K. Bhattacharyya, Geopolymer concrete: A review of some recent developments, Constr. Build. Mater.85(2015)78-90. 6. T. Xie, T. Ozbakkaloglu, Behavior of low-calcium fly and bottom ash-based geopolymer concrete cured at ambient temperature, Ceram. Int. 41(2015), 5945– 5958. 7. M.T. Junaid, O.Kayali, A. Khennane, J. Black, A mix design procedure for low calcium alkali activated fly ash-based concretes, Constr. Build. Mater.79 (2015) 301– 310. 8. A.A. Adam, Horianto, The effect of temperature and duration of curing on the strength of fly ash based geopolymer mortar, Procedia Eng. 95 ( 2014 ) 410 – 414 9. Faten Slaty, Hani Khoury, Hubert Rahier, Jan Wastiels, Durability of alkali activated cement produced from kaolinitic clay, Appl. Clay Sci. 104(2015)229–237. 10. Ping Duan , Chunjie Yan, Wei Zhou , Wenjun Luo , Chunhua Shen , An investigation of the microstructure and durability of a fluidized bed fly ash–metakaolin geopolymer after heat and acid exposure, Mater. Des. 74(2015)125–137. 11. F.G.M. Aredes, T.M.B.Campos, J.P.B.Machado, K.K.Sakane, G.P.Thim, D.D.Brunelli, Effect of cure temperature on the formation of metakaolinite-based geopolymer , Ceram. Int. 41(2015)7302–7311. 12. Isil Ozer,SezenSoyer-Uzun, Relations between the structural characteristics and compressive strength in metakaolin based geopolymers with different molar Si/Al ratios, Ceram. Int. 41(2015)10192–10198. 13. D Dutta, S Thokchom, P Ghosh and S Ghosh, Effect of silica fume additions on porosity of fly ash geopolymers, ARPN J. Eng. App. Sci. 5(2010)74-79. 14. D. Adak, , M.Sarkar, S. Mandal, Effect of nano-silica on strength and durability of fly ash based geopolymer mortar, Constr. Build. Mater. 70 (2014) 453–459. 15. Phoo-ngernkham, T. Maegawa, A. Mishima, N. Hatanaka, S. Chindaprasirt P, Effects of sodium hydroxide and sodium silicate solutions on compressive and shear bond strengths of FA–GBFS geopolymer , Constr. Build. Mater.91 (2015) 1–8. 16. S. Kumar, R. Kumar, S.P. Mehrotra, Influence of granulated blast furnace slag on the reaction, structure and properties of fly ash based geopolymer. J. Mater Sci. 45(2010)607–15. 17. B.Prabu, A. Shalini and J. S. Kishore Kumar, Rice Husk Ash Based Geopolymer Concrete – A Review, Chem. Sci. Rev. Lett. 3(2014)288-294. 18. M. Vukevi, D. Turovi, M. Krgovi, I. Boskovic, M. Ivanovi, R. Zejak, Utilization of Geopolymerization for obtaining construction materials based on red mud, Materials and technology 47 (2013)99–104 19. F.N.Okoye, J.Durgaprasad and N.B.Singh, Mechanical properties of alkali activated flyash/Kaoline based Geopolymer Concrete, Constr. Build. Mater.98(2015)685-691. 9

20. B.V. Rangan, Low calcium fly ash based geopolymer concrete. In: Concrete Construction Engineering Handbook. E.G. Nawy (Ed.). Taylor and Francis, London, U.K. 2008. 21. P. Chindaprasirt, T. Chareerat, V.Sirivivatnanon, Workability and strength of coarse high calcium fly ash geopolymer, Cem. Concr. Compos. 29(2007) 224-229. 22. D. Bondar, Geo-polymer Concrete as a New Type of Sustainable Construction Materials , Third International Conference on Sustainable Construction Materials and Technologies,http://www.claisse.info/Proceeding.htm. 23. F. Skvara, L. Kopecky, J. Nemecek, Z. Bittnar. 2006. Microstructure of Geopolymer Materials based on fly ash, Ceram. Silik. 50(2006) 208-215. 24. B.A. Latella, D.S. Perera, D. Durce, E. G. Mehrtens, J. Davis. 2008. Mechanical properties of metakaolin-based geopolymers with molar ratios of Si/Al ≈ 2 and Na/Al ≈1, J Mater Sci. 43(2008) 2693-2699. 25. P. Wei Ken, M. Ramli, C.C. Ban, An overview on the influence of various factors on the properties of geopolymer concrete derived from industrial by-products , Constr. Build. Mater.77 (2015) 370–395 26. T. Bakharev Resistance of geopolymer materials to acid attack. Cem. Concr. Res. 35(2005)658–70 27. K. Komnitsas , D. Zaharaki, G. Bartzas, Effect of sulphate and nitrate anions on heavy metal immobilisation in ferronickel slag geopolymers, Appl. Clay Sci. 73 (2013) 103109

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28. Table 1 Chemical composition of binders Composition (%) Constituents OPC Fly ash SiO2 CaO MgO P2O5 Na2O K2O MnO Al2O3 Fe2O3 SO3 Loss of ignition

19.01 66.89 0.81 0.08 0.09 1.17 0.19 4.68 3.20 3.00 2.48

50.70 2.38 1.39 0.84 2.40 28.80 8.80 0.30 3.79

Silica fume 93.67 0.31 0.84 0.40 1.10 0.84 0.83 1.30 0.16 2.10

Table 2 Physical properties of gravels and sand Sample

Specific Gravity 2.5 2.4 2.6

20 mm aggregate 10 mm aggregate Sand

Water absorption (%) 0.17 0.87 -

Fineness modulus 2.7 2.8 2.1

Table 3 Mix proportion of geopolymer concrete MIX NO

Quantity of ingredients (kg/m3)

GPF GP1

Coarse Aggregate 20mm 10mm 862 431 862 431

Fine Sand 554 554

Fly Ash 400 380

Silica fume OPC 0 0 20 0

GP2 GP3 GP4 GP5 GP6 M40

862 862 862 862 862 862

554 554 554 554 554 554

360 340 320 280 240 0

40 60 80 120 160 0

431 431 431 431 431 431

0 0 0 0 0 400

SS 113 113

NaOH S P (14M) 45 4.0 45 4.0

ALK/ W/S FA 0.4 0.2 0.4 0.2

113 113 113 113 113 N.A

45 45 45 45 45 N.A

0.4 0.4 0.4 0.4 0.4 N.A

4.0 4.0 4.0 4.0 4.0 4.0

0.2 0.2 0.2 0.2 0.2 0.3

FA-Fly ash, SS-Sodium silicate, SP-Superplasticisers, ALK-Alkaline, W/S-Water/Solid ratio 11

Table 4 Percentage increase in compressive strength at different time interval with reference to control (M40) Mix Increase in compressive strength in relation to control (%) at different days 3d 7d 14d 21d 28d M40 GPF 0 2.9 14.6 5.9 0 GP1 46.5 26.6 26.4 19.6 1.5 GP2 22.9 40.6 39.9 32.3 9.3 GP3 46.5 26.3 27.3 23.8 12.4 GP4 110.0 82.9 90.5 79.9 51.2 Table 5 Compressive strengths Mix

M40

Comr.Strength Unexposed (N/mm2) 28d 48.5

GP4

69.7

Compressive strength exposed in different chemical environment (N/mm2) 3d 43.8 (2% H2SO4) 38.3 (5% Na2SO4) 51.4 (5%NaCl) 59.9 (2% H2SO4) 58.3 (5% Na2SO4) 61.2 (5%NaCl)

28d 41.2 (2% H2SO4) 35.3 (5% Na2SO4) 50.4 (5%NaCl) 59.8 (2% H2SO4) 60.8 (5% Na2SO4) 61.52 (5%NaCl)

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56d 33.1 (2% H2SO4) 34.0 (5% Na2SO4) 44.0 (5%NaCl) 58.3 (2% H2SO4) 63.2 (5% Na2SO4) 59.9 (5%NaCl)

90d 32.6 (2% H2SO4) 33.0 (5% Na2SO4) 42.0 (5%NaCl) 58.0 (2% H2SO4) 57.6 (5% Na2SO4) 63.1 (5%NaCl)

Fig. 1 Grading curve of 20mm and 10mm coarse aggregates and sand

Fig 2 Slump of fly ash based geopolymer concrete with various amounts of silica fume in relation to control

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Fig 3 Compressive strength of fly ash based geopolymer concrete in presence of silica fume in relation to control

Fig.4 Effect of silica fume on compressive strength 14

GPF

GP4

Fig. 5. SEM picture of GPF and GP4 hydrated for 28 days

Fig 6 Flexural strength of fly ash geopolymer concrete with different proportions of silica fume 15

Fig 7 Tensile strength of fly ash based geopolymer concrete with different proportions of silica fume

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Fig. 8 Durability properties of fly ash based geopolymer concrete blended with silica fume

Fig. 9 SEM picture of GP4 stored in 2%H2SO4 for 90 days

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