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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012

Influence of Silica Fume on Strength Characteristics of Fly Ash Alumino Silicate Concrete R.Gopalakrishnan1 and K .Chinnaraju2 1

Asst.Professor, Department of Civil Engineering, Sri Venkateswara College of Engineering, Sriperumbudur- 602 105, India.

E-mail: rajagopalan.gopalakrishnan0@gmail.com,1 2 Associate Professor, Division of Structural Engineering, Anna University, Chennai-25, India. geological origin or by product material such as fly ash. In terms of reducing global warming, geo polymer technology could reduce approximately 80% of CO2 emission to the atmosphere caused by cement and aggregate industry [3] compared with ordinary Portland cement concrete, geo polymers show many advantages. Geo polymers show substantially superior resistance to fire [4] and acid attack [5] and much less shrinkage than OPC Concrete. Geo polymer concrete can obtain 70% of the final compressive strength in the first four hours of setting. The compressive strength after 14 days was found in the range of 5-50 MPa. [6] The tensile strength of geo polymer concrete falls within the range observed for OPC based concrete. Also, the flexural strengths are generally higher than the standard model line for OPC based concrete. This favourable behaviour can be attributed to the type of matrix formation in geo polymer concrete.[7] It has been reported that the stress strain relationship of fly ash based geo polymer concrete is almost similar to that of ordinary Portland cement concrete.[8). These advantages make the geo polymer concrete a strong alternative for replacing ordinary Portland cement concrete.

Fresh concrete made with silica fume is more cohesive and therefore less prone to segregation than concrete without silica fume. The main benefit from increased cohesion can be seen in shot Crete whether it is for new construction, repair of existing structures or ground support in tunnelling operations. Because of the very high surface area of the silica fume and the usually very low water content of silica fume concrete, there will be very little, if any bleeding. Silica fume gained initial attention in the concrete market place because of its ability to produce concrete with very high compressive strength. Improvements in other mechanical properties such as modulus of elasticity or flexural strength are also seen.

This paper presents the results of an experimental investigation on strength characteristics of geo polymer concrete prepared by using fly ash and NaOH and Na2Sio3 as alkaline liquid which was taken as activator and a partial replacement to fly ash by Silica fume(Micro silica) with various percentages were investigated.

Abstract - The consumption of Ordinary Portland cement (OPC) caused pollution to the environment due to the emission of CO 2. As such, alternative material had been introduced to replace OPC in the concrete. Fly ash is a by-product from the coal industry, which is widely available in the world. Moreover, the use of fly ash is environmental friendly and save cost compared to OPC. Fly ash is rich in silicate and alumina, hence it reacts with alkaline solution to produce alumina silicate gel that binds the aggregate to produce a good concrete.

The compressive strength increases with the increasing of fly ash fineness and thus the reduction in porosity can be obtained. Fly ash based geo polymer also provided better resistance against aggressive environment and elevated temperature compared to normal concrete. In this present investigation the strength properties of different concentration of molarity of fly ash- geo polymer concrete by partially replacing fly ash by Silica fume was studied. It is concluded that higher concentration of molarity and a partial replacement of silica fume to the fly ash increases the Compressive Strength and Tensile strength of geo polymer concrete.

Index terms - Fly ash, silica fume, geo polymer, alkaline solution, Compressive strength, Tensile strength

I. INTRODUCTION Demand for concrete as construction material is on the increase so as the production of cement. The production of cement is increasing about 3% annually. [1] Among the green house gases CO2 contributes about 65%of global warming.

The production of one ton of cement liberates about one ton of CO2 to atmosphere. Furthermore, it has been reported that the durability of ordinary Portland cement concrete is under examination, as many concrete structures, especially built in corrosive environments start to deteriorate after 20 to 30 years, even though they have been designed for more than 50 years of service life. Although the use of Portland cement is unavoidable in the foreseeable future, many efforts being made to reduce the use of Portland cement in Concrete. In this respect geo polymer is very promising technique.The term Geo polymer describes a family of mineral binders with chemical composition same as Zeolite. Hardened geo polymer concrete has an amorphous microstructure [2] which is quite similar to that of ancient structures such as Egyptian pyramids and Roman amphitheatres. Geo polymer is produced by a polymeric reaction of alkaline liquid with source material of

II. E XPERIMENTAL PROGRAMME The experiments were carried out using two different concentrations of NaOH solution. In the fly ash-geo polymer concrete, the fly ash was replaced by silica fume at the rate of 20 and 40%. Compressive strength and Tensile strength were

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Assistant Professor and Cu rrently Research Schola r @ Anna University, Chennai (corresponding author).

E-mail: rajagopalan.gopalakrishnan0@gmail.com

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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012 conducted at 3, 7, 14 & 28 days. Tests also conducted for Conventional Concrete (M30 Grade) for comparison.

A. Materials 1. Coarse aggregates Coarse aggregates of 20 mm downgraded and specific gravity of 2.67 were used. [9] 2. Fine aggregates Sand confirming to Zone II as per IS 383 [9] was used as fine aggregate for the tests conducted. Locally available river sand having a specific gravity of 2.52 was used as fine aggregate for Geo polymer concrete mixes.

3. Fly ash Low calcium (ASTM Class F) fly ash is preferred as a source material than high calcium (Class C). The presence of calcium in high amount may interfere with the polymerization process.

The chemical analysis of fly ash reveals that the fly ash confirms to the various specifications of IS 3812-1997.

4. Cement The OPC Cement was used for preparing the Conventional concrete having a specific gravity of 3.13 5. Alkaline Solution The sodium hydroxide used in the study was in flake form of 98% purity. The sodium silicate solution (Na2Sio3) used is available locally. The composition of the solution is Na2O=14.7%, Sio2-29.4%, H2O-55.9% by mass.

6. Super plasticizer To improve the workability of the fresh Geo polymer concrete, a super plasticizer Naphthalene sulphonate (DEGUSA) is used.

7.Silica fume The ELKEM Micro silica (920 D) confirming to ASTM C 1240 is used throughout the investigation.

B. Test Variables The variables taken for preparation of geo polymer concrete mixes were 8 M, 12 M concentration for a curing time of 48 hrs. For each of the NaOH concentration, a partial replacement of fly ash was made by micro silica at 20% and 40%. Hence properties of total six concrete mixes have been studied and compared with the conventional concrete of Grade M30. The mix designations have been shown in Table I. The ratio of cement: fine aggregate: Coarse aggregate is 1:1.33:2.76 were taken for the design of M30 Grade concrete.

The ratio of sodium silicate solution to sodium hydroxide solution by mass was kept as 2.5, while the ratio of Activator solution to fly ash by mass was kept as 0.35. The mix proportions were shown in Table II. C. Preparation of Geo polymer Concrete 1. Preparation of solution Separate solutions of NaOH and Na 2Sio3 of required concentrations were prepared 24 h prior to the casting. Both the solutions were mixed together at the time of mixing.

2. Mixing Weighed quantity of fly ash, fine aggregates and coarse aggregates were dry mixed in a pan for about 3 min. After dry 166 Š 2012 ACEE DOI: 02.AETACE.2012.3.505

mixing, wet mixing was done for another 3 min. 12 cubes of 10 x 10 x10 cm3 were prepared for each variable in total 72 cubes were prepared. 24 cylinders of 10 x 20 cm were prepared for testing Split Tensile Test. Compaction was done by applying a manual strokes, followed by compaction on a vibration table for 20 seconds. A handling time of 60 minutes was adopted for the study.

3. Curing After casting the cubes, they were kept (with moulds) in a hot air oven at 100oC for 24 h. Then the cubes were remoulded and the cubes were placed at the oven for another 24 h. Then the cubes were kept at the room temperature for the required curing time until the period of testing (Fig 1 and Fig.2) 4. Tests performed Compressive strength test and Split Tensile test for different concentrations, for different variables were conducted. [10] TABLE I. MIX DESIGNATION

Fig. 1. Cubes in Oven for Curing

Fig. 2. Cubes in ambient condition after Curing in Oven

Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012 TABLE II. MIX PROPORTION KG/ CU.M.

concrete at 28 days.There is an increase of 7.5% in the higher concentration 12M of GPC when compared to 8M concentration at 28 days. The same results were also observed at 7 days. In replacement of 20% by silica fume to the fly ash in GPC, in higher concentration there is an increase of 5.2% when compared to the lower concentration at 28 days.

Similarly, the replacement of 40%, in higher concentration there is an increase of 4.7% when compared to the lower concentration at 28 days. The results indicate that there is an increase of 41.7% of GPC (8M) when compared to conventional concrete (M30). When a partial replacement of 20% by silica fume to the fly ash GPC is done, the percentage increase is 48%.

Similarly a partial replacement of 40% by silica fume to the fly ash in the fly ash GPC, the percentage of increase is 53.4%. In the higher concentration the result indicates that there is an increase of 52.4% of GPC (12M) when compared to the conventional concrete (M30). A partial replacement of 20% by silica fume to the fly ash GPC, the percentage of increase is 56.5%. Similarly, a partial replacement of 40% by silica fume to the fly ash in the fly ash GPC, the percentage increase is 60.7%. The results are shown in (Fig.4 and 5)

Fig. 3. Comparision of Tensile Strength

III. RESULTS AND DISCUSSIONS A. Tensile Strength: Higher concentration gives a higher tensile strength value when compared to the conventional concrete. There is an increase of 5.78% for 12M concentration when compared 8M concentration. (Fig.3).

B. Compressive Strength: The compressive strength test was performed taking three cubes from each set after 3, 7, 14 and 28 days. In 8M concentration, there is an increase of 5% for the 20% replacement of silica fume to the fly ash in geo polymer concrete when compared to the fly ash geo polymer concrete.

There is an increase of 8.3% for the 40% replacement of silica fume to the fly ash in geo polymer concrete compared to the fly ash geo polymer concrete at 28 days .The rate of increase from 7 days to 28 days is the same in GPC as well as 20% and 40% replacement of silica fume.In 12M concentration, there is an increase of 3% for the 20% replacement of silica fume to the fly ash in geo polymer concrete when compared to the fly ash geo polymer concrete. There is an increase of 5.5% for the 40% replacement of silica fume to the fly ash in geo polymer concrete compared to the fly ash geo polymer Š 2012 ACEE DOI: 02.AETACE.2012.3.505

Fig. 4. Overall Comparison of Compressive Strength

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Full Paper Proc. of Int. Conf. on Advances in Civil Engineering 2012 yields better results when compared to lower concentration 6. Further investigation is needed for the complete replacement of fly ash geo polymer concrete to the silica fume geo polymer concrete.

REFERENCES [1]

Rangan, B.V., Hardjto, D, “Development and properties of low calcium fly ash based geo polymer concrete”. Research report GC-1, Faculty of Engineering, Curtin’s University of technology, Perth, Australia, 2005 [2] Bakharev, T, “Geo polymeric materials prepared using class Fly ash and elevated temperature curing”, Cement and concrete research, 35; 1224-1232, 2005.

[3]Rangan, B.V., Wallah, S.E, “Low calcium fly ash based geo polymer concrete long term properties”, Research report GC2, Faculty of Engineering, Curtin University of Technology, Perth, Australia, 2006.

[4]Cheng, T.W. and J.P. Chiu, “Fire resistant Geo polymer”, produced by GGBS, 2003.

[5]Bakharev,T., “Durability of geo polymer materials in sodium and magnesium sulphate solutions”, Cement and Concrete Research,35:1233-1246.

[6] Van Jaarsveld, J.G.S.,”Factors affecting the immobilization of metals in Geo polymerized fly ash”, Metallurgical and Material transactions, 29B(1):283-291.

[7]Soft,D;Van Deventer, Engineering properties of inorganic polymer concretes (IPCs), Cement and Concrete Research.

[8] Hardjito,D and Rangan,B.V.”Development and properties of low calcium fly ash based Geo polymer concrete”, Research report GC-1, Faculty of Engineering, Curtin’s University of technology, Perth, Australia, 2005 [9] IS 383-1970, Specifications for coarse and fine aggregates from natural sources for concrete, 2nd revision [10] IS 516-1959, Methods of tests for strength of concrete

Fig. 5. Rate of development of Compressive Strength

CONCLUSIONS 1. Geo polymer concrete is more environmental friendly and it is suitable for many applications such as precast units to replace conventional concrete.

2. Higher concentration (in terms of molar) of sodium hydroxide solution results in higher compressive strength of fly ash-based geo polymer concrete.

3. There is an increase in the compressive strength of fly ash geo polymer concrete when fly ash is partially replaced by silica fume both in the 8M and 12M concentration respectively.

4. When compared to 8M, 12M concentration shows good results in compressive strength values.

5. Tensile strength values indicates that higher concentration

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