Resistance to sodium sulphate and sodium chloride attack of fly ash based geopolymer concrete

Transcription

1 Resistance to sodium sulphate and sodium chloride attack of fly ash based geopolymer concrete Bhagia Maria Joshy 1 Dr.Mathews M Paul 2 1P G Student, Mar Athanasius college of engineering, Kothamangalam, Kerala 2 Professor, Mar Athanasius college of engineering, Kothamangalam, Kerala Abstract Among the hydraulic binders common in our modern world which remain the most used is Portland cement. Even though cement is a versatile construction material and is being used worldwide extensively, the green house gas (CO 2) produced during its manufacturing process causes environmental impact. However concrete made out of geopolymer technology replaces cement completely in it and thereby reduces the said environmental deterioration. Geopolymers are formed by alkaline activation of an aluminosilicate material like fly ash, slag, metakaolin, rice husk ash etc. They are also known as alkali activated concrete. This paper presents the experimental investigation done on performance of fly ash based geopolymer concrete subjected to severe environmental conditions. The grade chosen for the investigation was M 25, the mix was designed for Molarity (M) of 10 M. The test specimens were 150x150x150 mm cubes heat-cured at 90 C in an oven for 24 hours. Durability of specimens in terms of chloride and sulphate attacks were assessed by immersing GPC specimens in 0.25 M NaCl, 0.5 M NaCl, 0.75 M NaCl, 4% Na 2So 4, 5% Na 2So 4, 6% Na 2So 4 solutions separately and periodically monitoring surface deterioration, changes in weight and compressive strength over a period of 14, 28, 56 and 90 days. The test results indicate that the heat-cured fly ash-based geopolymer concrete has an excellent resistance to chloride and sulphate attack when compared to conventional concrete. Keywords geopolymer concrete,durability,fly ash,sulphate attack I. INTRODUCTION The definition of concrete is the mixture of cement, water, aggregates, additives or sometimes superplasticizers. Among hydraulic binders common in our modern world, which remain the most used is Portland cement. So the most important part of concrete is cement. However, production of Portland cement is a resource exhausting, energy intensive process that releases large amounts of the green house gas CO 2 into the atmosphere. The most effective way to decrease the CO 2 emission of cement industry is to substitute a proportion of cement with other materials. These materials are called supplementary cementing materials. Usually used supplementary cementing materials are Ground Granulated Blast Furnace Slag, Fly Ash, Silica Fume, Trass, Metakaolin etc. Nevertheless, many studies are being carried out to design new hydraulic binders which can not only substitute Portland cements, but can also be produced in a way as to safeguard our environment. Among these hydraulic binders, geopolymer cements present interesting potential. Geopolymer concrete is also known as alkali activated concrete. The geopolymer technology is proposed by Davidovits [1] and gives considerable promise for application in concrete industry as an alternative binder to the Portland cement. In 1970 s Davidovits stated that, geopolymers are a new class of three dimensional silico-aluminate materials. The raw materials commonly used are those rich in alumino-silicate such as natural pozzolan, like fly ash, blast furnace slag, and calcined kaolinite clays. Geopolymers are formed by alkaline activation of an aluminosilicate material like fly ash, slag, metakaolin, rice husk ash, activated bentonite clay, red mud etc. There are two main constituents of geopolymers, namely the source materials and the alkaline liquids. The source materials for geopolymers based on alumina-silicate should be rich in silicon (Si) and aluminium (Al). The alkaline liquids are from soluble alkali metals that are usually Sodium or Potassium based. The most common alkaline liquid used in geopolymerisation is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate or potassium silicate. Geopolymerization involves the chemical reaction of alumino-silicate oxides (Si 2O 5,Al 2O 2) with alkali polysilicates yielding polymeric Si O Al bonds. This investigation presents the sulphate attack and chloride attack on geopolymeric materials prepared using Class F fly ash and sodium silicate, sodium hydroxide and cured thermally at 90 C for 24 hours. For sulphate attack 4% Na 2So 4, 5% Na 2So 4, 6% Na 2So 4 were used and for chloride attack also three different Molarities, namely 0.25 M NaCl, 0.5 M NaCl, 0.75 M NaCl. A. Scope II. SCOPE, SIGNIFICANCE AND OBJECTIVE Literature survey reveals that there exists a gap area in the durability study of geopolymer concrete with fly ash as source material. Hence to evaluate the durability of geopolymer concrete mixture with flyash in terms of sulphate and chloride attack shows a potential area of study. For making workable, high strength and durable geopolymer concrete containing fly ash without usage of ordinary portland cement. 22

2 B. Significance This investigation aims to reduce the usage of ordinary Portland cement and to improve the usage of the other by product fly ash. This product helps in reducing the carbon emissions caused by the manufacturing of cement. This also produces high strength concretes with the use of nominal mixes when compared to conventional concrete C. Objective Durability can be investigated by considering different parameters such as mechanical behavior, chloride migration, carbonation, leaching, permeability, desorption isotherms, porosity etc. In the present investigation, the performance of geopolymer concrete mixture in terms of chloride attack and sulphate attack was investigated. The approximate molarity of sodium chloride in sea water is 0.5 M, so chloride solution of strengths 0.75 M, 0.5 M and 0.25 M were used for the study of chloride attack. Similarly the percentage of sulphate content in the soil is approximately 5%, so sulphate solution of strengths 4%, 5% and 6% were used for the study of sulphate attack. A. Fly ash(source material) III. MATERIALS AND METHODS One constituent of geopolymers is the source material. In this study fly ash is used as source material. Low calcium (ASTM class F) fly ash is used as source material. The presence of calcium in high amount may be interfering with the polymerization process. Fly ash used in the study is having specific gravity 2. B. Alkaline liquid The most common alkaline liquids used in geopolymerisation are a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and Sodium Silicate or Potassium Silicate. A combination of sodium hydroxide (NaOH) and Sodium Silicate solutions were used in the present study. The sodium hydroxide pellets with 97% to 98% purity was used. The concentration of Sodium Hydroxide solution can vary in the range between 8 Molar and 16Molar, however 8 M solution is adequate for most applications. In this investigation 10 Molar solutions was used. The sodium silicate solution with SiO 2-to-Na 2O ratio by mass of approximately 2 was used. The Ratio Sodium Silicate Solution to Sodium Hydroxide Solution by mass was taken as 2.5 C. Aggregates Aggregates used in this study are fine aggregate and course aggregates. Manufacturer s sand was used fine aggregate. It was water washed properly and then sieved well. Coarse aggregate occupies more than 70% of the volume of concrete. 20 mm and 12.5 mm coarse aggregates were used in the study in the ratio 60:40. The physical properties of the course and fine aggregate were tested as per relevant IS Specifications [12,13] and it was observed that all the physical properties are satisfied and can be used for concrete manufacture. D. Water Water used in the study was potable water. The only purpose of adding water to geopolymer concrete is to provide workability. After the curing process, water is released as a byproduct. Studies show that as the amount of water increases, the strength of geopolymer concrete decreases. E. Methodology One typical grade of concrete was identified (M 25). Using the available mix design procedure the mix was designed. Then required number of concrete cubes of size 150 x150 x150 mm was prepared. The cubes were demoulded on the third day and some specimens were cured in ambient conditions of the laboratory and the other specimens were subjected to temperature curing at 90 C for 24 hours. Each concrete cubes were weighed and the weights were noted. Visual observations were done for the edges and corners. On the 7 th day sufficent number of temperature cured cubes were immersed in NaCl and Na 2So 4 solutions of three different strengths for specified number of days ( 14, 28, 56 and 90 days). The control specimens were not immersed in the solutions, instead they were subjected to air curing at room temperature for specified number of days ( 14, 28, 56 and 90 days). The immersed cubes were removed from the respective solutions after specified number of days. Physical inspection for the edges and corners were done. The samples were weighed after drying. The compressive strength of the immersed cubes was found out after 24 hours. The results were compared with the corresponding control specimens. 23

3 A. Mix proportions IV. EXPIRIMENTAL PROGRAMME Currently no standardized methods of mix design for geopolymer concrete mixes are available. So mixes are essentially designed by trials as of now. Many researchers has proposed many mix designs for development and manufacture geopolymer concrete. The Curtin University has [3] suggested several mix proportions for the development and manufacture of geopolymer concrete. In this study one of them was selected and many trials were done and Quantity of materials required in 1 m 3 of concrete is given in Table I Ratio of fly ash to alkaline liquids Fly ash TABLE I. Fine aggregate MIX PROPORTION Coarse aggregate Alkaline liquids NaOH Na 2SiO B. Prepartion of liquids The Sodium hydroxide (NaOH) solids were dissolved in water to make the solution. The molarity of NaOH solution used in this study is 10 molar i.e. 400gm of NaOH is dissolved in 1000 litres of water.the sodium silicate solution and the sodium hydroxide solution were mixed together at least one day prior to use. On the day of casting the specimens, the alkaline liquid was mixed together with the extra water to prepare the liquid component of the mixture. C. Manufacturing of fresh concrete, casting and curing Mixing of all the materials were done Using concrete pan mixer in laboratory at room temperature. The mixing of geopolymer concrete involves two phases. Initially the mixing of fly ash along with combined aggregate in dry condition was done. Later alkaline solution and extra water was added to commence the wet mixing. The fly ash and aggregate first mixed homogenously for 3 minutes and then alkali solutions which were made one day before and water was added to mixture of fly ash and aggregate. Then mixing was continued for about 4 minutes to get fresh geopolymer concrete. The mixing of total mass were continued until binding paste cover all the aggregate and become homogenous and uniform in colour. Before pouring into moulds the slump value of concrete was tested. Then cube were prepared. Each cube specimen was casted in three layers by hand compacting and then it was vibrated in table vibrator for 15 seconds. In each layer 35 strokes compaction by standard compaction rod was given. The cubes were demoulded on the third day and they were subjected to temperature curing at 90 C for 24 hours. Control concrete was cured by ambient curing. D. Durability study In order to study the durability charecteristics, that is, the sulphate attack and chloride attack the geopolymer concrete cubes were immersed in sodium chloride and sodium sulphate solutions. The cubes were demoulded on the third day and they were subjected to temperature curing at 90 C for 24 hours. Then the cubes were kept in the room temperature upto 7 th day and on the 7 th day the cubes were dipped in the respective solutions. The test specimens soaked in liquids were removed from the immersion container, after 14, 28, 56, 90 days and their surface were wipped out and then their corresponding compressive strengths were noted after 24 hours. As mentioned earlier sodium chloride solution of three different strengths were used. They are 0.25M NaCl, 0.5M NaCl and 0.75M NaCl solutions. Similarly sodium sulphate solution was also of three different strengths. They are 4% Na 2So 4, 5% Na 2So 4, 6% Na 2So 4 were used. The compressive strength of specimens without any exposure was taken as the reference compressive strength. E. Test conducted on concrete On fresh geopolymer concrete, workability was determined by slump test. The maximum slump obtained was 60 mm. Since workability could not be enhanced at the cost of compressive strength. The water to geopolymer ratio could not be increased beyond certain limit. On hardened concrete compressive strength test was conducted. Out of many test applied to the concrete, this is the most important one which gives an idea about all the characteristics of concrete. The test was conducted according to relevant IS codes [14]. 24

4 A. Compressive strength V. RESULTS AND DISCUSSION The variation of the cube compressive strength immersed in NaCl solution with respect to duration of immersion in days and strength of the solution are depicted in Figures 1 and 2. Figure 1: Variation of cube compressive strength immersed in NaCl solution with respect to Figure 2: Variation of cube compressive strength immersed in NaCl solution with respect to strength of solution Figure 3: Variation of cube compressive strength immersed in Na 2So 4 solution with respect to Figure 4: Variation of cube compressive strength immersed in Na 2So 4 solution with respect to strength of solution Figure 5. Variation of weight loss of cubes immersed in NaCl solution with respect Figure 6. Variation of weight loss of cubes immersed in Na 2So 4 solution with respect to The variation of the cube compressive strength immersed in Na 2So 4 solution with respect to duration of immersion in days and strength of the solution are depicted in Figures 3 and 4. The loss of strength 25

5 observed was connected to depolymerisation of the aluminosilicates. The visual appearance of the test specimens after soaking in sodium sulphate solution and sodium chloride solution up to 90 days revealed that there was no change in the appearance of the specimens compared to the condition before they were exposed. There was no sign of surface erosion, cracking or spalling on the specimens. Results show that compressive strength for both types decreases on exposure of 14, 28, 56 and 90 days duration. The surface deterioration was not observed in both sulphate and chloride immersion for geopolymer concrete. B. Weight loss The change in weight of geopolymer concrete cubes exposure to sodium chloride and sodium sulphate solution were observed. All the exposed specimens recorded weight loss and it was observed that the weight loss in case of sulphate attack was more when compared to chloride attack. The variation of the weight loss of cubes immersed in NaCl solution and Na 2So 4 solution with respect to are depicted in Figures 5 and 6. VI. CONCLUSION On the basis of results obtained during the experimental investigations, following conclusions were drawn: There was no substantial gain in the compressive strength of heat-cured fly ash based geopolymer concrete with age. Fly ash based geopolymer concrete cured in the laboratory ambient conditions gains compressive strength with age. The 7th day compressive strength of ambient-cured specimens depends on the average ambient temperature during the first week after casting; higher the average ambient temperature higher is the compressive strength. The geopolymer concrete indicated minor changes in weight and strength when the specimens were exposed to sodium chloride and sodium sulphate solutions of different molarities. The compressive strength loss for the geopolymer concrete specimens exposed in sodium chloride was in the range of 8% to 41%. The compressive strength loss for the geopolymer concrete specimens exposed in sodium sulphate was in the range of 7 to 38 %. Fly ash based geopolymer concrete have no visible signs of surface deterioration, and spalling of concrete after immersion in aggressive solutions for 14, 28, 56 and 90 days of exposure. The test result shows that heat-cured fly ash based geopolymer concrete has an excellent resistance to Chloride attack. There was no damage to the surface of test specimens after exposure to sodium chloride solution up to 90 days. There are no significant change in the mass and the compressive strength of test specimens after an exposure period of 90 days. This proves geopolymer concrete can be used in sea water area. The test results also demonstrate that heat-cured fly ash-based geopolymer concrete has an excellent resistance to sulphate attack. There is no damage to the surface of test specimens after exposure to sodium sulphate solution up to 90 days. The strength loss observed was connected to depolymerisation of the aluminosilicate. Heat cured geopolymer concrete possesses excellent mechanical properties and durability for aggressive environment when compared to ordinary Portland cement concrete. So, Fly ash based geopolymer concrete has excellent compressive strength and is suitable for structural applications. ACKNOWLEDGMENT We express our sincere gratitude to all the staff, teachers and students of M. Tech section of Civil Engineering Department of Mar Athanasius College of Engineering for their help and support. REFERENCES [1] T. Bakharev, Resistance of geopolymer materials to acid attack Cement and Concrete Research 35, 2004,pp [2] T. Bakharev, Durability of geopolymer materials in sodium and magnesium sulfate solutions Cement and Concrete Research 35,2004, pp [3] S. E. Wallah and B. V. Rangan, low-calcium fly ash-based Geopolymer concrete: long-term Properties, Research Report GC 2, Faculty of Engineering Curtin University of Technology Perth, Australia,2006. [4] Anuar K.A, Ridzuan A.R.M, Ismail S., Strength Characteristics of Geopolymer Concrete Containing Recycled Concrete Aggregate,International Journal of Civil & Environmental Engineering, 2011, IJCEE-IJENS Vol: 11 No:

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