PHENOMENOLOGICAL MODEL TO RE-PROPORTION GEOPOLYMER COMPRESSED BLOCKS

Transcription

1 PHENOMENOLOGICAL MODEL TO RE-PROPORTION GEOPOLYMER COMPRESSED BLOCKS Radhakrishna*, Dept of Civil Engineering, R.V.College of Engineering, Bangalore, India A Shashishankar, Dept of Civil Engineering, R.V.College of Engineering, Bangalore, India B C Udayashankar, Dept of Civil Engineering, R.V.College of Engineering, Bangalore, India 33rd Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2008, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 33 rd Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2008, Singapore PHENOMENOLOGICAL MODEL TO RE-PROPORTION GEOPOLYMER COMPRESSED BLOCKS Radhakrishna*, Dept of Civil Engineering, R.V.College of Engineering, Bangalore, India A Shashishankar, Dept of Civil Engineering, R.V.College of Engineering, Bangalore, India B C Udayashankar, Dept of Civil Engineering, R.V.College of Engineering, Bangalore, India Abstract Despite the extensive developments in the construction industry, bricks and blocks have still remained as the major building units. The traditional fired bricks still rule supreme since cost consideration is the prime influencing factor in the product choice. About 22 million tones of coal are consumed in the production of burnt bricks apart from 10 million tones of biomass. Clay bricks consume large amount of fertile top soil owing to fast depletion of soil. The alternative to these bricks is compressed cement blocks which have gained popularity recently. The main draw back of cement blocks is consumption of cement which is deterrent to sustainability. As an alternative to the above products a new technology is developed to manufacture fly ash based geopolymer compressed blocks. There are several advantages of these blocks. Marginal materials can be used to develop these blocks. No traditional curing methods are adopted and no cement is being used. In the present investigation alkali activated fly ash and ground granulated blast furnace slag (GGBFS) are used as binder materials in the compressed blocks at ambient and higher temperatures. Blocks of wide range of strength (1-20 MPa) are cast. The alkaline fluid-to-binder ratio (FBR) is optimized to get the maximum strength. A phenomenological model is advanced to develop mix proportion for the blocks. The pattern of strength variation was found to be in accordance with generalized Abrams law, for constant degree of saturation at various FBR. The strength data is being further analyzed within the framework of generalized Abrams law, which has already been validated for cement based composites. To formulate the phenomenological model, the strength data at a specific fly ash-to-fluid ratio is identified as reference value. The strength ratio used in the phenomenological model reflects the synergy between different ingredients in the microstructure of the geo-polymers. The validity of phenomenological model thus developed have been examined with an independent set of experimental data generated at reference state and the same set of values are used to find the strength development at any other FBR and compared with actual values. Key words: fly ash, geopolymer, compressed blocks, fluid-to-binder ratio, Abrams law, phenomenological model.

3 1 General Introduction. In recent times the emission of carbon dioxide into the air is being increased day by day. Considerable amount of fossil fuel, coal and oil are burnt for different reasons. This weakens the heat trapping blanket that surrounds the planet, causing global warming. Various alternatives can be considered to protect the planet. The rapid increase in the capacity of thermal power generation has resulted in the production of a huge quantity of fly ash. The prevailing disposal methods are not free from environmental pollution and ecological imbalance. On the other hand, the production of each ton of cement releases equal amount of carbon dioxide to the atmosphere. The usage of cement can be reduced by using the other possible cementing materials without compromising the strength and durability. The most basic building material for construction of houses is the usual burnt clay brick. A significant quantity of fuel is utilized in making these bricks. Also, continuous removal of topsoil, in producing conventional bricks creates environmental problems. There is strong need to adopt cost effective sustainable technologies using local materials and appropriate/intermediate technologies using materials with efficient and effective technology inputs. Different methods are adopted to produce the building blocks using cement, lime-fly ash, lime-slag bindings etc. There is a need to develop simple and highly effective technologies for producing the building blocks. The imperative need to produce more building materials for various elements of construction and the role of alternative options would be in sharp focus. This is in considering the short supply, increasing cost, energy and environment considerations for traditional and conventional materials. The possibility of using innovative building materials and technologies, using waste material like fly ash have been considered. 2 Background information. Inorganic polymers formed from naturally occurring aluminosilicates have been termed geopolymers by Davidovits [1]. Various sources of silica (Si) and alumina (Al), generally in reactive glassy or fine grained phases, are added to concentrated alkaline solutions for dissolution and subsequent polymerization to take place. Typical aluminosilicate precursors used are fly ash, ground blast furnace slags, and metakaolinite. Geopolymer or inorganic alumino-silicates polymer is synthesized from predominantly silicon and aluminum materials of geological origin or by product materials such as fly ash at ambient or higher temperatures [2][3]. Furthermore, the studies have been conducted to date on the possibilities of producing and marketing alkali activated fly ash (AAFA) paste, mortar and concrete need further research on the proportioning the mix. The recent studies on the total replacement of cement to produce No Cement Concrete by alkali activation of fly ash are very encouraging from the point of development of sustainable alternative construction materials which gives fillip to booming infrastructure growth. Geopolymers have excellent properties such as abundant raw resource, little CO 2 emission, less energy consumption, low production cost, high early strength and fast setting. These properties make geopolymer find great applications in civil engineering. Heat-cured low-calcium fly ash-based geopolymer concrete exhibit excellent resistance to sulfate attack, undergoes low creep, and suffers very little drying shrinkage. The elastic properties are comparable with Portland cement concrete. The bond of GPC with reinforcement is also better compared to OPC concrete [4]. It is well known that geopolymers posses excellent mechanical properties, fire resistance and acid resistance [5] [6]. The polymerization process involves a substantially fast chemical reaction under alkaline condition on Si-Al minerals, those resulting in a three-dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds [7]. According to Bakharev [8] [9] alkali activated fly ash concrete possess high acid and sulphate resistance. Low-calcium fly ash has been successfully used to manufacture geopolymer concrete. In geopolymer concrete the mix attains saturation and requires considerable amount alkaline fluid at higher fluid-to-binder ratios (0.4 onwards). This fluid consumes considerable amount of sodium hydroxide and sodium silicates and may not be economical all the times. On the other hand at lower fluid-to-binder ratios (0.15 to 0.25) the mix requires less amount of fluid and will be harsh. The mix needs compaction effort to transform into a block. The blocks developed using alkali activation of fly ash need thermal input for activation. But addition of GGBFS will eliminate the thermal input as the mix hardens at ambient temperature. The resulting blocks can be successfully used as masonry blocks in place of traditional burnt bricks and concrete blocks.

4 Binder 3 Scope of the Research. In conventional compressed cement blocks, the strength development is sensitive to watercement ratio at constant degree of saturation. The mix follows the Abrams and Bolomey s laws[10][11]. Does the strength development in geopolymer blocks is same as that of cement concrete blocks? Can a phenomenological model be advanced for assessment for the combination of ingredients to meet the specified strength development? The aim of the present study is to address these issues. 4 Materials and Methods. The physical and chemical properties of fly ash and GGBFS used in this investigation are shown in Table. No. 1. The XRD study on fly ash used confirms the presence of crystalline phases of Quartz and mullite in matrix of alumino silicate glass. SEM study of the ash indicates that almost all the particles are spherical. The fine aggregate used are sand and quarry dust having specific gravity of 2.60 and 2.65 respectively. Specific Gravity Percentage Of Residue left on 45µ Table 1 Properties of fly ash and GGBFS Loss on Ignition Chemical Composition in percentage Al 2 O 3 Fe 2 O 3 SiO 2 MgO SO 3 Na 2 O Total Chlorides Fly Ash (FA1) Fly Ash FA2) Fly Ash (FA3) Fly Ash (FA4) GGBFS Ca O Alkaline solution for different molarities was prepared using the tap water, sodium hydroxide flakes and sodium silicate powder. The ratio of sodium silicate and sodium hydroxide was maintained as 0.4. Standard Proctor test was conducted to find the optimum dry density and optimum fluid content required. Static compaction device was used in the experimental program for casting the cylindrical specimens for the required densities. The diameter and height of the specimen were 36mm and 72mm respectively. To cast the cylindrical specimens of geopolymer mortar, the fine aggregate and fly ash/ggbfs were mixed in dry condition in the specified ratio by weight. Then the alkaline solution of required quantity was added and mixed properly by hand to eliminate clustering.the fresh mortar mix was used to cast the cylindrical specimens. The degree of saturation of the mortar was maintained at 44 percent. The prepared samples of geopolymer cylinders were kept in oven at 60 0 C for 24 hours for thermal curing or at ambient conditions. Some specimens were wrapped with aluminum foil to prevent the moisture loss. After the period of thermal curing the samples were removed from the oven and kept at ambient temperature for one hour for cooling. The specimens were tested for unconfined compressive strength at the age of 1day, 3days and 7days. Table 2 Mix proportions Considered Series Molarity Binder Curing Temp. 0 C FA B:A Ratio Wrapped/ Unwrapped M1 14 FA1:GGBS = 1:1 25 Sand 1:2 UW M2 14 FA2GGBS = 1:1 25 Sand 1:2 UW M3 14 FA3:GGBS = 1:1 25 Sand 1:2 UW M4 14 FA4:GGBS = 1:1 25 Sand 1:2 UW M5 14 FA1:GGBS = 1:1 25 Sand 1:1 UW M6 14 FA1:GGBS = 1:1 25 Sand 1:2 UW M7 14 FA1:GGBS = 1:1 25 Sand 1:3 UW M8 10 FA1:GGBS = 1:0 60 Sand 1:1 UW M9 12 FA1:GGBS = 1:0 60 Quarry Dust 1:1 W

5 5 Results and Discussions. The consistency of the geopolymer mortar mix with fluid-to-binder ratio less than 0.3 would be too dry frictional material for placement by normal casting in a homogeneous manner. Hence, in this range of fluid-to-binder ratio, the mix is suitable for making compressed blocks. As the mix is not saturated, it cannot be transformed into a shape unless there is an external pressure. For easy making of the compression blocks optimum dry density is found by Standard Proctor test. The strength development at constant dry density did not follow the pattern of Abrams law. This is because, at the same dry density, the degree of saturation was not constant. Therefore it was decided to maintain same degree of saturation. At the degree of saturation of 44% it is found that it is practically possible to make the blocks. The different parameters considered to study the strength development are (Table No.2) Source of fly ash FA1, FA2, FA3 and FA4. Ratio of binder-to-fine aggregate 1:1, 1:2 and 1:3. Age of the mix 1, 3 and 7 days. Binder Fly ash and fly ash with GGBFS. Different aggregates Sand and quarry dust. Different wrapping conditions Wrapped with aluminium foil and unwrapped. Curing conditions Open air and 60 0 C in oven for 24 hours. Molarity of the activator solution 10, 12, 14. Figure No.1 indicates the strength development at seven days for various fluid-to-fly ash ratios with fly ash as the binder along with GGBFS cured at ambient temperature. There is considerable influence of the size of the particles on the strength development. As the particle size decreases the strength development increases. It is because of high surface area of the ash. For the same binder the strength increases as the ratio of binder-to- aggregate. Age of the specimen has positive effect on the strength. There is formation of more alumino silicate gel with fly ash and CSH gel with GGBFS with age. Therefore the compressive strength increases with age as in Figure No.2. It is noticed that there is marginal increase in the strength with the use of quarry dust in place of sand as fine aggregate. It is because of the better workability of the fresh mix. The wrapped cylinders exhibits better strength compared to unwrapped specimens. The strength development is marginally higher in alkaline fluid compared to dry alkali medium. At constant degree of saturation, the strength development follows the pattern of Abram s law for all the series of experiments. In all the above cases it is common interesting phenomena that fluid-to-binder ratio commands the strength at the constant degree of saturation. The strength pattern follows Abrams law as that of Portland cement concrete blocks [12].The higher fluid content may not be useful for the chemical reaction. As the moisture escapes fro the mix, air voids would be created resulting in the reduction of the strength. It is well known fact that each percent of air voids reduced about 5% of the strength in saturated Portland cement composites. Similar trend is observed even in geopolymer mix. If the strength is plotted with respect to binder-to-fluid ratio, the variation of the strength is linear as indicated in Figure No 3 and 4. There is clear indication that the pattern is same as Bolomey s law [11]. Again the trend is same as Portland cement composites. The models obtained by regression analysis are shown in Table No.3. It is interesting to note that the correlation coefficient is high in all the cases. As there are many parameters which influence the strength development, it becomes very difficult to decide the mix proportion for a given strength at any age. The simple way is to develop a model so that single input will take care of the required strength at a given age of the mix.

6 Compressive Strength, MPa M1-7 Days M2-7 Days M3-7 Days M4-7 Days M5-7 Days M6-7 Days M7-7 Days Fluid-to-binder Ratio Figure 1 Variation of Compressive Strength for ambient Cured Blocks Compressive Strength, MPa M8-1 Day M8-3 Days M8-7 Days M9-1 Day M9-3 Days M9-7 Days Fluid-to-binder Ratio Figure 2 Variation of Compressive Strength for Thermal Cured Blocks

7 Compressive Strength, MPa M1-7 days M2-7 Days M3-7 days M4-7 Days M5-7 Days M6-7 Days M7-7 Days Binder-to-fluid Ratio Figure 3 Variation of Compressive Strength with Binder-to-fluid ratio for ambient Curing 23 Compressive Strength, MPa Binder-to-fluid Ratio M8-1 Day M8-3 Days M8-7 Days M9-1 Day M9-3 Days M9-7 Days Figure 4 Variation of Compressive Strength with Binder-to-fluid Ratio for Thermal curing

8 Table 3 Regression Analysis Series Regression Equation ( x=b/f) Correlation Coefficient y = x M1-7 Days R 2 = R 2 = M2-7 Days y = x y = x R 2 = M3-7 Days y = x R 2 = M4-7 Days y = x R 2 = M5-7 Days y = x R 2 = M6 7 Days y = x R 2 = M7-7 Days y = x R 2 = M8-1 Day y = x R 2 = M8-3 Days y = x R 2 = M8-7 Days y = 2.784x R 2 = M9-1 Day y = x R 2 = M9-3 Days y = x R 2 = M9-7 Days A phenomenological approach is one by which the combination of parameters would be done within the basic framework of a scientific aw. The use of this model needs an input of experimental data of a single trial to account for the synergy between different constituents of a given set of materials. If any one or in combination the ingredients of a set changes for the new set, the new input data is to be generated again to use the phenomenological model to obtain the corresponding fluid to-binder ratio to arrive at the appropriate mix proportions in order to meet the specific strength and workability requirements. This exercise is identified as Re-proportioning Method. This is akin to making adjustments to the trial mix until the specified requirement is met with. Instead of repeated laboratory trials, introducing the experimentally determined reference strength value and making simple calculations result in desired results. This is a rapid exercise and has further potential to determine the parameter that would provide a wide spectrum of mixes that would have potential to develop strength over a desired range for a given set of materials. It is now intended to examine as to how far it is possible to formulate the phenomenological model for assessment of strength development at different fluid-to-binder ratios at different age with similar reference strength data within the range of value binder-to-fluid ratio considered. In this investigation inverse of fluid-to-binder ratio of 0.2 i.e. binder-to-fluid ratio of 5.0 is considered as reference for normalization of respective compressive strength as binder-to-fluid ratios of different series of experiments. This chosen value of binder-to-fluid ratio is arbitrary. There is no other significance. The following is the resulting phenomenological model after generalization is as indicated in Figure No. 5. S S B = B / F = 5 F

9 R 2 = n= Binder-to-Fluid Ratio M1 M2 M3 M4 M5 M6 M7 M8-1D M8-3D M8-7D M9-1D M9-3D M9-7D Figure 5 Generalization of compressive strength 6 Validation. To use this relation for a given set of materials the strength developed at a specified age for a binder-to-fluid ratio of 0.2 is to be determined. Using this as an input parameter in the equation the binder-to-fluid ratio for any other strength desired can be calculated using the phenomenological model. A separate series of experimental data was generated to examine the predictions made by the use of phenomenological model. From each of these sets, the compressive strength at reference binder-to-fluid ratio is taken into consideration in the denominator of the left hand side of phenomenological equation and strength developed at other fluid-to-binder ratios are calculated and tabulated for comparison with experimental values (Table. No.4) are very close match between the experimental and predicted values reinforcing the applicability of phenomenological model. With more data being generated for still wider range of binder-to-fluid ratio the scope of this phenomenological model can still be further enhanced.

10 Table 4 Comparison of Compressive Strength Series Binder-to-fluid Ratio Experimental Value in MPa Predicted Value in MPa M1-1Day Reference M2-3 Days Reference M3-1 Day Reference M4 3 Days Reference M5 1 Day Reference M6 3 Days Reference Conclusions. The following conclusions are made from this comprehensive study. The quality of fly ash, binder-to-aggregate ratio, molarity of the activator solution, fine aggregate type, curing conditions influence the strength development in geopolymer compressed blocks. If the air content is maintained constant then strength development is in accordance with Abrams law, used in concrete technology. It is possible to advance a phenomenological model similar to what has been done for cement based materials. For a given set of materials, the strength development at a particular age of the mix forms reference value for use in the normalized equation. With this input parameter, the binder-tofluid ratio for any other desired level of strength can be calculated. This procedure is simple and rapid. It is possible to develop compressed masonry blocks by using only marginal materials with out the use of conventional cement and thermal input. This is fast and simple method. The geopolymer compressed blocks can be the best alternative to bricks and concrete blocks in future. Wide range of strength is helpful to choose the parameters for the required strength at a given age.

11 8 Acknowledgement. The authors thank the authorities of Rashtreeya Shikshana Samithi Trust, Dr. S.C.Sharma, Principal, R.V.College of Engineering, Bangalore, for encouragement and Prof.B.L. Shivakumar, Head, Department of Civil Engineering, RVCE Bangalore. for extending facilities to carry out the research work at RV-Centre for Alternative Construction Materials and Technologies(RV-CALTech), RVCE. 9 References. [1] J. Davidovits, Geopolymers: Inorganic Polymeric New Materials, J. Therm. Anal., Vol. 37, , [2] Haradjito, D.,Wallah, S.E., Sumajiouw, D.M.J and Rangan, B.V. (2004a), Prpperties of Geopölymerconcrete with fly ash as source material : Effect of mixture composition. Seventh CANMET / ACI Int. Conf. on Recent Advances in Concrete Technology. May th 2004 : [3] Haradjito, D.,Wallah, S.E., Sumajiouw, D.M.J and Rangan, B.V. (2004b) Fly ash based Geopölymer concrete material for sustainable development. Invited Paper for Concrete World Conf. ACI India chapter Mumbai. Dec [4] Hardjito.D, Wallah, S.E, Sumajouw, D.M.J. and Rangan, B.V. On the development of fly ash based Geopolymer concrete. ACI Materials Journal Nov-Dec (2004c) Title no 101-MT, pp [5] J.Davidovits and M. Davidovics (1988) Geopolymer room temperature ceramic matrix for composites, Ceram. Eng. Sci. Proc. 9, [6] A. Palomo, A. Macias, M. T. Blanco and F. Puertas (1992) Physical, chemical and mechanical characterization of geopolymers, Proc of the 9th Internatl Congress on the Chem of Cem, [7] Davidovits, J. High Alkali Cements for 21 st Century Concretes. Concrete Technology, Past, Present and future, Proceedings, V. Mohan Malhotra Symposium, Editor: P.Kumar Mehta,ACI SP 144. P [8]T. Bakharev., Resistance of geopolymer materials to acid attack., Cement and Concrete Research 35 (2005a) [9]T. Bakharev.,Durability of geopolymer materials in sodium and magnesium sulfate solutions Cement and Concrete Research 35 (2005b) [10]Abrams D. Design of concrete mixtures. Bulletin No.1,1918,Structural Materials Research Laboratory, Lewis Institute. Chicago, p20. [11] Bolomey, J. Durecissenment des mortiers ets benton. Tech. Suisse Romande Nos. 16, 22 and ,1927 [12] K. Nagendra Prasad, M.L. Narasimhulu, T.S. Nagaraj, J.M. Naidu and Syed Ifthakaruddin Strength Development in Compressed Cement Blocks Analysis and Assesment ICI-J. April 2005, Volume 79,No 4 pp49-54 [13] Nagaraj, T.S. and Zahida Banu, Generalization of Abrams Law. Cement and Concrete Research, Elsevier Science. 26 (6) :