Continuous technological upgrading and

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2 Geopolymer Concrete - An Ecofriendly Concrete N. P. Rajamane Head, Concrete Composites Lab Nataraja M C Professor, SJ College of Engg, Mysore N Lakshmanan Former Director P S Ambily Scientist, Structural Engg Research Centre, CSIR, Chennai N. P. Rajamane Continuous technological upgrading and assimilation of latest technology has been going on in the cement concrete industry. But, the cement production is highly energy intensive and consumes significant amount of non-renewable natural resources such as lime stone deposits, coal, etc. Thus, the Portland cement industry does not quite fit the contemporary desirable picture of a sustainable eco-friendly industry. There is an urgent need to find an alternate binder which should be similar or superior to that of Portland cement for use in concretes in respect of parameters such as : processing conditions for production of concrete mixes, mechanical and durability properties, long term chemical stability of the binding system with common filler aggregate systems such as sand, crushed natural stones, etc. Towards this, a new binder material, known as 'geopolymer' is considered in this paper. Alkali activation of alumina and silica containing blast furnace slag powder, known as GGBS and fly ash is done to obtain inorganic polymeric binding material called geopolymer. Studies on geopolymers were carried out at SERC in early 2000s and several geopolymer cement concretes (GPCCs) were developed. From the test data generated at SERC, an overview of GPCCs is presented in this paper; it is seen that GPCCs are good candidates structural materials of constructions from both strength and durability considerations. Because of lower internal energy and lower CO2 emission contents of ingredients of geopolymer based composites compared to those of conventional Portland cement concretes, the new composites can be considered to be more ecofriendly and hence their utility in practical applications needs to be developed and encouraged The Indian cement industry is about 70 years old and is experiencing a boom on account of the overall growth of the Indian economy. Indian cement industry has become globally competitive; because, the industry had witnessed healthy trends such as cost control and continuous technology up-gradation. Global rating agency, Fitch Ratings, has commented that cement demand in India is expected to grow at 10% annually buoyed by housing, infrastructure and corporate capital expenditures [URL1]. The cement industry in India comprises 130 large cement plants and more than 300 mini cement plants. India is now the world's second largest producer of cement after China, with the total installed capacity of 219 MT in April 2009[URL2]. The cement industry is likely to add MT of capacity this fiscal year resulting in about 20 per cent increase over the installed capacity of With the boost given by the government to various infrastructure projects, road networks and housing facilities, a very high growth in the cement consumption can be anticipated in the coming years. With almost total capacity utilisation levels in the industry, cement despatches to the market have maintained a 10 per cent growth rate. For example, cement despatches were MT in March 2009, showing a growth of per cent as compared to MT in March During March 2009, cement production was MT, registering a growth of per cent as compared to MT in March Despite the concerns of recent economic 200 The Masterbuilder - November 2009

3 slowdown, led by a change in economic scenario along with excess supply pressure, the cement industry had ended the FY on a strong note. Government initiatives in the infrastructure sector, coupled with the housing sector boom and urban development, continue to be the main drivers of growth for the Indian cement industry. The cement business has been very steady for two decades with a cumulative annual growth rate (CAGR) of over 8%. It is now 1.3 times of the GDP [URL3]. This can be attributed to the following factors: Increased infrastructure spending has been a key focus area in India over the last five years indicating good times ahead for cement manufacturers. The Central Government has increased budgetary allocation for roads under National Highways Development Project (NHDP). A coal regulator has been appointed to facilitate timely and proper allocation of coal (a key raw material) blocks to the core sectors, cement being one of them. It may be noted that the per capita consumption of around 134 kgs in India compares poorly with the world average of over 263 kgs, and more than 950 kgs in China [URL4]. This, more than anything, underlines the tremendous scope for growth in the Indian cement industry in the long term. Moreover, the importance of the housing sector in cement demand can be gauged from the fact that it consumes almost 60%-70% of the country's cement; thus, continued demand in housing sector would itself be a significant factor in growth of cement industry [URL5]. Need for Alternate to Portland Cement Continuous technological upgrading and assimilation of latest technology has been going on in the cement industry. Presently, 93 per cent of the total capacity in the industry is based on modern and environment-friendly dry process technology and only 7 per cent of the capacity is based on old wet and semi-dry process technology. There is tremendous scope for waste heat recovery in cement plants and thereby reduction in emission level. The cement production is highly energy intensive (at 1.3 kwh/kg of cement), next only to steel and aluminium, also consumes significant amount of non-renewable natural resources such as lime stone deposits, coal, etc. A tonne of Portland cement production involves emission of about a tonne of CO2 which is a green house gas causing global warming. Among the greenhouse gases, CO2 contributes about 65% of global warming [McCaffery, 2002]. Thus, the Portland cement industry does not fit the contemporary desirable picture of a sustainable industry. There is an urgent need to find an alternate to P-C in order to make the construction industry eco-friendly. However, the new binder material should produce satisfactory strength and durability characteristics of concretes which are comparable, preferably even superior to those 'conventional concretes' (CCs) based on P-C. Requirements of New Binder The new binder material should be eco-friendly and it would be acceptable if it has following characteristics: It should be produced from widely available waste by-products from industries. 'Internal Energy Content' should be less Chemical activators for generating binding system should be common chemicals. The new binder concretes should be similar or superior to that of P-C based concretes in respect of :- processing conditions for production of concrete mixes time required for demoulding or formwork removal curing regimes and periods rate of strength developments with age mechanical properties such as compressive strength tensile strength flexural strength modulus of elasticity 202 The Masterbuilder - November 2009

4 durability related mechanical properties such as protection to embedded steel reinforcement diffusion of chloride ions, moisture/water, etc resistance against attack by sulphates, acidic solutions, etc cost per unit volume long term chemical stability of the binding system formed capable of accepting common filler aggregate systems such as sand, crushed natural stones, etc Choice of Geopolymer as a New Binder A new binder material, known as 'geopolymer' was developed in the mid 1970's by Davidovits [1991] and it had aluminosilicate gel performing as a binder. He utilised silica (SiO 2 ) and alumina (Al 2 O 3 ) available in the specially processed clay (metakaolin) to get inorganic polymeric system of aluminosilicates. Recently, Rangan and Hardijto, [2005] also exploited silica (SiO 2 ) and alumina (Al 2 O 3 ) of fly ash to produce three-dimensional polymeric chain and ring structure consisting of Si-O-Al-O bonds of geopolymeric binder which was found to be useful to prepare structural grade concretes. Alkali activation of alumina and silica containing blast furnace slag powder, known as GGBS, has been investigated since long, though, the term 'geopolymer' was not used to describe the binder generated. From above, it is clear that any minerals containing oxides of silicon and aluminium can be activated by suitably formulated highly alkaline liquid to obtain inorganic polymeric binding material. Preliminary studies in this direction were carried out at SERC in early 2000s and, both fly ash and GGBS, (either individually or combined in certain proportions) from indigenous sources were found to be suitable to produce geopolymeric systems to achieve sufficient strength levels in geopolymer cement concretes (GPCCs) made from these industrial wastes [Rajamane and Sabitha, 2005]. It was observed that the activation of FA and GGBS involved use of hydroxides and silicates of alkali (such as sodium, potassium) which are commonly available in India; the processing conditions for GPCCs were almost similar to CCs except that during mixing operations of concretes, instead of water, a premixed alkaline solution, known as 'Catalytic Liquid System' (CLS), was added. Basics of Geopolymers The term 'geopolymer' describe a family of mineral binders with chemical composition similar to zeolites but with an amorphous microstructure. Unlike ordinary Portland/pozzolanic cements, geopolymers do not form calcium- silicate-hydrates (CSHs) for matrix formation and strength, but utilise the polycondensation of silica and alumina precursors to attain structural strength. Two main constituents of geopolymers are: source materials and alkaline liquids. The source materials on alumino-silicate should be rich in silicon (Si) and aluminium (Al). They could be by-product materials such as fly ash, silica fume, slag, rice-husk ash, red mud, etc. Geopolymers are also unique in comparison to other aluminosilicate materials (e.g. aluminosilicate gels, glasses, and zeolites). The concentration of solids in geopolymerisation is higher than in aluminosilicate gel or zeolite synthesis. Composition of Geopolymer Cement Concrete Mixes Following materials are generally used to produce GPCCs: Fly ash, GGBS, Fine aggregates and Coarse aggregates Catalytic liquid system (CLS): It is an alkaline activator solution (AAS) for GPCC. It is a combination of solutions of alkali silicates and hydroxides, besides distilled water. The role of AAS is to activate the geopolymeric source materials (containing Si and Al) such as fly ash and GGBS. Formulating the GPCC Mixes Unlike conventional cement concretes, GPCCs are The Masterbuilder - November

5 a new class of materials and hence, conventional mix design approaches are applicable. The formulation of the GPCC mixtures requires systematic numerous investigations on the materials available. Preparation of GPCC Mixes The mixing of ingredients of GPCCs can be carried out in mixers used for conventional cement concretes - such as pan mixer, drum mixer, etc Mechanical Properties Compressive Strength: With proper formulation of mix ingredients, 24 hour compressive strengths of 25 to 35 MPa can be easily without any need for any special curing. Such mixes can be considered as self curing. However, GPCC mixes with 28 day strengths up to about MPa have been developed at SERC. Modulus of Elasticity: The Young's modulus or modulus of elasticity (ME), Ec of GPCC is taken as tangent modulus measured at the stress level equal to 40 percent of the average compressive strength of concrete cylinders. The MEs of GPCCs are marginally lower than that of conventional cement concretes (CCs), at similar strength levels. Stress Strain Curves: The stress-strain relationship depends upon the ingredients of GPCCs and the curing period. Rate of Development of Strength: This is generally faster in GPCCs, as compared to CCs. Reinforced GPCC Beams Load carrying capacity of GPCC beams, are up to about 20% more of CC beams at similar concrete strength levels. Cracking of concrete occurs whenever the tensile strength of the concrete is exceeded. The cracking in reinforced concrete is attributable to various causes such as flexural tensile stresses, diagonal tension, lateral tensile strains, etc. The cracking moment increases as the compressive strength increases in both GPCC and CC beams. Reinforced concrete structures are generally analyzed by the conventional elastic theory (Clause 22.1 of IS 456:2000) which is equivalent to assuming a linear moment-curvature relationship for flexural members. However, in actual behaviour of beams, non-linear moment curvature relationship is considered. The moment-curvature relation can be idealized to consist of three straight lines with different slopes. The slopes of these line changes as the behaviour of the beam is changed due to increasing load. Thus each straight line indicates different phases of beam history. The moment-curvature relations of GPCCs and CCs are essentially similar. The service load is generally considered as the load corresponding to a deflection of span/350 or maximum crack width of 0.2 mm, whichever is less. The deflections at service loads for the GPCC and CC beams are found to be almost same. Thus, at service loads, the behaviour of the GPCC and CC beams are similar. Ductility factor of the beams is considered as the ratio of deflection at ultimate moment (?U) to the deflection at yield moment (?Y). The ductility factor decreases as the tensile reinforcement increased. The ductility factor of GPCC beams could be marginally less than CC beams indicating higher stiffness of GPCC beams. The crack patterns observed for GPCC beams are similar to the CC beams. Reinforced GPCC Columns The concrete compressive strength and longitudinal reinforcement ratio influence the load capacity of columns. The load carrying capacity increases with the increase in concrete compressive strength and longitudinal reinforcement ratio. Crack patterns and failure modes of GPCC columns are similar to those of CC columns. Bond Strengths of GPCC with Rebars The bond strengths of GPCCs with rebars are higher compared to CC. Thus developmental length of steel bars in reinforced GPCC can be kept same, as in the case of reinforced CC. The bond strengths of GPCC and PPCC are significantly more and conservative than the design bond stress recommended in IS: 204 The Masterbuilder - November 2009

6 The GPCCs possess satisfactory bond with embedded steel bars so that the conventional design process of reinforced structural components can be applied conservatively to GPCCs also. Durability Aspects of GPCCs The GPCC specimens have chloride permeability rating of 'low' to 'very low' as per ASTM 1202C. GPCCs offer generaly better protection to embedded steel from corrosion as compared to CC. The GPCC are found possess very high acid resistance when tested under exposure to 2% and 10% sulphuric acids. Concluding Remarks on GPCCs From the test data generated at SERC, it can be concluded that GPCCs are good candidates materials of constructions from both strength and durability considerations. Geopolymer concrete shows significant potential to be a material for the future; because it is not only environmentally friendly but also possesses excellent mechanical properties. Practical recommendations on use of geopolymer concrete technology in practical applications such as precast concrete products and waste encapsulation need to be developed in Indian context. Because of lower internal energy (almost 20% to 30 % less) and lower CO2 emission contents of ingredients of geopolymer based composites compared to those of conventional Portland cement concretes, the new composites can be considered to be more eco-friendly and hence their utility in practical applications needs to be developed and encouraged. URL2 [2009], May 2009 URL3 [2009], Cement-demand-in-India-is-growing/articleshow/ cms, June 2009 URL4 [2009], cement.asp, 2009 URL5 [2009], cement/, July 2009 McCaffrey, R, [2002], Climate Change and the Cement Industry. Global Cement and Lime Magazine (Environmental Special Issue), pp Davidovits, J., [1991], Geopolymers: inorganic polymeric new materials, Journal of Thermal Analysis, Vol 37, pp Rangan, B. V, and Hardijto, D, [2005], Development and properties of low calcium fly ash based geopolymer concrete. Research report GC-1, Faculty of Engineering, Curtin University of Technology, Perth, Australia. Rajamane, N. P., Sabitha, D., [2005], Studies on geo-polymer mortars using fly ash and blast furnace slag powder, International Congress on Fly Ash, Fly Ash India, Chapter 6, pp 1-7. Acknowledgements The authors thank Director, SERC, Chennai, for permitting to publish this paper. The co-operation received from the staff of Concrete Composites Laboratory, SERC, in creating the test data and preparation of paper is gratefully acknowledged Refereces URL1 [2009], The Masterbuilder - November 2009 Velohar Infra Private Limited