31-01-2013, 10:23 AM
Geopolymer Concrete with Fly Ash
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ABSTRACT
Geopolymer concrete results from the reaction of a source material that is rich in silica and alumina with alkaline liquid. A summary of the extensive studies conducted on fly ash-based geopolymer concrete is presented. Test data are used to identify the effects of salient factors that influence the properties of the geopolymer concrete and to propose a simple method for the design of geopolymer concrete mixtures. Test data of various short-term and long-term properties of the geopolymer concrete and the results of the tests conducted on large-scale reinforced geopolymer concrete members show that geopolymer concrete is well-suited to manufacture precast concrete products that can be used in infrastructure developments. The paper also includes brief details of some recent applications of geopolymer concrete.
INTRODUCTION
Davidovits [1988] proposed that an alkaline liquid could be used to react with the silicon (Si) and the aluminium (Al) in a source material of geological origin or in by-product materials such as fly ash and rice husk ash to produce binders. Because the chemical reaction that takes place in this case is a polymerization process, he coined the term „Geopolymer‟ to represent these binders. Geopolymer concrete is concrete which does not utilize any Portland cement in its production. Geopolymer concrete is being studied extensively and shows promise as a substitute to Portland cement concrete. Research is shifting from the chemistry domain to engineering applications and commercial production of geopolymer concrete.
GEOPOLYMER PRODUCTION
Mixture Proportions of Geopolymer Concrete
The primary difference between geopolymer concrete and Portland cement concrete is the binder. The silicon and aluminium oxides in the low-calcium fly ash reacts with the alkaline liquid to form the geopolymer paste that binds the loose coarse aggregates, fine aggregates, and other un-reacted materials together to form the geopolymer concrete. As in the case of Portland cement concrete, the coarse and fine aggregates occupy about 75 to 80% of the mass of geopolymer concrete. The influence of aggregates, such as grading, angularity and strength, are considered to be the same as in the case of Portland cement concrete [Lloyd and Rangan, 2009]. Therefore, this component of geopolymer concrete mixtures can be designed using the tools currently available for Portland cement concrete.
Curing of Geopolymer Concrete
Heat-curing of low-calcium fly ash-based geopolymer concrete is generally recommended. Heat-curing substantially assists the chemical reaction that occurs in the geopolymer paste. Both curing time and curing temperature influence the compressive strength of geopolymer concrete. The effect of curing time is illustrated in Figure 2 [Hardjito and Rangan, 2005]. The test specimens were 100x200 mm cylinders heat-cured at 60oC in an oven. The curing time varied from 4 hours to 96 hours (4 days). Longer curing time improved the polymerization process resulting in higher compressive strength. The rate of increase in strength was rapid up to 24 hours of curing time; beyond 24 hours, the gain in strength is only moderate. Therefore, heat-curing time need not be more than 24 hours in practical applications.
Effect of Curing Time on Compressive Strength
Heat-curing can be achieved by either steam-curing or dry-curing. Test data show that the compressive strength of dry-cured geopolymer concrete is approximately 15% larger than that of steam-cured geopolymer concrete [Hardjito and Rangan, 2005]. The temperature required for heat-curing can be as low as 30 oC (Figure 1). In tropical climates, this range of temperature can be provided by the ambient conditions.
The required heat-curing regime can be manipulated to fit the needs of practical applications. In laboratory trials [Hardjito and Rangan, 2005] precast concrete products were manufactured using geopolymer concrete; the design specifications required steam-curing at 60oC for 24 hours. In order to optimize the usage of formwork, the products were cast and steam-cured initially for about 4 hours. The steam-curing was then stopped for some time to allow the release of the products from the formwork. The steam-curing of the products then continued for another 21 hours. This two-stage steam-curing regime did not produce any degradation in the strength of the products. A two-stage steam-curing regime was also used by Siddiqui [2007] in the manufacture of prototype reinforced geopolymer concrete box culverts in a precast concrete plant. It was found that steam curing at 80 ˚C for a period of 4 hours provided enough strength for de-moulding of the culverts; this was then followed by steam curing further for another 20 hours at 80 ˚C to attain the required design compressive strength.
DESIGN OF GEOPOLYMER CONCRETE MIXTURES
Concrete mixture design process is vast and generally based on performance criteria. Based on the information given in above, some simple guidelines for the design of heat-cured low-calcium fly ash-based geopolymer concrete have been proposed [Hardjito et al, 2004; Rangan, 2008; Sumajouw, 2007]. The performance criteria of a geopolymer concrete mixture depend on the application. For simplicity, the compressive strength of hardened concrete and the workability of fresh concrete are selected as the performance criteria. In order to meet these performance criteria, the alkaline liquid-to-fly ash ratio by mass, water-to-geopolymer solids ratio by mass, the wet-mixing time, the heat-curing temperature, and the heat-curing time are selected as parameters.
With regard to alkaline liquid-to-fly ash ratio by mass, values in the range of 0.30 and 0.45 are recommended. Based on the results obtained from numerous mixtures made in the laboratory over a period of six years, the data given in Table 2 are proposed for the design of low-calcium fly ash-based geopolymer concrete. Note that wet-mixing time of 4 minutes, and steam-curing at 60oC for 24 hours after casting are proposed.
GEOPOLYMER CONCRETE PROPERTIES
The elastic properties of hardened geopolymer concrete and the behavior and strength of reinforced geopolymer concrete structural members are similar to those observed in the case of Portland cement concrete [Sofi et al, 2007; Chang, 2009]. Heat-cured low-calcium fly ash-based geopolymer concrete also shows excellent resistance to sulfate attack, good acid resistance, undergoes low creep, and suffers very little drying shrinkage [Wallah and Rangan, 2006].
The behaviour and failure modes of reinforced geopolymer concrete columns and beams were similar to those observed in the case of reinforced Portland cement concrete columns [Sumajouw and Rangan, 2006; Sumajouw et al, 2007]. Test results demonstrated that the methods of calculations used in the case of reinforced Portland cement concrete columns and beams are applicable for reinforced geopolymer concrete columns.