GEOPOLYMER CONCRETE WITH FLY ASH AND GGBS
M.S.Sudarshan- Senior Director, CivilAidTechnoclinicPvt Ltd, Bangalore
Prof. R.V.Ranganath - Professor and H.O.D., Dept of Civil Engineering, BMSCE, Bangalore
Geopolymers are alkali activated alumino silicate binders formed by reaction of silica and alumina rich materials with alkaline solutions. The reaction results in a mixture of gels and crystalline compounds which harden into a strong matrix. A relatively low temperature environment of 60-800C is sufficient for geopolymerisation, unlike organic polymers. The invention of this organic binder by Davidovits opened up many avenues of engineering applications. One of the significant applications was use of geopolymer binder in place of Portland cement binder in concretes. Eventhough the experiments on geopolymer started with alkali activation of alumina silicate rich kaolinite, other similar amorphous silica rich materials such as flyash, ground granulated blast furnace slag(GGBS), rice husk ash and other natural pozzolonas were also successfully tried to arrive at geopolymer binders with desired properties.
| The basic ingredients for geopolymer concrete consists of reactive silica and alumina rich materials such as low calcium fly ash, GGBS, natural pozzolana, Alkaline solutions such as sodium/potassium hydroxide of known concentration and Activator solution such as sodium/potassium silicate. The coarse and fine aggregates form the inert fillers. The water required for reaction is made available from the alkaline solutions. Additional water can be added, if necessary. Water reducing admixtures, used for Portland cement concrete can also be added in geopolymer concrete to improve workability.The reactions involved consist of dissolution of alumino silicate into aluminate and silicate species in monomeric form by |
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alkaline hydrolysis. The dissolution is rapid at high pH concentration of alkali. The aluminate and silicate species, along with silicate which may be present in activator solution, are incorporated into the aqueous phase. This further leads to gelation through formation of large networks by condensation. The water consumed during dissolution gets released during condensation, which is stored in the pores of the gel.
The gel system interconnects on rearranging and reorganizing, to result in a three-dimensional alumino-silicate network of geopolymer. Unlike hydration reaction of Portland cement which consumes water, geopolymerisation releases water. The geopolymerisation occurs rapidly at an elevated temperature environment of 60-800C.
The use of geopolymer as binder in concrete was able to address two important issues to its advantage. The cement production, which contributes to 8% of the total carbon dioxide emission can be reduced by use of less cement in constructions. Every ton of cement production releases one ton of carbon dioxide to atmosphere. The alternative binders such as geopolymers can reduce the utilization of cement in concrete resulting in less pollution of the environment. The other advantage is the utilization of Industrial bye products having reactive silica in considerable proportions in making of geopolymer concrete. These materials which are otherwise considered as waste can be consumed in concrete after suitable processing. Thus, geopolymer concrete is a green concrete helping in reducing carbon dioxide emissions and also utilizing waste materials.
In view of the above factors, geopolymer concrete has generated interest among the concrete researchers to explore this material in terms of utilizing different siliceous materials for manufacturing and characterizing the engineering properties of such concretes. A significant work has been done by Hardjito and Rangan, in development and characterization of geopolymer concrete using low calcium flyash. The research involved the development, mix proportions, evaluation of short term and long term materials in comparison with cement concrete. The studies established geopolymer concrete as an excellent alternative to cement concrete with similar mix design procedures, short term and long term properties. Geopolymer concretes of strength 60-70 Mpa could be easily realized with proper mix proportioning. The properties such as low drying shrinkage and low creep, excellent resistance to sulphate attack and good acid resistance make this material a suitable choice as a durable concrete exposed to aggressive environment. A few applications in precast industry such as box culverts, pipes etc.and in pavements have also been implemented using geopolymer concrete.
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One of the practical difficulties in the in-situ application of geopolymer concrete is its requirement of elevated temperature curing. This concrete requires to be cured at a temperature of 60-800 C for 24 hours to achieve the required strength. However, experiments conducted by using a mixture of fly ash and GGBS as alumino silicate materials in making of geopolymer concrete have shown promising results wherein reasonable strengths are achieved by just exposing them to sun light. In a tropical country like ours, where the average temperature is about 25-300C is sufficient for the necessary reactions to occur in geopolymer with fly ash and GGBS. Reasonable strengths of 25-35 Mpa could easily be achieved by exposing such geopolymer concrete to sun-curing for seven days.
It is believed that the nature of reactions involve both hydration and geopolymerisation. Initially, calcium oxide in GGBS reacts |
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with alumino silicate to produce calcium alumino silicate hydrate (CASH) providing initial strength. The heat produced during this chemical reaction further aids the geopolymerisation. Water released during geopolymerisation may be used for further hydration reaction to produce CASH. Thus the reactions are complementary to each other resulting in strength development at ambient temperature environment.
| The trials carried out at BMS college laboratory and Civil Aid Laboratory using combination of fly ash and GGBS at different proportions and subjected to sun-curing have produced very encouraging results. The geopolymer concrete mix with fly ash and GGBS at 50:50 ratio and with water content of 120 kg/cum, 28 days compressive strength of 60 Mpa could easily be obtained.This geopolymer concrete with fly ash and GGBS which can produce good strengths with sun-curing appears to be a |
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promising material for construction and repairs which does not require conventional water curing.