Geopolymer Concrete - Properties

Geopolymer Concrete - Properties
Geopolymer Concrete - Properties

Geopolymer Concrete:-

 The emission of CO2 coupled with non absorption of the same on account of deforestation etc has caused tremendous environmental pollution leading to global warming and other bad effects. It is estimated that about 7% of greenhouse gas is being emitted into the atmosphere annually on account of production of OPC alone.

.Therefore, it is necessary to reduce the emission of CO2 into atmosphere by reducing the cement production and consumption. It is suggested that consumption of cement could be reduced by three ways. 

  • Through economical mix design
  • By replacing cement with fly ash by adopting high volume fly ash concrete (HVFC) or by using other supplementary cementitious materials.
  • By using alternate binding materials for concrete such as Bacterial concrete or Geopolymer concrete. (no cement in concrete)

What is Meant by Geopolymer Concrete ?

The term “Geopolymer” was coined by Davidovits in 1978. Geopolymer is an inorganic alumino-silicate polymer, synthesized from predominantly silicon and aluminium material such as fly ash. Alkaline solutions are used, to induce the silicon and aluminium atoms; in the source materials (fly ash), to dissolve to form gel. The polymerisation process may be assisted by applied heat followed by drying. The Geopolymer gel binds the loose coarse and fine aggregates to form geopolymer concrete. Geopolymer gel replace the C–S–H gel in cement concrete. Chemical reaction period is substantially fast and the required curing period may be within 24 to 48 hours. 

Davidovits claimed that the Egyptian pyramids were built by casting geopolymer on site. He also reported the geopolymer possesses excellent mechanical properties, does not dissolve in acidic solutions, and does not generate any deleterious alkali-aggregate reaction even in the presence of high alkalinity. Some applications of geopolymer concrete are for marine structures, precast concrete products such as railway sleepers, sewer pipes etc.

B.V. Rangan et al of Curtain University of Technology, Perth, Australia  have carried out pioneering experimental work for the production of geopolymer concrete with a view to develop optimum mix design process and to establish the engineering properties such as compressive and tensile strengths, stress-strain relations, modulus of elasticity etc. The studies undertaken by them also aimed at establishing shrinkage, creep and durability properties, in particular, the corrosion resistance. The information given under geopolymer concrete in this book is freely drawn from the article published by RV Rangan et al in the proceedings of the seminar, INCONTEST 2003 held at Coimbatore, India.

In the experimental work low calcium (Class F) fly ash has been used. Sodium hydroxide in flake form Na(OH) with 98% purity and sodium silicate solutions (Na2O = 14.7%), SiO2 = 29.4% and water 55.9% by mass) were used as alkaline activators. To improve the workability of fresh concrete, a commercially available naphthalene based superplasticizer was used Four types of locally available aggregates (at Perth, Australia) were mixed together.

 Aggregates and the fly ash were mixed dry in a pan mixer for 3 minutes. The alkaline solutions and the superplasticizer were mixed together, and then added to the solid particles and mixed for another 3 to 5 minutes. The fresh concrete had a stiff consistency and was glossy in appearance. The mixture was cast in 100 x 200 mm cylinder in three layers. Each layer was tamped 60 times and vibrated for 10 seconds on a vibrating table. Five cylinders were prepared for each test variable.

Immediately after casting, the samples were covered by a film to avoid the loss of water due to evaporation during curing at an elevated temperature. After being left in room temperature for 30 – 60 minutes, the specimens were cured in an oven at a specified temperature for a period of time in accordance with the test variables selected.

 Numerous trial mixes of geopolymer concrete were made and tested. The data collected from these studies indicated that the salient parameter affecting the compressive strength of geopolymer concrete are listed below : 

" Silicon oxide (SiO2) to aluminum oxide (Al2O3) ratio by mass in fly ash should preferably be in the range of 2.0 to 3.5 to make good concrete. 

" Activator liquid to source material (fly ash) ratio by mass

 " Concentration of sodium hydroxide NaOH liquid measured in terms of Molarity (M) in the range of 8 to 16 M.

 " Sodium silicate to sodium hydroxide liquid ratio by mass. The effect of the parameter depends on the composition of the sodium silicate solution.

" Curing temperature in the range of 30° to 90°C 

" Curing time in the range of 6 to 48 hours. 

" Water content in the mixture.

Effect of Parameter on Compressive strength

Effect of Parameter on Compressive strength

Table Parameter on Compressive strength

       
Effect of curing temperature on compressive strength
 Effect of curing temperature on compressive strength 

Effect of water contents in the Mix 

In order to study the effect of water content on the compressive strength of geopolymer concrete several tests were carried out. The dose of superplasticizer to the mass of fly ash was taken as 1.5%. 

The effect of water contents is shown in above diagram by plotting the compressive strength versus water to geopolymer solid ratio by mass. For a given geopolymer concrete the total mass of water in the mix is taken as the sum of the mass of water in the sodium silicate solution plus the mass of water in sodium hydroxide solution plus the mass of extra water, if any, added to the mixture. The mass of geopolymer solids is the sum of the mass of fly ash, the mass of sodium hydroxide flakes and the mass of sodium silicate solids.

Effect of water/Geopolymer solids ratio on compressive strength.
Effect of water/Geopolymer solids ratio on compressive strength. 

Remarks :-

Geopolymer concrete is a concrete made without using portland cement and as such it is environmentally friendly and energy efficient construction material with an enormous potential in many infrastrctural applications. The limited trial results show that geopolymer concrete undergoes very little drying shrinkage and moderately low creep, and possesses excellent resistance to sulphate attack.

Also Read :-Why sea water-is-not-used-in-concrete

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