17-08-2013, 04:47 PM
PROPERTIES OF GEOPOLYMER CONCRETE
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Drying shrinkage and creep:
Figure 10 shows the measured drying shrinkage and creep strains of geopolymer concrete produced using Mixture A-2.The test specimens were cured for 24 h at 60 °C. The percentage of high-range water-reducing admixture by mass of fly ash was 1.5%. There was no rest period before casting, and the delay time after mixing was 30 min. The 7-day compressive strength was 53.7 MPa. The creep specimens were loaded on the seventh day to produce a sustained stress of 22 MPa (approximately 40% of the compressive strength).
The details of test specimens and test procedure for creep tests were in accordance with the relevant Australian Standard for OPC concrete. Figure 10 shows that the drying shrinkage strains are extremely small indeed. The ratio of creep strain-to-elastic strain (that is, creep factor) reached a value of approximately 0.30 in approximately 6 weeks. Beyond this time, the creep factor increased only marginally. To evaluate the resistance of geopolymer concrete to sulfate attack, a series of tests were performed. The test specimens were soaked in a 5% sodium sulfate (Na2SO4) solution for periods of time. The test results reported elsewhere14 showed that after 12 weeks of exposure, there were no significant changes in the compressive strength, the mass, and the length of test specimens.
Sulfate Resistance:
Tests were performed to study the sulfate resistance of heat-cured low-calcium fly ash-based geopolymer concrete. The test specimens were made and heat-cured at 60°C for 24 hours after casting; they were immersed in 5% sodium sulfate solution for various periods of exposure up to one year. The sulfate resistance was evaluated based on the change in mass, change in length, and change in compressive strength of the specimens after sulfate exposure. The test specimens were 100x200 mm cylinders for change in mass and change in compressive strength tests and 75x75x285 mm prisms for change in length test7.
COMPRESSIVE STRENGTH:
The compressive strength of geopolymer concrete is significantly influenced by both curing and period. The highest compressive strength of 25 MPa was achieved at the blended ash ratio of 70:30%. It is evident that the concentration of NaOH regarding to curing period has an effect on the compressive strength of geopolymer concrete. This might be due to the acceleration in the geopolymerization process with the increase of the NaOH concentration or molarity in certain curing period. The strength is gradually increased with increasing curing period at high Molar ratio of 14.
Geopolymer concrete shows high compressive strength at Na2SiO3:NaOH ratio of 2.5. Geopolymer concretes give higher compressive strength at high temperature. This is because of complete polymerization process during heat curing at high temperature.
Setting time:
The setting time of geopolymer paste was affected by the curing temperature, type of alkaline activator, and the composition of the source material. The setting time of geopolymer paste was between 15 to 45 minutes at 60o C. The presence of compounds other than Al2O3 and SiO2 in the source material may also delay the setting. In materials of pure geological origin (say calcined kaolin), the dominant chemical contents are only Al2O3 and SiO2 , whereas by-product materials such as fly ash may contain other compounds, e.g. Fe2O3. It appears that pure geological materials may be more reactive to the alkaline activators, and hence a reduction in initial setting time.