- مبلغ: ۸۶,۰۰۰ تومان
- مبلغ: ۹۱,۰۰۰ تومان
This paper presents the effects of sodium and potassium based activators on compressive strengths and physical changes of class F fly ash geopolymer exposed to elevated temperatures. Samples were heated at 200 °C, 400 °C, 600 °C and 800 °C to evaluate the residual compressive strength after 28 days of curing. The fly ash geopolymer were synthesized with combined sodium silicate and sodium hydroxide solutions and potassium silicate and potassium hydroxide solutions by varying mass ratios of Na2SiO3/NaOH and K2SiO3/KOH of 2, 2.5 and 3. Results show significant improvement is compressive strength in the case of Na2SiO3/NaOH ratio of 3 than 2 and 2.5, where the residual compressive strengths are increased up to 600 °C. Better results on the geopolymer synthesized with potassium based activators are obtained where the residual compressive strength up to 600 °C are much higher than their sodium based counterparts. It is also found that the fly ash geopolymer synthesized with potassium based activators is more stable at elevated temperatures than its sodium based counterparts in terms of higher residual compressive strengths, lower mass loss, lower volumetric shrinkage and lower cracking damage. X-ray diffraction (XRD) and thermogravimetric analysis (TGA) results of sodium and potassium activator synthesized fly ash geopolymer also corresponds to the measured residual compressive strengths.
Based on limited experimental variables in terms of Na and K-based activators, the ratios of silicate to hydroxide of above activators, elevated temperatures on the residual compressive strengths, volumetric shrinkage and mass loss of geopolymer pastes the following conclusions can be drawn: 1. The geopolymer pastes containing Na-based activator exhibited higher compressive strength at ambient temperature and higher compressive strength at elevated temperatures up to 400 °C than its K-based counterpart. 2. At 600 °C the compressive strength of geopolymer containing Kbased activator is slightly higher than its Na-based counterpart. 3. The geopolymer paste containing K-based activator exhibited higher residual compressive strengths at all elevated temperatures compared to ambient temperature than its Na-based counterpart. 4. The geopolymer paste containing K-based activator with K2SiO3/ KOH ratio of 3 exhibited the highest residual compressive strengths at all elevated temperatures compared to ambient temperature than its Na-based counterpart. 5. The volumetric shrinkage and mass loss of geopolymer paste containing K-based activator is lower than its Na-based counterpart. 6. The geopolymer pastes containing K-based activator exhibited fewer surface cracks than that of Na-based activator. 7. XRD peaks of quartz and mullite are observed in both geopolymers after exposure to 400 and 800 °C temperatures like those observed in ambient condition. The presence of quartz and mullite peaks are believed to be the reason for maintaining residual compressive strength by both geopolymers after exposure to elevated temperatures. 8. The Na-based fly ash geopolymer exhibited higher weight loss in TGA and higher DTA peak at about 100 °C than its K-based counterpart after exposure to 400 °C. The higher weight loss and DTA peak is associated with the loss of absorbed and combined water in geopolymer gels, which indicate that higher geopolymer gels are remained in Na based geopolymer after 400 °C exposure than K-based system, which agrees well with the observed compressive strength results. An opposite trend is observed after exposure to 800 °C.