Geopolymer mortars were produced by replacement of high calcium fly ash with cement at the percentages of 5%, 10%, 15%, 20%, 25%, and 100%. Sodium hydroxide was used as the activator and sodium/binder ratios, by weight, were 10%, 12%, 14%, 16%, 18%, and 20% in mortar mixtures. Some physical properties such as unit weight, water absorption, apparent porosity and ultrasound pulse velocity tests were performed on specimens kept in ambient temperature. Flexural and compressive strength tests were made on specimens kept in ambient temperature, and exposed to temperatures of 200°C, 400°C, 600°C, and 800°C for residual performances of mortars. After exposing of specimens to high temperatures, ultrasound pulse velocity and loss in weights were also determined for deterioration levels. An experimental design is also achieved to find optimum solutions for Na/binder and cement/fly ash ratios by using D‐optimal design method after establishing response surface models for compressive and flexural tensile strengths. The highest compressive strength of 35.65 MPa was obtained in mortar specimens containing 14% sodium at 20% cement replacement with fly ash. Similarly, residual strengths were observed higher than those of other mortar specimens at the same sodium and cement replacement ratios.
In this study it was investigated the behavior of class F fly ash based geopolymer mortars subjected to elevated temperatures. Geopolimer composites were prepared with CEN-sand, water, fly ash as the binder, nSiO2Na2O and NaOH mixture combination as alkali activator. After mixing fresh geopolymer mixture, prismatic specimens were prepared using 40 mm × 40 mm × 160 mm prism molds. After molding, fresh geopolymer mortar samples with their mold were subjected to heat curing at 50 °C, 60 °C, 70 °C, 80 °C, 90 °C and 100 °C temperature for 48 hours in an oven. After 48 hours initial heat curing, the hardened samples were taken out of their mold. Then, they were further cured by leaving them in laboratory environment at about 22 ± 2 °C temperature, until 28 days together with heat curing duration. At the end of 28 days, geopolymer mortar samples developed flexural strength values between 2,9 MPa and 8,51 MPa. Geopolymer mortar samples developed compressive strength values between 7,63 MPa and 50,64 MPa. High temperature experiments were conducted to observe behaviour of geopolymer mortars at elevated temperatures. The control cement mortar mixtures were also prepared and subjected to high temperature exposure in comparison to geopolymer mortar mixtures. Control cement mortars were cured at laboratory environment for 28 days without initial temperature curing. Control cement mortar mixtures and geopolymer mortar mixtures were exposed to elevated temperatures of 200 °C, 400 °C, 600 °C and 800 °C temperature. The unit weight, ultrasonic pulse velocity, flexural and compressive strength of all mixtures were measured before and after high temperature exposure. Scanning electron microscope (SEM) images of the mixtures were taken and X-ray fluorescence (XRF) spectrometer analyses were carried out. It was observed that there was an increase in the flexural and compressive strengths of some geopolymer mortars after high temperature exposure. In general, geopolymer mortars exhibited better performance at elevated temperatures in comparison to control cement mortar mixture.
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