Influence of Molar Concentration of Sodium Hydroxide Solution on High Temperature Resistance of Geopolymer Paste

This study aims to investigate the influence of the molar concentration of NaOH solution on the high temperature resistance of geopolymer paste. Specimens were prepared with 55% of fly ash, 45% of ground granulated blast-furnace slag and alkaline solutions. Alkaline solutions were prepared with sodium silicate solution and NaOH solutions at different concentrations; 10, 12, and 14 M. Specimens were cured for 24 h at 70 °C after casting. Then, they were cured at 20 °C, demolded at the age of 48 h and stored at 20 °C and 60% RH. At the age of 28 days, specimens were exposed to high temperature of 500 and 950 °C. As a result, when the specimens were exposed to 500 °C, C-S-H gel was decomposed and their compressive strength decreased by approximately 50%. Furthermore, when the specimens were disclosed to 950 °C, recrystallization to the secondary mineral phases was promoted and the specimens lost approximately 80% of their compressive strength. On the other hand, there was no significant difference in the results obtained from the different molar concentrations of NaOH solution. It implies that the molar concentration of NaOH solution cannot remarkably influence the high temperature resistance of geopolymer paste.