Performance of geopolymer mortars prepared by different alkaline solutions under elevated temperature

Fire has severe effects on concrete as it causes a significant deterioration in its properties. Geopolymers (GP) are considered as an emerging alternative binder to Portland-cement OPC). This paper presents an investigation of the performance of fly ash (FA) based GP-mortars made with different alkaline solutions after exposure to elevated temperatures (200, 400, 600, and 800 ℃). The main investigated parameters were the type of hydroxide (NaOH and KOH), type of silicate (Na2SiO3 and K2SiO3), molarity levels of NaOH, Na2SiO3/NaOH ratios, alkaline-solution to FA ratio and sand to FA ratio. Compressive, flexural, and indirect tensile strengths were measured before and after exposure to elevated temperatures. Moreover, SEM analysis was conducted on GP and OPC mortars. The results showed that performance of GP mortars containing Na2SiO3 with KOH was better than GP mixes that contained Na2SiO3 with NaOH or K2SiO3 with KOH after exposure to elevated temperatures. A higher loss rate in mechanical properties after exposure to 800℃ was observed as the NaOH molarity increased. Increasing Na2SiO3/NaOH ratio up to 2.5 caused an increase in the mechanical properties, while increasing that ratio from 2.5 to 3 caused higher loss in the mechanical properties of GP mortars. After exposure to 800 ℃, sand to FA ratio of 2 and alkaline solution to FA ratio of 0.5 gave higher residual strengths than the other ratios. Behavior of GP mortar was found to be better than OPC mortar after exposure to elevated temperatures. SEM images showed that the structure of the GP mortar, after exposure to elevated temperatures, was denser than that of the OPC mortar. Empirical equations capable of predicting the residual mechanical properties of GP-mortars made using different alkaline solutions with good correlation have been proposed