High temperature performance of geopolymers as a function of the Si/Al ratio and alkaline media

•K- and Na-geopolymers were made using metakaolin with varying Si/Al ratios.•The samples were submitted to simulated firing conditions (23–1,200 °C).•Lower Si/Al resulted in higher compressive strength after firing for Na geopolymers.•Geopolymers are stable until 900 °C with no dimensional changes.•Na-geopolymers with Si/Al = 1.01 showed the better performance until 1,100 °C. Geopolymeric cements are thermally stable when their volume, strength and chemical composition do not change after heat treatment. However, the effect of the Si/Al ratio and alkaline media on the geopolymer behavior over 500 °C is not well known. Therefore, the influence of the Si/Al ratio (1.01, 1.31 and 1.79) and the alkaline activator (sodium or potassium) was evaluated in this study under simulated firing conditions (23–1,200 °C). As a result, the geopolymers are stable up to 900 °C, keeping an amorphous structure. At 1,100 °C leucite (K-geopolymers) and nepheline (Na-geopolymers) are formed. Their strength is decreased after heating depending on the Si/Al ratio. The results show the potential use of geopolymeric cements for more efficient structures under fire.