Performance of geopolymer concrete composed of fly ash after exposure to elevated temperatures

•The temperature performance of Fly Ash alkali-activated concretes was evaluated.•The residual strength of FA/GBFS concrete at 700°C was 23,71MPa.•The residual strength of FA/OPC and FA/GBFS at 1100°C was 5.5MPa and 15MPa respectively.•New crystalline phases at 1100°C were identified.•These concretes perform better at high temperatures compared to Portland concrete. Alkaline activated concretes based on a mixture of fly ash (FA) and blast furnace slag (GBFS), as well as FA and Portland cement (OPC), both at a ratio of 80:20, were exposed to temperatures between 25°C and 1100°C. Then, the physicochemical and mechanical changes were evaluated. The results indicate that the activated concretes have better performance than the reference ones (100% OPC). At temperatures of 1100°C, the residual strengths of the FA/GBFS and FA/OPC concretes are 15 and 5.5MPa, respectively, unlike the OPC concrete that lost 100% of its strength. At temperatures above 900°C, the activated matrix densified, and the crystalline phases, such as sodalite, nepheline, albite and akermanite, are identified.