•Thermal behaviour of alkali activated slag binder exposed to temperatures up to 1200°C was studied.•Superior mechanical performance at temperatures above 800°C was observed.•High temperature XRD revealed the crystallization of akermanite at 710°C.•Changes on molecular level during heating are described based on results of IR and MAS NMR measurements. The paper covers the results of a study of microstructural changes in alkali activated granulated blast furnace slag exposed to high temperatures. Paste prepared from slag activated with solid sodium silicate glass was subjected to temperatures in the range of 200–1200°C for a period of 1h. The microstructural changes caused by the heat were investigated by means of SEM, HT-XRD analysis and FTIR and MAS NMR spectroscopy. Based on the latter two methods, only partial dehydration and decomposition of C–A–S–H phase can be observed up to 600°C. The principle changes in the microstructure of alkali activated slag occur between 600 and 800°C, when the dehydration of C–A–S–H phase is complete and new phases start to crystallize, among which akermanite is dominant. This significant phase transformation is reflected in the morphology, pore distribution, and mechanical properties of the AAS paste.