This study aims to understand the thermal behaviour of argillite based alkali-activated materials. Thermally heated argillites with different processes (furnace and flash process) and two alkaline solutions based on potassium or on a mix of potassium and sodium were used to synthesize consolidated materials. Then, the thermal resistance of the synthesized samples was evaluated by thermal treatment and to thermal shock at 800 degrees C. Whatever the sample, a good resistance to thermal treatment with an increase of the compressive strength after thermal treatment was noticed. Indeed, the compressive strength ratio (sigma(after)/sigma(before)) varies between 1 and 2.2. However, all samples do not resist to thermal shock. In order to understand these behaviours, structural investigations by Al-27 MAS-NMR, XRD, FTIR spectroscopy and DTA-DTG were performed. As the temperature increases, the nature of formed phases, strongly depends on the chemical composition of the geopolymer materials (type of carbonates, nature of alkali cation and alkaline earth) and on the thermal treatment conditions (heating rate and exposure time). The formation of crystalline phases such as wollastonite and zeolite result from the reaction between the available species like Ca2+ and Mg2+ from carbonate compounds decomposition and K+, Na+, Si4+ and Al3+ from the amorphous geopolymer network. These phases seem to improve the resistance to thermal treatment (ramp of 1 degrees C/min, 800 degrees C, 1 h). However, the formation of metastable phases such as kilchoanite during a rapid thermal treatment (800 degrees C, 10 min) decreases the mechanical resistance (sigma/10). In conclusion, the use of potassium alkaline solution (Si/K = 0.58) and flash calcined argillite induce better thermal resistance.