Here, the valorization of Callovo-Oxfordian (COx) argillite from the geological layer focused by the Cigeo project (Industrial Center for Geological Disposal of radioactive waste) is studied by Andra. Its reuse by alkali-activation is studied in order to develop thermally resistant binders. The thermal properties of various argillite-based samples are studied at 800 degrees C, and they present a good resistance to a slow firing environment but a lower resistance to thermal shock. This article discusses the influence of the sample compositions on the thermal resistance. First, the thermal activation of argillite has a significant impact on the thermal resistance of the alkali-activated samples. Indeed, a higher resistance is reached with the use of furnace-calcined argillite due to the complete dehydroxylation of clay minerals and decomposition of carbonates. However, the use of flash-calcined argillite (partial dehydroxylation and decompositions) leads to different species present in the mixture and therefore may be conducive to higher structural evolutions during sample firing. Then, the influence of the nature of the alkaline silicate solution is highlighted. It appears that the use of sodium-potassium silicate powder-based solution (Si-Na-K) is conducive to lower reactivity and more viscous flow than a liquid-based potassium solution (Si-K). Finally, the addition of filler, sand (SiO2) and calcined clay (Al2O3-SiO2) allows for better thermal resistance but limits the development of mechanical properties.