The scientific community's interest in geopolymers keeps on growing under environmental and climate change pressures. Although most studies concern metakaolin or industrial by-products as aluminosilicate precursors, few are those concerning clay cuttings characterized by little or no kaolinite content. This study proposes a method to activate illitic clay in order to obtain a geopolymer with high mechanical performances. The tested illitic material contained 53 wt% of illite/muscovite, 22 wt% quartz, 7 wt% feldspars and low content of calcite. The innovation consists in coupling thermal and mechanical activations favoring both the clay dehydroxylation and the destruction of the clay crystalline structure. Such increase of amorphous rate in the precursor is required to make the material reactive. The evolution of the precursor material after activation was followed by ThermoGravimetric Analysis (TGA/DTG), X-Ray Diffraction (XRD), Fourier-Transform InfraRed spectroscopy (FTIR) and Scanning Electron Microscopy coupled to Energy Dispersive X-ray spectrometry (SEM-EDX). The impact of the variations of the mechanical activation duration and ball:powder mass ratio (b:p) were mainly explored, as well as the Liquid:Solid mass ratio (L:S) which is a key parameter of the geopolymerization. Finally, an optimized geopolymer with a mechanical compressive strength reaching 126 MPa after 28 days has been manufactured with a grinding duration of 240 min, a b:p = 15 and a L:S = 0.5. The demonstration was done that a geopolymer can be manufactured from 100% illitic clay precursor, in addition to a relatively low amount of alkaline solution (L:S mass ratio of 0.5) then lowering the cost and the environment impact of the final geopolymer material. The resulting illitic clay based geopolymers can display very high mechanical resistance (compressive strength up to 126 MPa after 28 days), fully competitive with conventional building materials.