Abstract In the cement industry, it is usual to use calcined clays due to structural changes caused by thermal activations; however, there are other activation methods, such as mechanical activation by grinding. This method is not so common and has been relegated to the second level. Thus, in this work, the physical and mechanical changes in the structure of clays, induced by comminution processes, of standard clays and a low‐grade kaolinitic multicomponent clay, derived from weathering of metamorphic rock, were evaluated. These changes were measured using X‐ray fluorescence, X‐ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, specific surface area (SSA) by Brunauer–Emmett–Teller method, and lime fixing as a measure of pozzolanic activity. It was found that the milling process modified the structure of the clays and the pozzolanic activity, since the main reflection (001) disappeared, the dehydroxylation temperature decreased to a maximum differential of 150°C with 120 s of milling time, especially in T:O:T clays, and the SSA increased along with the lime fixing to a maximum of 119 m 2 /g and 46%, respectively. In this way, the milling process is effective for the mechanical activation of the clay, especially in multicomponent clay, making it interesting for use as supplementary cementitious material.