Excavation and removal of unstable clayey soils before construction works, e.g., in infrastructure, residential or commercial buildings, etc., can generate vast amounts of waste clay deposits. Treatment of those clays to achieve suitable supplementary cementitious materials (SCMs) for use in concrete production can extensively contribute to a circular economy. Mechanical activation (MC) by ball milling has shown suitability to be an alternative and sustainable method to process clays and to obtain dehydroxylation at reduced temperatures and without the addition of chemicals. Furthermore, BM can induce amorphization, increased chemical reactivity, and improved pozzolanic properties. Amorphization of crystalline phases can be achieved also for poorly reactive materials such as air-cooled blast furnace slags (ACBFS), which can be further utilized as a precursor in sodium silicate alkali-activated systems. This study shows how mechanical activation is promoting the reactivity of clay and ACBFS, and their potential to be used as a replacement for cement in concrete production. Evaluation of the pozzolanic activity before and after treatment was performed for the treated clay, suggesting increased pozzolanic properties. While alkali-activated systems based on mechanically treated ACBFS reached, after 28 days, comparable compressive strength values with the commonly used ground granulated blast furnace slag (GGBFS)