The use of supplementary cementitious materials (SCMs) is an effective strategy for reducing cement consumption and associated CO₂ emissions. This study investigates the potential of coal mine tailings (CMT) obtained from a coal washing plant as an alternative SCM through selective activation methods. Three activation approaches were evaluated: mechanical activation, thermal activation at 600 and 800 °C, and pozzolan-assisted blending with fly ash and trass. Mortar specimens were prepared according to EN 196-1 with cement replacement levels of 5-20 wt.%, and compressive strength was determined after 28 and 90 days of curing. The results showed that mechanical activation reduced particle size but did not improve compressive strength. In contrast, thermal activation significantly enhanced the cementitious performance of CMT. The highest compressive strength, approximately 50 MPa, was achieved at 90 days with 10% CMT replacement after thermal activation at 800 °C, representing an approximately 100% increase compared with untreated CMT at the same replacement level. Pozzolan-assisted blending mainly improved long-term strength development. The binary blend containing 2.5% CMT and 2.5% trass achieved the highest 90-day strength among the pozzolan-assisted mixtures, reaching approximately 41 MPa and representing an increase of nearly 32% compared with the 5% untreated CMT mixture. Overall, the findings show that coal mine tailings can be transformed into effective SCMs through appropriate activation, particularly thermal treatment, providing a sustainable pathway to convert coal-processing wastes into value-added construction materials and support circular economy objectives.