The cement industry is under growing pressure to reduce production costs while minimizing its environmental footprint. Among the strategies showing promising prospects to address this issue is the use of natural pozzolans, particularly mordenite-rich tuffs. This study investigates the effect of moderate thermal activation on the reactivity and performance of mordenite-rich tuff (TB) when used as a partial replacement for Portland cement in standardized mortar formulations. The tuff was activated thermally to 300 and 500 °C prior to its use at 0, 5, 10, 15, and 20% by weight replacement levels. Several parameters were evaluated, including workability, total shrinkage, pore size distribution, dynamic Young’s modulus, mechanical strength, and microstructure by SEM analysis. The results showed that thermal activation had a positive influence on mordenite-based mortars. Calcination improved flowability, reduced shrinkage, increased mechanical strength, and refined the pore structure by lowering the proportion of large capillary pores. These findings suggest that controlled thermal activation enhances the pozzolanic reactivity of mordenite-rich tuff, making it more suitable as an eco-friendly supplementary cementitious material.