Marls, widely available globally, are under research as supplementary cementitious materials. This study investigated metamontmorillonite reactivity and optimal calcination temperatures. The complex composition of marls, including minerals like montmorillonite and dolomite alongside chemical oxides such as CaO and MgO, posed challenges in determining the ideal calcination temperature. Material transformations at varying temperatures were tracked using X-ray diffraction, scanning electron microscopy, and nuclear magnetic resonance techniques. Comparative analysis of static and rotary kilns evaluated mortar compressive strength after 28 days, determining suitable replacement levels and the best calcination temperature. The study determined that calcination at 850 °C maximized pozzolanic reactivity, resulting in the formation of β-C2S. Static furnace calcination showed a slight advantage over rotary kiln. Mortar with 30% calcined marl replacement at water: binder ratio (W/B = 0.45) exhibited the highest compressive strength. Hydration analysis revealed the formation of calcium carboaluminate and hydrotalcite hydrates, with Al/Si = 0.18 composition.