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One of the promising routes to make the cement-based materials eco-friendly is to use alkali-activated materials. In general, any material that has certain amount of reactive silica and alumina can be alkali activated; yet, the process and resulting material properties are dependent on the composition of the solid precursor. There is a wide range of reactive aluminosilicate materials, which can be used for the synthesis of alkali activated materials, calcined clay being the most abundant and widespread aluminosilicate source. The reactivity of the calcined clay depends on the type and the content of the clay mineral. Metakaolin, which is the dehydroxylated form of kaolinite, is one of the most frequently used precursors in alkali activation, due to its amorphous phase and high reactivity. However, other clay minerals such as 2: 1 clays, which are found in natural clay deposits, have been rarely studied for the synthesis of alkali activated materials probably due to their complexity of mineralogy and structure, presence of interlayer cations and isomorphic substitution. This study investigates the reactivity of thermally treated pure 2: 1 clay minerals such as Ca-and Na-montmorillonite through alkali activation. The aim of this study is to understand the influence of the pure clay type and the influence of the interlayer cations on the activation potential and early age reaction mechanism and kinetics of montmorillonite. Three pure natural calcined clays were used: kaolinite (> 95 wt%) as a reference material, as well as Ca-and Na-montmorillonite (> 90-100 wt%). The two-montmorillonite clays were subjected to a purification process using the Jackson-treatment …