413 The crystallographic structures of the pristine Mnt and the Mnt/CaCO₃ mixture were analyzed using XRD patterns (Fig. 2 and Fig. 3). The pristine Mnt showed a characteristic diffraction peak at 2θ= 7.25°, corresponding to the (001) reflection with a basal spacing (d001) of 1.22 nm. In the presence of CaCO₃, this characteristic peak shifted to a lower angle of 2θ= 6.40°, indicating an expansion of the interlayer distance to 1.38 nm. This expansion is attributed to the ion exchange of interlayer Na+ with dissolved Ca2+ from limestone, forming a Camontmorillonite phase with a larger hydrated ionic radius [11]. Additionally, sharp diffraction peaks assigned to calcite (CaCO₃) were clearly observed in the mixture, confirming the coexistence of crystalline limestone. Figure 2 – X-ray diffraction pattern (a) pristine Mnt and (b) Mnt + CaCO3 Figure 3 – The 001 reflections from Mnt FE-SEM observations revealed the changes in the microstructure of Mnt due to the coexistence of CaCO3. Without CaCO3, Mnt particles formed an Edge-Face (E-F) stacked orientation, known as a 'house-of-cards' structure [12]. In contrast, the addition of CaCO3 (001) (100) (005) (a) (b)
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