Table 1
This is well supported by the marked depletion of the XRD belonging to Quartz in bentonite and montmorillonite patterns (denoted as Q), and the presence of the main XRD lines of crystalline silica in the pattern of the residual ash (Fig. S10 ). A shift of the 001 XRD line from a 2θ value of 7.357˚ for bentonite to 9.142˚ was noticed for NaMt, indicating a decrease in the interlayer spacing. This shift is a special feature of the lamellar structure and is due to the replacement of bulky impurity particles by sodium cations. This was confirmed by the noticeable sharpening and increased intensity of the main XRD lines, which is a special feature of homo-ionic form of montmorillonite and almost perfect parallel arrangements of the clay sheets.
The Na+ cation has a lower polarizing power and thereby lower intrinsic acidity and weaker capacity to attract water molecules than multivalent cations. This somehow explains the lower WRC value observed for NaMt compared to bentonite. MNP incorporation induced a marked decrease in CRC, due to at least two kinds of competitive interactions. Some MNP interactions occur with the lattice oxygen atoms (-Si-O-Si- with strong Lewis basicity). Other interactions take place between the OH groups of both terminal in-plane silanols and out-of-plane Si-OH with weaker basicity and MNPs on the surface (Sulpizi, Gaigeot, & Sprik, 2012).