One of the governing mechanisms for the colloidal behaviour of clays is isomorphous substitution, i.e. the exchange of cations of similar size and valence whilst the crystal structure is preserved. This process is completed during the formation of minerals and is the factor that determines the permanent charge of the mineral and thus its cation exchange capacity. Combined with the pore water chemistry, this permanent charge of the clay mineral governs the behaviour of the colloid system. Isomorphous substitution is significant in expandable minerals. Smectite minerals have high cation exchange capacities (CEC), in the range of 80-150 meq/100g [24] and are vulnerable to charge changes. Therefore, the emerging scale length in the SAXS **data** could be attributed to replacement of cations, especially from the interlayer exchangeable cations due to the new charge equilibrium of the global clay stacked structure. Large pore water gradients, and hence flow rates, present in the monotonic loading stages would not allow a stable ionic composition around the particles, required for an equilibrium state. On the other hand, illite, which is a low cation exchange capacity mineral, does not show similar behaviour. Nonetheless, average increase in spacing of values around 1 Å is relatively small, close to the bond length of covalent bonds.