The capillary and osmotic forces acting between the particles cause a clay to absorb water during moistening. The resulting increase in volume is referred to as swelling. The water molecules are bonded to the clay minerals by surface activity and develop a water shell surrounding the particles.

In the presence of excess water a distinction is made between two types, inter-crystalline and intra-crystalline swelling. Inter-crystalline swelling is characterized by attachment of the water dipoles to the free surface of the clay minerals. Interchangeable ions (e.g. Ca2+, Mg3+, Na+, K+) are adsorbed here. These cations have a strongly varying water binding capacity. In addition to the size of the specific surface and the existing free valances, this results in the extent of the inter-crystalline swelling capacity of the different clay minerals.

Intra-crystalline swelling is of particular relevance for the practical use of montmorillonite clay minerals. Intra-crystalline swelling refers to the phenomenon of water entering in between the multi-layer packages of the clay particles which may result in a significant expansion of the distance from on layer package of the matrix to the other. With the montmorillonite, the multi-layer packages are only bonded by weak secondary valence forces allowing for the inclusion of water.

The amount of the swelling capacity is largely influenced by the type of the ion deposits and their packing density (ion exchange capacities). Bentonies with Na+ ion deposits (s. Sodium bentonite) can virtually swell without limit. -The swelling capacity of Ca2+ bentonites (s. Calcium bentonite), however, is limited.

According to W. Tilch, the swelling of bonding clays can be divided into three stages:

1. Capillary absorption of the water (corresponds to inter-crystalline swelling)

2. Entering of the water into the space between the multi-layer packages (corresponds to intra-crystalline swelling)

3. Hydratation of the interchangeable cations