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Clay minerals are hydrous aluminum phyllosilicate minerals with a layered structure. Each layer consists of tetrahedral sheets of silicon-oxygen and octahedral sheets of aluminum-oxygen or magnesium-oxygen. They are held together by weak van der Waals forces and hydrogen bonds, which allows the clay minerals to have a high affinity for water.

When water is added to clay minerals, it can penetrate between the layers, forming hydrogen bonds with the oxygen atoms in the silicon-oxygen and aluminum-oxygen sheets. This process is called hydration.

Upon the addition of water, the hydration process causes an increase in the interlayer distance between the clay layers. This phenomenon weakens the van der Waals forces and hydrogen bonding between the layers, allowing them to slide past each other more easily when a force is applied.

Hydroplasticity is an important property of clay minerals, as it allows them to be easily molded and shaped when wet. As the clay dries, the water evaporates, causing the layers to move closer together and the hydrogen bonding and van der Waals forces to strengthen again. This results in the hardened and rigid structure of the clay, retaining the desired shape. In conclusion, clay minerals become hydroplastic when water is added due to the hydration process. The water molecules penetrate between the clay layers, increasing their interlayer distance, weakening the existing interlayer forces, and enabling the layers to slide past each other easily when a force is applied. This property is vital for the ability to mold and shape clay materials.