ABSTRACT
Liquids are effectively incompressible when subjected to direct stress, which implies that the elementary particles are in direct contact. However, liquids obviously flow under the action of the shear stress, which shows that the particles are able to move relative to each other i.e. there are no primary bonds between them. There are two important consequences of viscoelasticity. The first is that the stress–strain relationship is non-linear. The second consequence is known as Boltzmann’s superposition theory. Gels are formed when a liquid contains a fairly concentrated suspension of very fine particles, usually of colloidal dimensions. Adsorption is a reversible process; when the gel is saturated it may be heated to drive off the water and its drying powers regained. The most important engineering gel is undoubtedly hardened cement, which develops a highly rigid but permeable structure of complex calcium silicates by the chemical reaction between the fresh, powdered cement and water.
