Role of disjoining pressure in cement based materials

During the hydration process of Portland cement a nanoporous gel is formed. The high internal surface of the hydration products, mainly calcium-silicate hydrates, and the numerous adsorbed cations interact with water. At an equilibrium moisture content below 50% adsorbed water molecules reduce the surface tension of the solid particles. Water adsorbed at higher relative humidity is at the origin of disjoining pressure acting in small gaps between particles. Both processes are major mechanisms of shrinkage and swelling of hardened cement paste. Results of investigations into the disjoining pressure are summarised. The relevance of disjoining pressure for the behaviour of cement-based materials will be discussed.