The microstructural nature of Portland cement-based products is complex and gives rise to time-dependent physical and mechanical properties, with significant changes taking place over periods of many months or even years. These effects are more fundamental than the continuing hydration reaction of the initial unhydrated cement, and involve slow ageing processes in the colloidal hydration products. In addition, the chemical binding of water in the hydrates takes on a series of energies which result in ready loss of moisture over a wide range of humidities, and a readjustment of water molecules between different sites within the microstructure in response to small changes in environmental temperature and humidity. All such changes, some of which are irreversible, are reflected in changes in physical and mechanical properties. Conventional physical and mechanical attributes suitable for describing the behaviour of solids are allotted to hardened cement pastes and concretes, for example, thermal expansivity or elasticity, but it should be recognised that these functions have only approximate validity for these systems and are dependent on the length of time for which the external stimulus is applied.