Beyond the sorptivity: definition, measurement and properties of the secondary sorptivity

Capillary imbibition in brick, stone and concrete occurs in two stages. The primary process, which occurs in the standard test to measure sorptivity, is a spontaneous imbibition in which air is displaced by the invading liquid (usally water). In primary imbibition, the displacement of air is incomplete, and some air is trapped. The residual air content lies usually in the range 0.1–0.4 of the volume-fraction porosity. Primary imbibition is followed by a much slower secondary process in which trapped air in the interior of the material dissolves in the liquid phase and diffuses to the unsealed external surfaces where it escapes. As air is lost, there is further imbibition of liquid to replace it. Eventually, all trapped air is lost, and the material reaches saturation. There is current interest in using the rate of secondary imbibition to define a secondary sorptivity, and speculation that this may be a useful property for characterizing porous construction materials, particularly in relation to durability. This paper analyses the secondary imbibition process, and provides a definition of the secondary sorptivity which is independent of the dimensions of the test specimen. The analysis is supported by experimental data on Ancaster and Portland limestone test materials.