The relative humidity dependence of the permeability of cement paste measured using GARField NMR profiling

The relative humidity (RH) dependence of the water permeability of cement is calculated from the water concentration profile of a paste exposed to an RH gradient and the desorption isotherm. The profile is measured using GARField, standing for Gradient at Right Angles to Field, NMR. The isotherm is derived from other earlier NMR measurements. The Darcy equation gives the intrinsic permeability as 4.6 × 10− 21 m2. The apparent intrinsic permeability to water flow shows a broad “U” shape dependence on RH, with a minimum of 7 × 10− 22 m2 at RH 55%. The “U” shape is attributed to the fact that the transport mechanism involves a coupling of liquid and vapour modes. The data is further analysed in terms of a model of coupled liquid and vapour diffusion and Darcy flow due to Baroghel-Bouny et al. (Cem. Concr. Res. 2011 41 828) from which the relative liquid water and vapour permeabilities are calculated. They are strongly RH dependent. The former increases with increasing RH; the latter decreases.