Prestudy on diffusion and transient condensation of water vapor in cement mortar

The kinetics of sorption of water vapor in mature cement mortar are examined experimentally by the use of a sorption balance (DVS1000). The relative humidity in the sample chamber is obtained by mixing dry and saturated air using flow regulators with very high accuracy. A predefined change of relative humidity in the sample chamber can be obtained in about 30 s. The time needed for equilibration in terms of the mass of a finely grained mortar sample is shown to be in the order of 2500 min for a 0-96% step in relative humidity at 25 degreesC. This fact calls for a refinement of modeling of diffusion of water vapor in materials such as mortar to take into account the kinetics of sorption. The established model involves a transient mass exchange term that explicitly describes the rate of condensation at internal material surfaces. This function becomes zero when located at the equilibrium sorption isotherm. The 'global' mass diffusion velocity of water vapor in the air-filled porosity is assumed to be significantly affected by the mass concentration of liquid water in the pore space. A satisfying match between the proposed model and measurements presented by U. Daian [Transp. Porous Media 4 (1989) 1.] was obtained by using nonlinear diffusion and sorption characteristics. In order to get a satisfying match, the sorption kinetics was, however, forced to be much slower than the recorded values obtained by the sorption balance. Some speculations as to the reason of this marked difference are performed. (C) 2002 Elsevier Science Ltd. All rights reserved.