Water vapor diffusion properties of Obernkirchener sandstone: Analysis of DVS data

In this paper, experimental characterization by dynamic vapor sorption (DVS) experiments on Obernkirchener sandstone samples is presented, giving access to the sorption isotherm of the porous building material. The latter may be classified as sigmoidal type II with a hysteresis in the high-humidity range. As the specimens were prepared with a defined geometry (circular plate, thickness of 7 mm), the recorded mass history in response to a step-wise increase or decrease of the sample-enclosing humidity was employed to backcalculate diffusivity of the porous material. Hereby, modeling focuses on vapor diffusion in the unsaturated pore space with water vapor concentration gradients as the driving force. We disregard capillary transport in the model, which, hence, is only valid in the low-humidity range, i.e. modeling in the scopes of a porous, hygroscopic material. The vapor diffusion coefficient of the porous material is estimated as 2600 mm2/h (at a temperature of 25 °C), what relates well to the bulk diffusion coefficient of water vapor in air taking into account tortuosity and porosity of the considered sandstone. As regards the influence of surface resistance to mass transfer, the samples may be characterized as borderline thick with a Biot number of approximately 50.