Influence of Environmental Factors and Microstructure on Water Vapor Permeability of Gypsum-Based Inorganic Mineral Materials

The water vapor permeability coefficient is a key parameter describing moisture transport in building materials, exerting a crucial influence on their hygrothermal performance and indoor humidity regulation. However, uncertainties remain in existing experimental methods and parameter-correction approaches. In this study, three gypsum-based inorganic materials—gypsum–zeolite (G-Z), gypsum–diatomite (G-D), and gypsum–magnesium aluminum silicate (G-A)—were systematically investigated using the wet-cup method to measure their vapor permeability coefficients. The effects of the average relative humidity (RH) and RH gradient on the measured results were quantitatively analyzed. The water vapor transport characteristics of gypsum-based inorganic mineral materials were also analyzed. The results indicate that the influence of the RH gradient is comparatively minor, whereas the average RH is the dominant factor. Within the average RH range of 22–90%, the vapor permeability coefficients of the three materials exhibit a nonlinear dependence that can be accurately described by a binomial function. In the low-humidity range (RH < 70%), the coefficients remain nearly constant, while in the high-humidity range (RH > 70%), they decrease significantly with increasing RH. Microstructural characterization further reveals that G-Z exhibits a well-developed microporous structure and large specific surface area, leading to high moisture adsorption but low permeability; G-D shows superior permeability due to its well-connected meso-and macroporous channels; and G-A forms a dense interlocking structure with few connected pores, resulting in the highest vapor resistance among the three materials.