Zur Ermittlung der Sorptionsenthalpie von Baustoffen

On determining the sorption enthalpy of construction materials.

Although processes of water adsorption have been analysed intensively for a long time, technical literature provides only scarce information regarding the sorption enthalpy of building materials. However, such data is required for calculations of transient coupled heat and moisture transfer. In this context, latent energy is relevant, which includes the enthalpy of evaporation during the water/vapour phase change and the sorptive bonding enthalpy during water ingress in the pores of building materials. For higher moisture contents the sorption enthalpy does not depend on the material, but only on the gas constant and on the temperature. The sorption enthalpy becomes the smaller the higher the moisture content is. If we have hygroscopic materials with small moisture content the sorption enthalpy depends on the material properties. It is increasing with decreasing moisture content and can be determined either by conducting calorimetric measurements or by measuring two sorption isotherms at two different temperatures. Due to a linear-logarithmic relationship, the measurement of merely two isotherms will be sufficient; it is not necessary to determine the entire isotherm family. The procedure of determination is exemplified specifying the individual calculation steps. From this, a simple formula for determining the sorption enthalpy can be derived.