The Adsorption Characteristics of the Enclosing Construction Materials

Abstract

The knowledge of the numerical values of the adsorption characteristics of the materials of the building envelope is necessary for the calculation of the thermal performance of these structures. The adsorption characteristics of the material are: constant с of the Brunauer-Emmett-Taylor equation (BET) of the water vapor sorption, the equilibrium sorption moisture w_m of the material, corresponding to the filling of the first monolayer of adsorbed water molecules (monolayer capacity), relative air humidity ф_т , corresponding to the filling of the first monolayer, and specific surface A of the material. There are two methods for processing isotherms of sorption of water vapor by the studied material in order to determine these characteristics. The first method is based on the use of the BET equation. The second method, called the N-method, is a modification of the first.

The BET equation is applicable for analysis of the sorption isotherm only in the range of relative adsorbate pressures from 0.05 to 0.35. The N-method is applicable for the analysis of sorption isotherms in the range of relative air humidity from 0.05 to 0.5. Thus, the main advantage of the N-method compared to the method based on the BET equation is that the N-method allows one to obtain more accurate information on the adsorption characteristics of the material under study, since this information is obtained by processing alarger number of experimentally obtained equilibrium values sorption humidity of this material.

In order to compare the two above-mentioned methods, these methods were used to process the sorption isotherms of eight building materials obtained in a vacuum sorption installation at a temperature of + 20 °C: cement stone, heavy concrete, expanded clay concrete, azerit concrete, red brick, cement-sand mortar and foam concrete. Based on the processing results, the adsorption characteristics were determined and the equations of the initial sections of the sorption isotherms of the studied materials were obtained at a temperature of+20 °C.