PRESSURE MEASUREMENT AS A TOOL TO IDENTIFY MOISTURE TRANSPORT MECHANISMS IN CONVECTIVE DRYING OF NON-SHRINKING MATERIAL

摘要

The drying process is one of the most important stages in the production of building materials. The choice of the drying method affects the chemical and physical properties of the final product. The aim of this research is to measure and analyze the dynamic changes of internal pressure in non-shrinking, porous material during convective drying. In this work the problem will be discussed with special attention to the behavior of rewetted plaster. A commercial gypsum of company PIOTROWICE II (Alpol brand), typically used in construction and decorative plastering was applied. Gypsum was mixed with water in recommended proportion of 0.6 water/gypsum and drying experiments were performed at 50°C. The changes in sample overall mass as well as pressure and material temperature on the midpoint of sample axis were monitored. On the basis of the obtained experimental data of axial pressure, it is possible to perform a more detailed analysis of mass and heat transfer mechanisms than based on the drying kinetics alone. The pressure trends in the sample allow one to determine the moment of transition from the first to the second drying period, without the need to determine the kinetics of drying. The element of novelty consists of using a direct internal pressure measurement to provide information on the variation of the actual drying rate and mass transfer mechanisms.