The Moisture Absorption/desorption Behavior And Effective Thermal Conductivity Of Autoclaved Aerated Concrete Toward Energy-efficient Buildings

The moisture absorption/desorption behavior and thermal conductivity is the most important basic physical parameters of porous building materials while studing the thermal performance of building enclosur and energy consuming. In this paper, autoclaved aerated concrete(AAC) has been chose as the typical self-thermal-insulation enclosure structure material, by means of scanning electron microscope, X-ray diffractometer and mercury injection apparatus, the basic physical parameters of AAC such as the microscopic pore structure, bulk density, skeleton density, open porosity, total porosity have been characterized.In order to determine the absorption/desorption behavior of AAC, the paper aims to study the dynamic isothermal absorption/desorption curves of AAC with various porosity under different ambient relative humidity. For the adsorption process, the dynamic water absorption of AAC increases with raising relative humidity under the same porosity. When the relative humidity reach80%RH or more, the absorption growth more intense. For the desorption process, the moisture content of AAC change more with larger porosity. For the same porosity AAC, the time to reach dynamic desorption equilibrium will increase with the increasing of relative humidity. In addition, this paper also study the effect of temperature on the desorption rate of AAC, and found that the higher the temperature, the greater the desorption rate, witch means that when the temperature decreases, the internal moisture is hard to release. For perennial high humidity area, we should concern more about the interior condensation of the building wall in order to improve the thermal insulation of the building and indoor thermal comfort of human body.In order to study the effective thermal conductivity of moist AAC, this paper determine the optimum parameters by adjusted the probe model, the test of time and the output power of the Hot Disk thermal constants analyzer. With the optimum parameters, the thermal conductivity of dry and moist AAC is determined by using the Hot Disk. The result showed that the thermal conductivity of AAC is influenced by the composition of the pore structure and internal components and it increases monotonously with raising the moisture content but decrease monotonously with raising the porosity of AAC. When moisture content is below15%, the thermal conductivity of AAC increases rapidly with the increase of moisture content while the moisture content is greater than15%, the change of the thermal conductivity gradually slows down. It is worth noting that, when in the hot-sumer and cold-winter zone with the annual average relative humidity of about80%RH, the equilibrium moisture content of AAC would reach more than5%, which means that the thermal conductivity of AAC upgrade nearly twice than the dry state.Furthermore, on the basis of the fractal theory, this paper describes three method to calculate the fractal dimension of AAC. Since the image method is more workable, low cost, easy data qcquisition than mercury intrusion method, it is more suitable for the calculation of fractal dimension of porous building materials with continuous solid phase. Besides, based on Feng’s research, the paper had predicted the thermal conductivity of AAC under dry state by using a simplified model. By comparing with the experimental data, this model proved to be suitable to predict the effective thermal conductivity of the solid-phase continuous porous materials (such as AAC).