Investigation On The Measurement Methodology Of Water Vapor Transfer Properties Of Autoclaved Aerated Concrete (AAC) And The Basic Thermal Properties Of AAC-based Phase Change Composites

With the increase of the building energy consumption,it is imperative to promote the ‘Energy Efficient Buildings' strategy,which is likewise essential to fulfill the Chinese “Dual Carbon” goal.The coupled heat and moisture performance of building envelopes is the foundation for their hygrothermal design and consumption analysis.To measure and improve the basic thermal and hygric properties is also of great significance to guide and fulfill the “Energy Efficient Buildings' strategy.There are two main research directions towards the coupled heat and moisture transfer of building envelopes.One is investigating the heat and moisture transfer properties of porous materials precisely in order to provide the analysis of coupled hygrothermal performance with accurate input parameters.Although we have developed measurement approaches towards the heat transfer properties of porous materials,the investigations on the moisture transfer properties are still developing.There is a lack of a rapid and accurate means of measurement on the water vapor transfer properties of porous building materials.Another is the improvement of the heat and moisture adjustment performance of building envelopes.As the moisture buffering is inherent to the porous materials,attentions should be paid to the improvement of their thermal mass.The main approach is to involve phase change materials(PCMs).However,the thermal adjustment performance is restricted for current PCM-embedded porous building materials due to the limited PCM loading.Meanwhile,there still lack the investigations on the basic hygrothermal properties of PCM-embedded porous building materials.Therefore,in this paper,the methodology of water vapor transfer property towards the hygrothermal transfer performance,and the improvement of the heat storage property towards the heat and moisture adjustment performance are investigated,taking autoclaved aerated concrete(AAC),which is one typical type of porous building material,as an example.Firstly,an important improvement was made on the transient measurement approach of water vapor transfer property by concerning the hygroscopicity of the porous material in its basic equation.Investigations based on both original and improved methodologies were made on three different types of AAC with four lengths of each type.Comparisons were also made on the water vapor permeabilities from improved transient and standard steady-state measurements.Results showed that the water vapor diffusivities from the original transient measurement decrease monotonously by increasing the sample length,which contradicts the basic concept as the diffusivity is the inherent property of the material.With respect to the diffusivities and permeabilities within one AAC type from the improved measurement approach,good agreement was found among the results of various sample length.Plus,the transient-measured permeabilities match well with the ones from the steady-state measurement.All in all,this improved methodology is good enough to be further developed,and to measure the water vapor transfer properties in a large scale in order to fulfill the basic hygroscopic database towards the coupled heat and moisture transfer performance.Secondly,in order to improve the hygrothermal adjustment ability of building envelopes,PCMs was directly impregnated into the pores of AAC to manufacture PCM-AAC composites.The impregnation performance was evaluated,and the thermal conductivities and volumetric heat capacities,as the basic thermal properties of coupled heat and mass transfer,was measured in order to evaluate their thermal storage performance.Results showed that the optimal PCM loading is two thirds of the corresponding saturated loading of each porosity,with the improvement of the thermal storage performance by 100% as compared by the pristine AAC materials.Plus,the liquid PCM is unlikely to leak under this loading.Lastly,although the thermal storage performance has indeed been improved by impregnating PCMs,some pores of AAC are hence filled,which affects its thermal insulation performance.Therefore,a novel PCM-composite aerated concrete(AC)was formed by adding microencapsulated PCM as an ingredient at various loadings.Results showed that the microencapsulated PCM could cooperate with the aluminum powder to further increase the porosities of AC,which are all beyond 50%.The best thermal storage performance,as well as high porosity and compressive strength of the composite was found with the mass fraction of the aluminum powder and micro-encapsulated PCM of 1% and 3%,respectively,which is deemed to be potential to be utilized in building envelopes.