Study On Moisture Buffer Capacity Of Building Hygroscopic Materials And Its Influencing Factors

The relative humidity of indoor air is one of the important indicators that affect indoor air quality and thermal comfort.Different from the traditional active humidity control method that consumes a lot of energy,the frequency and magnitude of peak of indoor relative humidity can be reduced by absorbing or releasing the moisture in the surrounding air through the porous structure of the moisture buffer material that is a kind of passive humidity adjustment methods.Obtaining the practice buffering capacity of the hygroscopic material and clarifying the effect of different factors on it is not only the key to knowing the actual application of the moisture buffer material,but also the basis for the combination of the humidity control material and other temperature and humidity control methods.In this paper,diatomaceous earth,sepiolite,palygorskite and biomass fiber are selected as the moisture buffer matrix,and gypsum is used as the cementing material to make the samples of moisture buffer material.The practical moisture buffer value of the samples are produced.Moisture buffer materials can be divided into three categories:A,B,and C through the vapor permeability coefficient and the equilibrium moisture content.Through the method of numerical simulation,the influence of material thickness,moisture boundary conditions and surface mass transfer coefficient on the humidity control performance of different types of materials is studied.And the micro-topological structure of different porous media is constructed by random generation method.Based on Fick’s law,the diffusion law of water vapor in porous media with different porosity and different pore structure is analyzed.The research results show that the four moisture buffer materials selected in this paper all have a porous structure with a larger specific surface area.The practical moisture buffering capacity of palygorskite and biomass fiber samples reached the good level in the Nordtest Project’s classification of material humidity control performance,while sepiolite and diatomite reached the medium and limited levels.For Class A materials with large vapor resistance factor and small moisture capacity,when the thickness is only about2mm～5mm,the humidity control ability basically reaches the maximum;Class B materials with small vapor resistance factor and small moisture capacity need to be the thickness of 20mm～30mm can make the material reach the maximum humidity control ability;because of the characteristics of small vapour resistance and large moisture capacity,the humidity control capacity of C type materials is larger,and the thickness required to achieve the maximum humidity control capacity is smaller than that of B materials.Under different humidity boundary conditions,the moisture absorption and sorption capacity of various materials are different.So there is certain limitations to evaluate the moisture buffer effect of materials to take a practical moisture buffer value tested in only one kind of humidity range.The surface mass transfer coefficient has a significant impact on the humidity control ability of the material.The surface mass transfer coefficient between 1×10^-8 kg/（Pa·m²·s）to 7.35×10^-8kg/（Pa·m²·s）has a great influence on the moisture buffer effect of the material.the humidity control ability of the material does not change significantly,as the surface mass transfer coefficient increases.For porous media with different pore structures,which the better the connectivity of the pores,and the direction of the pores is consistent with the direction of water vapor diffusion,the shorter the time required for water vapor diffusion to reach a steady state,and the higher the water vapor concenrtation on right side.