| In this paper,cement-based coatings and gypsum-based coatings with different thickness(1mm,2 mm,3 mm,5 mm)were prepared by adding heavy calcium carbonate,ash calcium,talc and rubber powder,cellulose into white cement and gypsum as inorganic gelling agent,respectively.The diffusion depth and adsorption and the diffusion behavior of formaldehyde and toluene in the coatings were studied by static dual chamber method,and the mechanism of adsorption diffusion were discussed as well.The phase,microstructure,pore size distribution and surface groups of coating samples were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),mercury intrusion porosimetry(MIP),gas adsorption apparatus and Fourier transform infrared spectroscopy(FT-IR).The adsorption and diffusion mechanism was discussed combined with the material properties,structure and adsorption diffusion results of gas in the coatings.The experimental results show that the formaldehyde can be absorbed by the inorganic gel composite coatings,and the adsorption mount were related to the time,coating thickness and gelled material directly.It shows that the adsorption mount present exponential law with time.The adsorption mount in the cement-based coatings has a tend of exponential growth with coating thickness,while the adsorption mount in the gypsum-based coatings has a tend of linear growth with coating thickness.Under the same thickness,the adsorption mount in cement-based coatings is 2-3 times to gypsum-based coatings.Meanwhile the diffusion behavior of formaldehyde exists in the coatings,and the diffusion concentration were related to the time,coating thickness and gelled material directly.The diffusion concentration of formaldehyde present exponential growth with time and linearly decreases with the coating thickness.For different types of coatings,the diffusion concentration of formaldehyde in gypsum-based coating is higher than that of cement-based coating.The diffusion depth in the cement-based coating is 3 mm,while in gypsum-based coating arrives to 5 mm.It is found that inorganic gel composite coating has a poor adsorption ability to toluene.The adsorption mount were related to the time,coating thickness and gelled material as well.It shows that the adsorption mount present exponential law with time and linearly decreases with the coating thickness.Under the same thickness,the adsorption mount in cement-based coating is 1 to 2 times than in gypsum-based coating.Meanwhile the diffusion behavior of toluene exists in the coatings,and the diffusion concentration were related to the time,coating thickness and gelled material aswell.The diffusion concentration of toluene present exponential growth with time and linearly decreases with the coating thickness.For different types of coatings,the diffusion concentration of toluene in gypsum-based coating is 2 times than that in cement-based coating.It found that the adsorption quantity of two kinds of gas in the coating is basic quite from the contrast analysis of formaldehyde/toluene adsorption diffusion in the same thickness.While the diffusion amount of toluene were higher in two coatings than formaldehyde.The main reason is that toluene is water-soluble nonpolar gas,and the coating can not be absorbed it easy.XRD results show that the gelled material takes effect from the hydration in the coating,the coating is porous from SEM.Two coating belong to class II adsorption isotherm which means the adsorption mechanism is monolayer adsorption to the transition of multimolecular adsorption from Nitrogen adsorption.FT-IR shows that the cement based surface coating contains a lot of activity groups and the absorption peak decreased significantly after adsorption formaldehyde,that should be the result of physical adsorption.But the absorption peak has no obvious change in gypsum based coating after adsorption formaldehyde,and it can not reflect the gas adsorption.The main adsorption mechanism of formaldehyde in the coating is mineral surface adsorption,including the chemical monolayer adsorption to multilayer physic adsorption,pore capillary condensation and the absorption of water molecules.Formaldehyde is water-soluble gas polarity which can be adsorbed to the mineral surface and dissolved in water molecules easily.But toluene is nonpolar gas that can be adsorbed hardly,so the adsorption quantity in coating is small.The gas diffusion in coating mainly related to the pore structure and material property,and the diffusion mechanism was analyzed from the micro structure and pore size.It can be founded from the testing results that the putties are porous.The MIP shows that the cement-based coatings has a total porosity of 36.5 %,and the pore size of the composites is in the range of 3.6 nm to 1 μm.The pore structure is mesopore,and the main diffusion mechanism is transitional diffusion.The main aperture is 1 ~ 100 μm,and the pore structure is macropore for gypsum based coatings with a total porosity of 51.6 %.The diffusion mechanism were fick diffusion and transitional diffusion.Different pore structure leading to different diffusion mechanism for two kinds of coatings,so the diffusion amount is different obviously.The porous structure of the coating is the main reason for its adsorption and diffusion ability.The absorption and diffusion of VOC can be affected by the adsorption on mineral surface and the internal diffusion. |
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