Study On Preparation And Thermal Insulation Properties Of A Novel Expanded Vermiculite/Xonotlite Composite

Energy consumption in buildings is an important part of the global energy consumption structure,and the development of building energy efficiency technologies is an important way to cope with the global energy crisis and solve the energy shortage problem in China.Building insulation materials are materials or material composites that have significant resistance to heat flow,and the development of new building insulation materials is one of the most important measures to achieve energy efficiency in buildings.Vermiculite is a naturally occurring multi-layered silicate mineral that expands easily with heat to form expanded vermiculite.Expanded vermiculite is an ideal thermal insulation material for buildings,but the disadvantage of poor high temperature insulation limits its wide application.In this paper,a new expanded vermiculite/xonotlite composite was prepared by hydrothermal synthesis using vermiculite as the matrix material,and the high-temperature thermal insulation performance of expanded vermiculite was enhanced by intercalation of xonotlite fibers.The main research elements were as follows:(1)Study of the expansion properties of vermiculite.The effect of heating temperature and heating time on the expansion properties of expanded vermiculite was examined and the relationship between the expansion rate of vermiculite and its thermal insulation properties was analyzed.Characterization and test results show that the optimum expansion temperature of vermiculite is900 ℃,and the expansion rate of expanded vermiculite prepared by heating at this temperature for 60 s is maximum.The thermal conductivity of expanded vermiculite decreased as its expansion rate increased,and the lowest thermal conductivity of expanded vermiculite sample was 0.474 W/m K at 800 ℃.(2)Study of the preparation process of xonotlite.The differences between xonotlite fibers prepared by two hydrothermal synthesis processes were examined and the synthesis mechanism of nano xonotlite was analyzed.The results show that the two-step hydrothermal synthesis process with intermediate insulation followed by warming can produce xonotlite fibers with smaller diameter and secondary particles than the conventional direct hydrothermal synthesis process,which was 75 nm and 85 μm,respectively.What’s more,the thermal conductivity of xonotlite decrease with the decrease of diameter.(3)Study of preparation and properties of expanded vermiculite/xonotlite composite.Expanded vermiculite/xonotlite composite was prepared via hydrothermal method.The optimum process parameters for the intercalation experiments and the effect of raw material ratios on the morphology and thermal properties of the composites were investigated.On this basis,opacifiers were added to the composite to further improve the thermal insulation properties.And the effects of the particle size,type and percentage of opacifiers on the extinction capacity were further investigated.The intercalation of xonotlite greatly improved the thermal insulation of the expanded vermiculite at high temperatures.When the mass ratio of calcium silica raw material to expanded vermiculite was 4:1,the thermal conductivity of the composite at 800 ℃ was0.2116 W/m K.In addition,the use of Si C as an opacifier could further improve the high temperature insulation properties of the composite.The thermal conductivity of the composite decreases from 0.2116 W/m K to 0.149 W/m K after compositing with Si C.(4)Study of the heat transfer mechanism of composite.A construction of heat transfer model of composite was built through the theoretical treatment of heat transfer in composite materials with common models and methods of heat conduction.The results showed that gas-solid coupled thermal conductivity was the dominant mode of heat transfer in composite at low temperatures.As the temperature rising,radiative heat transfer gradually dominated the total thermal conductivity,which was 93.07% of total thermal conductivity at 800 ℃.A comparison of the calculated data with the test results showed that the heat transfer model was in general agreement with the experimental results,with a maximum error of only 11.82%.