Study On Mechanical And Anti-freeze Performance Of Cement-based Encapsulated Capsule Phase Change Materials

With the further advancement of the national One Belt & One Road strategy,the construction of major concrete projects has been extended to the wider western region.The harsh natural environment in the northwest,especially the freeze-thaw environment,is very easy to cause freeze-thaw damage to bulk concrete projects,aggravate the damage and deterioration of concrete structures and functional degradation,resulting in a significant shortening of the service life of buildings and even causing terrible safety accidents or major loss of economic property.Therefore,it is of great significance to improve the active frost resistance of concrete structures in order to alleviate or solve the problem of freeze-thaw damage of concrete structures in cold areas.This article focuses on breaking through the technical bottleneck of "traditional concrete material passive anti-freezing",based on the principle of phase change heat storage and release,the capsulated phase change materials(CPCMs)are prepared through a controllable assembly method,and at the same time,it is introduced into the cement matrix to explore its anti-freezing performance of concrete materials.Finally,based on the heat and mass transfer theory of concrete,a finite element model of cement-based CPCMs were constructed to simulate the phase change process of encapsulated phase change material and the temperature field distribution of cement matrix.The main contents and relevant conclusions of this study are as follows:(1)Liquid paraffin,methyl laurate and n-tetradecane were selected as pahse change materials,diatomite,red mud,MSWI fly ash and expanded graphite were selected as carrier materials,and ethyl cellulose(EC)was used as coating materials.Based on the idea of "hard and soft phase particle assembly design",the CPCMs were assembled in an orderly manner.The physical and chemical properties of the CPCMs were investigated by means of scanning electron microscopy(SEM),gas adsorption,X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FT-IR),laser Raman spectroscopy(LRS),differential scanning calorimetry(DSC)and thermogravimetric analysis(TG).The results show that the encapsulated PCMs have good chemical stability and thermal stability,and the ethyl cellulose film is uniformly coated.The phase change temperature is near 0 ℃,the maximum latent heat value of phase change can reach 160 J/g,and it can exist stably at high temperature of 80 ℃.(2)The effects of CPCMs on thermal conductivity,mechanical properties and freeze-thaw resistance of cement matrix were studied by adding CPCMs into cement at mass ratios of 10%,15% and 20%.The results show that the addition of n-tetradecane/expanded graphite CPCMs significantly improves the thermal conductivity of cement matrix,and the thermal conductivity of cement matrix is proportional to the content of the cement matrix,and the content of the other four kinds of CPCMs are inversely proportional to the thermal conductivity.The incorporation of CPCMs into the capsule will weaken the mechanical properties of cement-based test blocks,and the higher the content,the greater the loss of mechanical properties.However,the incorporation of CPCMs can reduce the mass loss and strength loss of the cement test block in the process of freezing and thawing,and effectively enhance the anti-freezing and thawing performance of the cement matrix.The mass loss and strength loss of the cement matrix with CPCMs can be reduced by up to 75% and 186% respectively.(3)In COMSOL Multiphysics,the solid heat transfer model of cement-based test block of CPCMs was built.The temperature field distribution,average temperature and phase change process were simulated based on the apparent heat capacity method.The simulation results show that when the ambient temperature is lower than the phase change temperature,the curing heat release of the CPCMs.The average domain temperature of the cement-based test block of the CPCMs is the highest of 1.02 ℃,which is higher than the solidification temperature of the pore water solution in the concrete,which can effectively prevent the freeze-thaw damage of the concrete structure.