Study On The Adsorption Law Of Loess Particles In The Process Of Steam Humidification

As a special geological body,loess has special physical properties such as large pores,vertical joint development,high salt content and strong collapsibility.Compared with the water-rich state,natural loess has higher strength and lower compressibility,but once water is encountered,the physical and mechanical properties deteriorate significantly.Under the action of natural forces such as rainfall,evaporation and dry-wet/freeze-thaw cycles,geological disasters such as slope collapse,ground collapse and uneven settlement occur frequently in loess areas,which directly affect the safety function and long-term service performance of engineering structures.In the natural environment,water vapor adsorption is one of the important sources of non-precipitation moisture on land surface,especially in arid and semi-arid areas,and its influence on soil moisture change can sometimes even reach the same magnitude as precipitation.Therefore,the effect of water vapor adsorption on the moisture content of loess and its hydraulic-mechanical properties should not be ignored,and there is an urgent need to investigate the water vapor adsorption properties of unsaturated loess under the effect of different influencing factorsIn this paper,water vapor isothermal adsorption tests under different humidity environments were conducted on unsaturated loess to investigate the effects of temperature,mineral composition and content,and dry density on the adsorption performance of the soil.Based on the results of isothermal adsorption tests,four adsorption thermodynamic models,namely Langmuir,Freundlich,BET and GAB,and four adsorption kinetic models,namely proposed first-order,proposed second-order,double first-order and single-pore diffusion,were used to fit the isothermal adsorption curves and adsorption kinetic curves of unsaturated loess water vapor,and the applicability of each model was evaluated in order to study the adsorption mechanism and adsorption law of unsaturated loess water vapor from the perspective of adsorption thermodynamics-kinetics.Based on the laboratory test,the water vapor adsorption process of unsaturated loess was simulated by COMSOL Multiphysics software,and different working conditions were set up for calculation as a supplement to the internal temperature and moisture distribution of unsaturated loess,which was difficult to measure in the test part,and compared with the actual test results.The main research conclusions obtained in this paper are as follows:(1)The water content of unsaturated loess after water vapor adsorption equilibrium increases with the increase of relative humidity,and the whole process contains three stages:monolayer adsorption,multilayer adsorption and capillary condensation.There is a significant negative correlation between the equilibrium water content of soil samples and temperature.When the relative humidity is constant,the equilibrium water content decreases with the increase of temperature.The water vapor adsorption of unsaturated loess is closely related to the mineral composition,and the clay mineral content directly affects the water vapor adsorption capacity.In addition,the effect of dry density on the equilibrium water content of soil samples can be divided into two stages.When the relative humidity RH<80%,the equilibrium water content increases with the increase of dry density.Until entering the capillary coalescence stage,the equilibrium water content after adsorption no longer increases but decreases with the increase of dry density.(2)GAB model is the best model to describe the adsorption isotherms of unsaturated loess-water vapor,which can roughly predict the trend of adsorption isotherms in the whole relative humidity range.The water vapor adsorption kinetic curve of unsaturated loess is in the form of Langmuir isotherm,and the fitting accuracy of the double first-order kinetic model is the highest,indicating that the unsaturated loess-water vapor adsorption process is composed of surface adsorption dominated by initial external diffusion and pore adsorption dominated by later internal diffusion.The control effect of internal diffusion on water vapor adsorption is not obvious,which means that external diffusion is the main rate control step of unsaturated loess-water vapor adsorption process.(3)The numerical calculation results show that the external ambient temperature has a significant effect on the temperature and moisture transfer process of water molecule adsorption in unsaturated loess.The larger the difference between the external ambient temperature and the initial temperature of unsaturated loess,the longer the time required for temperature transfer.At the same time,the higher the temperature,the shorter the time for moisture transfer,because the increase in temperature will increase the diffusion coefficient of water molecules,thus promoting the moisture diffusion rate.Different external environmental humidity has little effect on the temperature transfer of unsaturated loess,but it will affect the moisture transfer process,the higher the environmental humidity,the longer the time needed to reach the equilibrium of adsorption.(4)The curves of water vapor moisture absorption with time obtained from the experimental measurements and numerical simulations are similar in characteristics,and their changes go through two stages: rapid growth and stabilization.In the rapid growth phase,the moisture absorption obtained from numerical simulation is lower than the measured value,while in the later stabilization stage,the two curves basically overlap.In the range of RH<90%,the error between the measured and numerical simulation results of the test is less than1%,but when the RH>90%,because the moisture diffusion coefficient is a function of the relative humidity in the numerical simulation calculation,the moisture diffusion coefficient increases significantly with the increase of humidity under the high relative humidity condition,which leads to a large error between the measured and simulated values of moisture absorption.