Molecular Dynamics Study On Water And Ion Transport Behavior In Unsaturated Calcium Silicate Hydrated Gel Pore

C-S-H(calcium silicate hydrate)is the main component responsible for the durability of cement-based materials.The unsaturated transport of water and ions in the material channel is the main cause of durability deterioration.It is very important to study the transport properties of water and ions in C-S-H gel for analyzing the physical and chemical behaviors of cement-based materials and concrete structures.Although the macroscopical experimental study of water molecule and ion transport is very important,it is difficult to reveal the mechanism of its transport characteristics only by experimental means.In this paper,based on molecular dynamics simulation,we construct a C-S-H gel nano-channel transport model based on previous experiences.The transport properties of water and salt solution through unsaturated gel pores and the structural and kinetic characteristics at molecular scale are studied,and the basic theories of statistical thermodynamics,physical chemistry and hydrodynamics are properly combined.The mechanism behind the phenomena is revealed and the following results are obtained:(1)The distribution of water in C-S-H gel pore is mainly related to saturation and wall position,and the wall has hydrophilicity;When the saturation of gel pores is about 53%,the connected channels on both sides will trigger percolation effect,which will greatly improve the diffusion ability of water.The self-diffusion coefficient of water molecules in the interlayer region,adsorption region and free zone will show the rule of free zone > adsorption region > interlayer region due to the difference of geometric constraint and bonding.In the initial stage of water absorption,the adsorption zone of gel pores has a relatively high transport rate,and then the adsorption zone shows obvious velocity stratification,and the velocity increases along the vertical direction of the wall.However,in the process of water absorption,the water molecules will fill the whole channel with priority.Then it enters into the interlayer near the wall;Under the influence of percolation effect,with the increase of water saturation in the gel pores,the overall diffusion coefficient of water increases first and then decreases,while higher temperature will amplify the effect of the effect on the transmission.The pore size of gel pore is positively related to the diffusion ability of water.Higher initial saturation and curvature can inhibit the water absorption rate of gel pore.(2)In the transport system of ions in gel pores,the double layer effect exists and can well explain the transport of ions in gel channels;Sodium ions are more easily stabilized in the first layer,while chloride ions are difficult to stay in the adsorption zone for a long time.The adsorption and desorption behavior of chloride ions in the adsorption zone will be more frequent.Because of the exclusion of Coulomb potential and the poor stability of the second electric layer based on the first electric layer,the overall diffusion coefficient of chloride ion is higher than that of sodium ion,and the Ca / Si ratio will significantly affect the adsorption capacity of chloride ion on the surface of C-S-H.Because the transport of ions in gel pores needs to be attached to water molecules,the transport speed of ions is obviously lagging behind that of water.With the increase of saturation,the percolation effect will significantly promote the diffusion ability of ions.But it corresponds to the extreme value of ion diffusion coefficient,not the maximum value.(3)In unsaturated gel pores,saturation is the primary factor affecting ion diffusion ability,while in saturated gel pores,when the ion concentration reaches stability,the concentration difference gradually weakens.The polarity of C-S-H wall began to be the main factor affecting ion transport.Both types of curvature models show that a higher degree of curvature will obviously hinder the transport of ions in the gel channel,and the velocity of ion reaching the target concentration under pure diffusion(the second model)is slightly higher than that in the convection-diffusion state(the first type model).The interaction of calcium chloride increases the diffusivity of ions in sodium chloride solution,whereas potassium chloride increases the diffusion ability of ions in sodium chloride solution.This paper has 86 diagrams,1 tables and 127 references.