Research On Unsaturated Transport Of Water And Ions In Cement-based Materials Based On Molecular Dynamics

Water and ions are almost the reason of all concrete durability degradation reactions.At present,there are many macroscopic studies on the transport of water and ions were quite comprehensive,and few on mechanism investigation at micro-scale.Exploring the reactions and properties of matter atoms from the microscopic scale helps us to understand the nature of things more deeply,and thus better guide the development of concrete.Therefore,the research on the microscopic mechanism of concrete has received more and more attention in recent years.This paper is based on molecular dynamics theory to study the unsaturated transport of water and ions in cement-based materials.The unsaturated model is mainly composed of two parts.C-S-H as the main hydration products of cement was selected for the construction of the upper channel,combined with the etched solution at the bottom,creates a more realistic unsaturated transport model.First of all,the characteristics and mechanism of unsaturated transport are explored by comparing with the saturation model.Then for the unsaturated model,we constructed different sizes of gel pores to explore the effect of pore size on the unsaturated transport of water and ions.At the same time,we have constructed different bottom solution models to explore the coupling transfer characteristics of chloride and sulfate ions in the unsaturated state.The main research contents are as follows:?1?Molecular dynamics was utilized to study the capillary transport of water molecules and ions in unsaturated and saturated state,and then to explore the transport law of water and ions in the nanopore and the interaction characteristics with the C-S-H interface.In the unsaturated state,the transport of water molecules and ions is mainly driven by capillary action.The meniscus interface invades along the 3.5nm hydration calcium silicate channel,matching well with classic LW capillary adsorption theory.In the saturated state,the transport of water molecules is mainly due to the interactive transmission process formed by the diffusion of water molecules in the upper and lower parts,which slows down the diffusion of water molecules along the channel.And in the saturated state,the hydration shell at the interface is more tight and stable,which leads to the formation of ion clusters,directly hinders the subsequent entry of water molecules and ions.As a result,in the unsaturated state,the transport of water molecules in the nanometer pores is much faster than the saturated state.?2?For the unsaturated model,the capillary transport behavior of NaCl solution capillary transport through the calcium silicate hydrate?C-S-H?gel pore with pore size of 3.5 nm,2.5 nm,1.5 nm and 1 nm was investigated.The progressively reduction for the solution/C-S-H contact angle reflects the hydrophilic nature of the C-S-H surface.With decreasing of C-S-H gel pore size,the ions are filtered by the small C-S-H gel pore,with water invading deeply in the gel pore,and chloride and sodium ions remaining in the region of gel pore channel entry.While the silicate chains in C-S-H surface can provide non-bridging oxygen sites to associate with the sodium ions by Na-Os connection,the courter calcium ions in C-S-H surface can capture the chloride ions,forming the ionic pair.Furthermore,the transition region with slow mobility is formed at the channel entry region due to the coupling immobilization effect by both surfaces normal and parallel to the transport pathway.The presence of transition zones results in the necking of channel entry,which inhibits the ions with larger hydration shell from penetration.Additionally,at channel with size less than 2 nm,the Ca-Cl and Na-Cl ionic pairs accumulate to cluster and are highly concentrated in the central of the gel pore,blocking transport pathway for water molecules and ions.?3?Sulfate and chloride attacking lead to the deterioration of cementitious material serving in marine environment.To study the ions coupling transport mechanism,molecular dynamics is utilized to study the capillary adsorption of 1mol/L NaCl solution,Na₂SO₄ solution,and mixed solution in the 3.5 nanometer channel of calcium silicate hydrate.In pure NaCl solution,the invading depth of water with meniscus shape follows the parabolic relation as the function of time,and the Na⁺and Cl^-closely pursue the advancing frontier of water.The capillary transport rate of water and ions in different solutions ranks in following order:NaCl>mixed solution>Na₂SO₄.Different from the simultaneous transport of water and ions in NaCl solution,the ions immigrate dramatically slower than that of water in Na₂SO₄ and mixed solution,indicating that the addition of sulfate ion in NaCl solution can significantly reduce the transmission rate of sodium ion and chloride ion.Due to the strong connection between Na-SO₄,the Na-SO₄ pair can further attract neighboring ions,accumulate and grow to ionic cluster with maximum size around 2 nm.The mobility of ions caged in cluster state is significantly reduced as compared with free sulfate ions.Additionally,the formed large cluster blocks the nanometer channel and prevents the water from invading in the gel pore,retarding the ingress of solution species.