Study On The Mechanism Of Interfacial Action Of Waterborne Epoxy-concrete Repair Materials Based On Molecular Dynamics

Waterborne epoxy-concrete repair material is an organic-inorganic composite material,epoxy resin mixed into concrete can form a three-dimensional network structure with cement hydration products,thus can improve the mechanical properties and durability of cement-based materials.The repair effect of waterborne epoxy-concrete materials depends on the combined action of organic and inorganic phases.Due to the diversity of physical phases in cementitious materials and the variability of service environments,the complex interfacial interaction mechanism between cementitious materials and epoxy resins has not been clarified.As the main cementitious phase of cementitious materials,calcium silicate hydrate（C-S-H）gels are the source of cementitious strength of cementitious materials,and the study of the interaction between epoxy resin and C-S-H gels is important for understanding the complex interfacial interaction between cementitious materials and epoxy resin.To this end,this thesis investigates the mechanism of interfacial bonding between epoxy resin and C-S-H,the main cementitious phase of cementitious materials,based on molecular dynamics simulations,and explores the effects of carbonation and aggressive ion intrusion on epoxy resin/C-S-H interfacial interactions.The main research of this paper is as follows:（1）Simulation study on the interfacial bonding performance of epoxy resin and cementitious material with main hydride phase C-S-H.C-S-H gel has variable chemical composition,design epoxy resin model with high crosslinking degree and C-S-H model with different calcium-silica ratios,construct epoxy resin and C-S-H interfacial system with different calcium-silica ratios,simulate epoxy resin on C-S-H with different calcium-silica ratios by molecular dynamics method,and then investigate the mechanism of influence of calcium-silica ratio on epoxy resin/concrete interfacial bonding performance.It is found that the interfacial bonding performance tends to decrease with the increase of calcium-silica ratio,and the C-S-H structure with low calcium-silica ratio has more and denser silica-oxygen tetrahedral structure,which is favorable for the interfacial bonding between epoxy resin and C-S-H.From the interfacial local structure,the C-S-H surface with high calcium-silica ratio has more free hydroxyl groups,and the free hydroxyl groups are easy to form Ca-Oh ion pairs with surface Ca,resulting in the adsorption sites of O_epoxy are reduced,the ion pair strength of Ca-O_epoxy is lower,and the strength of the hydrogen bonding network formed between C-S-H and epoxy is lower and less stable,which is not beneficial to the adsorption of epoxy resin on C-S-H.（2）Simulation study of the bonding properties of epoxy resin and C-S-H interface by carbonation.The C-S-H part of calcium ions dissolved by carbonation and combined with carbonate to form calcium carbonate crystalline phase.Therefore,the epoxy resin/C-S-H interface system and epoxy resin/calcium carbonate interface system are constructed separately to investigate the mechanism of the effect of carbonation on the interfacial bonding performance of epoxy resin and cementitious materials.It is found that:epoxy resin has stronger interfacial bonding performance than epoxy resin/C-S-H at calcium carbonate interface,epoxy resin exfoliates more deformation on the surface of calcium carbonate.From the local structure of interface,the surface of calcium carbonate provides more adsorption sites for O_epoxy in epoxy resin,forming more Ca-O ion pairs with stable structures,while the epoxy resin as a hydrogen donor forms a stronger hydrogen bonding network with calcium carbonate.（3）Effect of water molecule and ion intrusion on the epoxy resin/C-S-H interface interaction.A pore transport model of water molecules and ions in the epoxy resin/C-S-H composite system is constructed to simulate the effects of water molecule and ion intrusion on the epoxy resin/C-S-H interfacial interactions.The local structure analysis of the interface shows that after the external solution passes through the epoxy resin covered area,the epoxy resin is partially exfoliated due to the water molecules occupying the epoxy resin adsorption sites,while the exfoliated part of the epoxy resin has hydrogen bonding interactions with the water molecules,resulting in the slowing down of the water molecule transport.The ions can interact with the epoxy resin after passing through the epoxy resin spalling area and form a cluster-like structure,which leads to the reduction of the ion intrusion depth.Through kinetic analysis,it can be found that the movement ability of water molecules and ions is slowed down in the system covered with epoxy resin.