Study On Interface Performance Experimental And Molecular Dynamics Simulation Of Modified EPS Concrete

EPS（Expanded Polystyrene,polystyrene foam）concrete has excellent thermal insulation,durability and fire resistance.It is an organic-inorganic composite building concrete material with broad engineering application prospects in solid waste building materials.Many adhesive interfaces between EPS particles and cement-based materials play a decisive role in the overall mechanical performance of EPS concrete.Although many scholars have done a lot of research on complex and variable interface problems,most of them are limited to phenomenological engineering methods,ignoring the potential interaction mechanisms in molecular structures,which to some extent,limits the improvement of the performance of EPS composite concrete materials.This article is based on the special fund project for basic scientific research business expenses of Henan provincial universities（2020ZKCJ21）and the natural science project of Zhengzhou Science and Technology Bureau（21ZZXTCX09）.The surface of EPS particles was modified with EVA（ethylene vinyl acetate）lotion to prepare EPS concrete test blocks.It is assumed that EVA acts as a bridge between EPS particles and cement-based two-phase bonding at the interface and can improve the overall composite’s interfacial bonding performance and mechanical strength.Under macro-scale regulation guidance,EPS concrete’s mechanical properties before and after EVA modification were tested.At the microscopic level,modern testing techniques such as scanning electron microscopy（SEM）,X-ray diffraction（XRD）,and Fourier transform infrared spectroscopy（FTIR）were used to characterize the structure and morphology of the EPS/cement-based interface before and after EVA modification.At the nanoscale,molecular dynamics simulations were used to introduce the molecular structures of EPS,aggregates,and hydrated calcium silicate（C-S-H）into the study of EPS concrete and the effect of EVA modifiers on the interfacial adhesion mechanism between EPS/C-S-H and aggregates/C-S-H.Although the compressive and flexural strength of EPS concrete has been improved by chemical modification of EVA,its ductility still needs to be significantly improved.Therefore,to enhance the ductility of EPS concrete,another chemical modifier,silane coupling agent（SCA）,and a physical modifier,polyvinyl alcohol（PVA）fibre,were used to modify EPS concrete synergistically.A multiscale research method was adopted to explore the synergistic reinforcement effect of SCA and PVA fibre on EPS concrete.The main conclusions are as follows:（1）Through testing the mechanical properties of EPS concrete before and after EVA modification and through a series of microscopic experiments to compare and analyze the microscopic morphology characteristics of the EPS/cement-based interface with or without EVA,the formation mechanism of adhesive force between EPS particles and cement matrix was preliminarily revealed.The results showed that EVA significantly improved the compressive strength and flexural strength of EPS concrete and improved the hydrophilic properties of EPS particles at the surface conducive to forming interfacial hydration products.In addition,ester and carboxyl groups were found in the interfacial transition region containing EVA,and C-S-H gel and Ca（OH）₂gradually increased with the increase of EVA and decreased,respectively.The effects of EVA on filling weak interfacial gaps,increasing the density of the interfacial transition zone,and enhancing the overall mechanical properties were verified.（2）Based on molecular dynamics,six interface models of EPS/C-S-H,Si O₂/C-S-H,and Ca O/C-S-H with or without EVA were constructed,and the static structure,dynamic characteristics,and adhesion mechanism of the interface were simulated at the molecular scale.The results show that EVA can produce stable hydrogen and ion bonds with water molecules and Ca in C-S-H,respectively,effectively bridging EPS polymer materials and inorganic C-S-H substrates and improving the interfacial bonding strength.In addition,EVA has a small impact on the adhesion of the Si O₂/C-S-H interface.On the contrary,EVA chains are easily adsorbed on the C-S-H surface at the Ca O/C-S-H interface,hindering the exchange of Ca ions and the formation of new hydration products in the C-S-H and Ca O aggregates,thereby weakening the interfacial adhesion of Ca O/C-S-H.（3）Based on the fact that using EVA modifier alone does not significantly increase the ductility of EPS concrete,a method of synergistic improvement of weak ductility of EPS concrete using SCA coupling agent and PVA fibre was adopted,and the microscopic morphology characteristics of EPS/C-S-H and PVA/C-S-H interfaces with or without EVA were analyzed.The results show that the mechanical strength order of the three types of concrete is EPS/PVA/SCA>EPS/PVA>EPS,and SCA modifiers and PVA fibres have cracking resistance and toughening effects on EPS concrete.In addition,SCA-modified polymer films can be used as bridge ligaments for the interaction between organic materials（PVA fibres and EPS particles）and cement-based bonding,which plays an important role in reducing the modulus difference between organic and inorganic materials and improving the overall ductility of concrete.（4）Based on molecular dynamics methods,four interface models of EPS/C-S-H and PVA/C-S-H with or without SCA were constructed,and molecular scale simulations were conducted for the static structure,dynamic characteristics,and adhesion mechanism of the interface.The results show that SCA can increase interfacial hydrophilicity,reduce interfacial effects,and enhance interfacial hydrogen bonding.In addition,the stable interaction between Ca and H can make PVA/SCA/C-S-H and EPS/SCA/C-S-H have relatively stable interface systems.The intrinsic mechanism of SCA increasing the adsorption energy of organic/inorganic interfaces and improving the interfacial adhesion quality was further revealed.The hypothesis that SCA improves the interfacial adhesion quality was verified.