Study On The Microstructure And Properties Of Calcium Silicate Hydrates Based On Molecular Dynamics Simulation

As the main hydration product of ordinary Portland cement,the microstructure and properties of calcium silicate hydrates(C-S-H)govern the service quality and life of the widely applied cement-based materials.So far,the microstructure and nanostructure of C-S-H has not been elucidated completely,while the rapid development of modern infrastructures calls for civil engineering materials with both high performance and low energy consumption.Therefore,by the combination of molecular dynamics(MD)simulation and experimental techniques,this dissertation focuses on four main parts,including the characterization of the basic microstructure and mechanical properties of C-S-H;in order for the durability improvement,study on the transport and adsorption of chlorides in the pore solutions of C-S-H;and for the purpose of mechanical property improvement,research on the interfacial connections between CS-H and polymers,and the microstructure and performance enhancement mechanisms of the inorganic C-S-H/ organic polymer binary system.The following are the main conclusions: 1)Obviously,C-S-H owns the multi-scale feature: at micro-scale,the structure and mechanical properties of C-S-H are governed by calcium to silicon ratio(C/S)and the packing density of particles,respectively.At nano-scale,C-S-H gels are poorly crystalline and possess layered structure,atoms of which are arranged regularly in short-distance range but irregularrly in long-distance range;At molecular-scale,the calcium silicate sheets are aligned alternately,constituting C-S-H with the interlayer region,which is filled with water molecules,calcium hydroxyls and silanols generated by water dissociation.When subjected to uniaxial tensile loading,C-S-H tends to be brittle if the low-strength ionic bonds 4)- ? (6 bonds are elongated till broken to be responsible for the strain evolution.However,if the strong covalent bonds Si-O-Si are stretched open during the tensile process,high ductility can be obtained.2)The aggregation degree of calcium in the vicinity of C-S-H surface affects greatly the chloride binding capacity of C-S-H.In the case of high surface C/S,more calcium ions are attracted to the interfacial area as a result of the increase in the effective space of channel area and number of the non-bridging oxygen.These calcium ions are tightly limited and able to further attract larger number of chlorides by ion clusters.The diffusion coefficients of chlorides tends to be lower as approaching the channel area.3)MD simulation shows,the affinity between C-S-H and polymers depends on the polarity of polymer functional groups,the diffusion and aggregation of polymer chains.PAA adsorb stably on the surface of C-S-H due to the high polarity of its carboxyl group.However,in spite of the relatively high polarity of hydroxyl group,the adsorption of PVA is far less stable because of the strong aggregation effect among hydroxyl groups from PVA chains.The functional group C-O-C of PEG is the least polar,which contributes to the free movement of PEG molecules with unfolded configurations.4)MD simulation indicates that the intercalation of polymers dramatically improves the ductility of C-S-H substrate.In the real C-S-H/polymer composite,the presence of PAA and PVA disturbs the regular arrangement of calcium silicate sheets of C-S-H,and changes the mean length of silicate chains,which implies those polymers may adsorb on the defective sites of silicates either on the surface or in the interlayer region of CS-H.However,there is low probability for the intercalation of PEG molecules.Instead,they are more likely to situate in the midst between C-S-H particles,with a special configuration.In consideration of the small particle size of C-S-H,this special configuration of PEG chains can lead to the high stiffness and high creep modulus of the composite.Therefore,by means of study on the microstructure and properties of C-S-H at micro-,meso-,and nano-scale,the mechanisms on the chloride adsorption,interfacial connections between C-S-H and polymers,mechanical performance enhancement in the C-S-H/polymer binary system are unraveled,which may provide theoretical support and guidelines to the mix design of concrete used in marine projects,the determination of functional groups in the preparation of concrete admixtures,and the fabrication of high mechanical performance cement-based materials,respectively.