Molecular Simulation Study On The Structural Characteristics Of C-S-H Gels

As the main product of cement hydrates, C-S-H gel has great influence on microstructure and macroscopical properties of cementitious materials. Though its composition, structure and properties have been widely investigated, people today still can not get the full information accurately of C-S-H gel for its small particle, uncertain component and amorphous phase. To study the microstructure characteristics effectively of C-S-H gel by advanced testing technique and computer simulation method, together with to investigate the relationship between its properties and structure, which is not only of great academic value, but also can be used to guide the development of high-performance concrete.Based on obtained configurable parameter by advance testing technique, we could establish the initial model and simulate the annealing process which is widely used in glass production through computer simulation method to get the amorphous phase finally. Radial distribution function (RDF), mean square displacement (MSD) and coordination number(CN) were chosen to discuss the different structural characteristic among various systems. Besides, the structure under crystal phase, liquid phase and amorphous phase and the change of Ca/Si ratio were also analysed. Futhernore, we could validae the obtained system by comparing the computed mechanical properties (including the Young's moduli, the bulk moduli, the shear moduli and the Poisson's ratio) with experimental data. The main results are as follows:(1) As the analog of C-S-H gel,9A Tobermorite,11A Tobermorite,14A Tobermorite and Jennite can be used as the initial structure to simulate and represent its structural characteristic, and some good results can be obtained.(2) After annealing process, there has been obvious change in the initial structure. Although silica tetrahedron still exists as the basic unit, the connecting way between each other has changed a lot. The long chains already twist, rotate and even rupture, and the former layered structure becomes disordered. The result of RDF shows that the peaks only appear in short range while other peaks in long range no longer exist. As the radius increases, the change of peak value tends to be balance and fluctates at about1. All the results indicate that the systems after annealing have transferred from ordered structure to disordered structure.(3) Tobermorite and Jennite can represent C-S-H gel when Ca/Si is0.83and1.5respectively for their different composition. Compared to9A Tobermorite and14A Tobermorite, the computed results of11A Tobermorite fit better with the experimental value. So11A Tobermorite and Jennite would be chosen as the input structure in our contrastive simulation. Though there is an obvious structural difference between the two models, the studied systems tend to be similar after annealing.(4) Compared to the systems before annealing process, the bulk moduli, the shear moduli and the Young's moduli of targeted systems after molecular dynamic simulation decrease largely, which agrees well with the experimental data of amorphous C-S-H gel, especially for11A Tobermorite and Jennite systems.(5) As the same as molecular dynamic simulation, the C-S-H systems obtained by Monte Carlo method are still amorphous structure, which belongs to the exploring research.Though this paper can get the amorphous C-S-H gel successfully by computer simulation and provide enough information of its structural characteristic. However, considering the complex and uncertain factor of C-S-H gel, more work still need to be done to improve such research.