Molecular Dynamics Study On Mechanical Properties Of Hydrated Calcium Silicate Composites At Different Temperatures

Though a series of single-particle models and cement hydration models has been proposed to reveal the mechanism of cement hydration,there is no support from the molecular dynamics theory.The main hydration product of cement is hydrated calcium silicate;so far,its mechanical properties and development mechanism still have no unified conclusion.In the studies of mechanical properties of hydrated calcium silicate from molecular dynamics,the simulation ensemble and pressure control algorithms chose by different scholars are not the same.Meanwhile,the influence of ensemble and pressure control algorithms on the mechanical properties of hydration calcium is also in a grey area.As the mechanical properties of concrete decreased seriously under high temperature,the effect of temperature on the mechanical properties of hydrated calcium silicate should not be ignored among the many influencing factors.Besides,as the proportion of the Tobermorite 9?,Tobermorite 11?,Tobermorite 14? and Jennite crystals in the C-S-H composites materials is also uncertain,the mechanical properties of the C-S-H composite materials can not be deduced by the mechanics properties of four kinds of crystal.So the consideration of porosity is also neened when discussing the mechanical properties of the C-S-H composite materials under different proportion of four kinds of crystal.Therefore,the study of the calcium silicate’s mechanical properties at normal temperature is very necessary,as well as the influence of different ensembles on the mechanical properties of hydrated calcium silicate under different temperatures.A certain reference values and theoretical basis for the establishment of cement hydration models is suggested.In this paper,the mechanical properties of C-S-H composites materials at different temperatures were studied by molecular dynamics(MD)simulation;meanwhile,the influences of proportion of different crystals,porosities and temperatures on the mechanical properties of C-S-H composites were also considered.Based on the Tobermorite 9?,Tobermorite 11?,Tobermorite 14? and Jennite crystals,the initial structures of the crystal models were built by the Materials Studio software,and then the geometry of the models were optimized before MD simulation at normal temperature.Following the MD simulation,the mechanical properties were calculated.The 23 different proportions of four crystals were considered when analyzing the influences of the crystals on the mechanical properties of C-S-H composites while the mesoscopic mechanics methods of SC,MT and Voigt were used to derive the mechanical properties of C-S-H composites.The porosities corresponding to high-density C-S-H(HD C-S-H)and low-density(LD C-S-H)should be taken into account as it has a huge influence on the mechanical properties of C-S-H composites.A reasonable agreement was found between the simulated results and those from the nano-indentation tests.After contrastive analysis of the results between the calculation and the nano indentation test,we found that the mechanical properties of C-S-H vary with the change of the porosity and proportions of the components in a certain range.Meanwhile,the change trend of mechanical properties has nothing to do with whether the effects of porosity were took into account.With the proportion of crystals components,the different mechanical parameters of C-S-H composites are different.The geometry of the supercells with 3a×3b×3c models was optimized based on the foundation of the crystals models.The MD simulation were performed under NVT and NPT ensemble,and the temperature were set at 25℃,80℃,and every 100 degrees between 200~1500℃,respectively,then the mechanical properties were calculated.The intermolecular interaction,the changes of cell parameters and the mechanical properties of the four crystals were analyzed at different temperature.The methods of SC,MT and Voigt were used to derive the influence of crystals proportions and porosities on the mechanical properties of C-S-H composites at five representative temperatures.It can be found that the bulk modulus,shear modulus and Young’s modulus of the C-S-H composites decreased with the increasing of temperature while the Poisson’s ratio increased slightly with the increasing of temperature.Through a same variation tendency of the mechanical properties of the same crystal exist in NPT and NVT;the numerical values have a big difference.The ensemble has serious inference on the mechanical properties of C-S-H composites.Considering the influence of porosity,certain proportion relationships were foud among the results of SC method,the MT method and the limit value method of the Voigt.