Molecular Dynamics Simulation Study On The Structure And Mechanical Properties Of The Main Components Of Cement

Cement as an inorganic material has been broadly used in the preparation of concrete,accounting for the major component of concrete strength.In order to reduce the environmental damage caused by the production of cement and produce high-performance cement,it is necessary to comprehensively understand the composition,structure and mechanical properties of cement.The main components of Portland cement are dicalcium silicate,tricalcium silicate,tricalcium aluminate and dicalcium iron aluminium oxide,and their crystal structures have been already solved accurately.While,in the process of cement hydration,Calcium Silicate Hydrate gel(C-S-H)accounts for 60%-70% of hydration products and is an important component.Due to the complexity of its composition,the microstructure is still controversial.The existing research shows that in the initial stage of cement hydration,the microstructure of C-S-H is similar to that of the mineral tobermorite family in nature,However,in the middle and late stages,its microstructure cannot be accurately solved.As research scales continue to deepen,molecular simulation has become an effective tool for investigating material structure and predicting material properties.The use of molecular simulation technology to study the microstructure and mechanical properties of cement-based materials can lead more understanding of cement-based materials and improve the properties of cement.Based on previous researches,three atomic structure models of calcium silicate hydrate,dicalcium silicate,tricalcium silicate,tricalcium aluminate,dicalcium iron aluminium oxide and ettringite were established.The parameters of Clayff force field,COMPASS force field and INTERFACE force field were introduced and employed in the geometric optimization and mechanical property computation on the above structures.Through the analysis of the computing results and comparison with the experimental values and other literature values,it is found that the selection of the force field has an important influence on the structural optimization and the computation of the mechanical performance parameters.The potential function type and atom type of force field have a direct relation of computing results.In terms of geometric optimization,the Clayff force field,COMPASS force field and INTERFACE force field can be used to reconstruct the structure more accurately.The error between the optimized result and the experimental value is less than 10%.Among them: COMPASS force field is most suitable for tobermorite 9 ? optimization,INTERFACE force field is most suitable for tobermorite 11 ? and 14 ?,dicalcium silicate,tricalcium silicate,tricalcium aluminate and ettringite optimization,Clayff is most suitable for dicalcium iron aluminium oxide optimization.In the calculation of mechanical parameters,the calculation results of the above structure using the Clayff force field deviate greatly from other literature values except for dicalcium iron aluminium oxide,the COMPASS and the INTERFACE force field are similar to the calculation results of the tobermorite family and are close to other literature values.The INTERFACE force field is more suitable for the calculation of dicalcium silicate,tricalcium silicate,tricalcium aluminate and ettringite.On the basis of the above research,tobermorites' family were duplicated three times through three direction,the supercell models then were used to model the C-S-H with low C/S ratio and the Pellenq's model were used to model C-S-H with high C/S ratio.After that,by performing the uniaxial tension simulation in the interlayer direction,the structural and mechanical properties of two type models under the loading state were studied.Stress-strain relationships of two type models were obtained,and the structure analysis of two type models was carried out in terms of the relative concentration profile and mean square displacement.In order to reduce the errors in the tensile simulation,the tobermorite 9 ?,11?,14? and Pellenq's model were carried on several tensile simulations,all the results show the similar stress-strain curve.Also,the results show that the strength of the structure in the interlayer direction is related to the interlayer spacing and the content of water molecules in the interlayer.The increase of the interlayer spacing will weaken the interaction between the O-Ca-O layers and the tensile strength of the structure in the z-axis direction.At the same time,the presence of interlayer water molecules will replace the bond between the layers of O-Ca-O to form weakly hydrogen bonds and Ca-O bonds,which also leads to a significant reduction in the tensile strength of the structure.As the water molecule content increases,meanwhile,the fracture form of the structure is changed,the C-S-H with low calcium to silicon ratio is structurally broken in the form of brittle fracture,while the high calcium to silicon ratio structure demonstrated a slippage behavior during the crack state due to the large amount of water molecules between the interlayer.In addition,the Ca ions will contribute to increase the strength of the structure,Ca ions increase the interaction between the adjacent silicon-oxygen tetrahedrons and thereby improve the strength of the structure.