Molecular Dynamics Study On Graphene Oxide Reinforced Cement Composite

Cement composites are the most important building materials in the modern construction industry.It has the advantages of low cost and high compressive strength,but the brittleness of cement based composute materials has also plagued people.The traditional method is to use materials such as steel and fiber to resist tensile stress and delay the expansion of microcracks.However,these methods do not improve the performance of the cement matrix itself.In recent years,nano-modified materials have developed rapidly,which provides a new method for the modification of cement-based materials.Graphene oxide has the advantages of large specific surface area,excellent mechanical properties and good water solubility,so it has become a popular cement-based modified material.Many scholars have found that graphene oxide can effectively regulate the shape of cement hydration products,promote cement hydration,and increase the compactness of cement hydration products.It can effectively improve the tensile and flexural properties of cement-based materials.However,the experimental method is affected by factors such as the rate of cement hydration,the dispersion effect of the graphene oxide solution.Moreover,key information such as chemical reactions and chemical bond strengths at the interface between graphene oxide and cement hydration products cannot be obtained.The above problems are difficult to solve from an experimental point of view.Molecular dynamics is a good method to study the arrangement of water and ions on the surface of graphene oxide,the kinetic behavior and the chemical bond information of the interface between graphene oxide and cement hydration products.This method can be used to study the micro-toughening mechanism of graphene oxide on cement-based materials and to guide the design of high-performance graphene oxide-based materials.The detailed content of this paper are listed as following:（1）The hydrogen network formed between the water molecules and functional groups,plays a critical role in bridging the water and GO sheet.The diffusion coefficient of GO-COO surface water is 1.20×10^-9m²/s,and the diffusion coefficient of GO-OH surface water is 1.48×10^-9m²/s.The above results show a significant decrease in the diffusion coefficient of 4.13×10^-9m²/s for large volume water.（2）The carboxyl functional group will form a Ca-Oc bond with calcium ions,so calcium ions will show obvious adsorption on the surface of graphene oxide.Sulfate ions are stably adsorbed on the surface by forming a Ca-Oso₄ bond with calcium ions,resulting in obvious clustering on the surface of GO-COO.This is similar to the experiment in which graphene oxide provides a site for cement hydration and accelerates the crystallization process of hydrated products.（3）The chemical bond between the graphene oxide and the main cement hydration product（Calcium-Silicate-Hydrates,C-S-H）was studied by reaction field and the source strength of the interface was analyzed.The Ca ions,both coordinate with oxygen in the silicate chain（Os）and oxygen（Oc）in the functional group,play mediating role in the bridging of C-S-H and the GO sheet.Besides the Ca-O bond,the silicate chains in C-S-H provide oxygen sites to form H bonds with functional groups,which strengthen the interfacial connection.The bond strength of Oc-Ca-Os and the lifetime of the H bonds are closely related to the polarity of the functional groups:GO-COOH>GO-OH>GO-Oo.（4）Due to the strong Ca–O and H bonds,C–S–H reinforced with GO–COOH and GO–OH have better mechanical performance when subjected to tensile loading.The weaker mechanical behavior of the G/C–S–H,GO–Oo/C–S–H and GO–SO3H composites is attributed to poor bonding,dissociation of the functional groups and instability of atoms in the interface region.