Study On Durability Of Graphene Modified Cement-based Materials In Chloride Environment

Cement-based materials are one of the most widely used materials in construction and infrastructure construction.However,the poor crack resistance limits their application,which adversely affects the durability of the structure.In order to improve these problems,the introduction of nanomaterials into cement-based materials has become a research hotspot.Graphene has advantage that other nano materials unmatched,such as extraordinary physical and chemical properties,significant nanosize effect and surface effect.It is widely used in the manufacture of high-performance composite materials to improve their durability.With the support of the National Natural Science Foundation of China Project "Research on the Technology and Design Theory of Durability Improvement of Offshore Prestressed Concrete Structures(51778272)",this paper takes graphene nanoplatelets(GNPs)as the research object to prepare modified cement-based materials.The influence of GNPs on the salt corrosion resistance of cement-based materials is studied and the mechanism of GNPs improvement is also explored by macro-test and micro-test.The main research work and conclusions are as follows:(1)Research on the durability of GNPs modified cement mortar under the environment of chloride salt immersion.Modified cement mortar with GNPs(0.00wt%,0.02wt%,0.06wt%,0.10wt%)were immersed in Na Cl solution(concentrations of 3%,5%and 10%,mass percentage)for 120 d,180 d and 240 d.The free chloride concentration was detected by the Rapid Chloride Test(RCT)and the influence of Na Cl concentration,immersion time and GNPs content on it were studied.The results show that,1)the free chloride concentration is still positively correlated with solution concentration and corrosion age.The transport of chloride ions in cement-based materials is driven by the concentration factor and has the characteristics of unstable diffusion.Compared with immersing in 3% Na Cl for 240 d,the free chloride ion concentration in 5% Na Cl can be increased by less than 30%,while 10% Na Cl can increase by nearly 80%.Compared with immersing in 10% Na Cl for 120 d,the free chloride ion concentration for 180 d and 240 d increased 46%～72% and 93%～124%,respectively.2)The free chloride ion concentration first decreases and then increases with the GNPs content.The enhancement effect of0.06wt% GNPs is relatively good and the chloride ion concentration at the three concentrations can be reduced by more than 50%.(2)Research on Durability of GNPs Modified Cement Mortar in Salt Spray Environment.Four kinds of GNPs modified cement mortar are placed into the salt spray box(salt solution is 5% Na Cl)and atomized for 3 d,5 d and 7 d.The free chloride ion concentration was also detected by RCT test.The influence of atomization time and GNPs content on it were studied.The results show that,1)the chloride ion concentration along the depth direction of GNPs modified cement mortar increases with the increase of the atomization time,and the corrosion depth deepens.Compared with 3 d,the free chloride ion concentration increased by 13%～40% and 21%～47% respectively after atomization for5 d and 7 d.2)Compared with ordinary cement mortar,the chloride ion concentration along the depth direction of 0.06wt% GNPs modified cement mortar decreased by8%～31%,2%～41% and 6%～37% respectively after atomization for 3d、5 d and 7 d.(3)carry out research on the influence of GNPs on the microstructure of cement mortar.Based on the microscopic damage and fracture mechanism of cement-based materials,samples of GNPs modified cement mortar were sampled and the microscopic morphology after chloride corrosion was analyzed by scanning electron microscope(SEM)and energy spectrum(EDS).The results show that,under the effects of the two environments,compared with the large number of microscopic cracks and holes in the cement mortar of the normal control group,GNPs can promote cement hydration very well.The microstructure of cement mortar mixed with GNPs is more compact,especially 0.06wt%GNPs modified cement mortar shows good resistance to chloride ion erosion.(4)Study on the diffusion of chloride ions in GNPs modified cement mortar.The chloride ion diffusion coefficients D under the effects of two environments are fitted and evaluated.The results show that D of GNPs modified cement mortar also has a time dependence.When the GNPs content is 0.06wt%,the strengthening effect of this content is the most obvious.After the long-term natural immersion corrosion of chlorine salt for 240 d and the accelerated corrosion of salt spray for 7 d,the resistance of cement mortar to chloride ion corrosion increased by 58.2% and 30.7%,respectively.This paper mainly studies the influence of GNPs on the corrosion resistance of cement mortar.The main considerations are the concentration of the corrosion solution,the corrosion time and the amounts of GNPs.Relevant research results can lay the foundation for structural durability evaluation and life prediction in the future.