Study On The Effect Of C-S-H And AFm On Chloride Ion Binding And Initial Corrosion Of Reinforcement

Chlorine salt attack is one of the leading causes of damage to reinforced concrete structures,especially in marine and de-icing salt environments.The chloride ions invade the concrete and accumulate to the chloride threshold value on the surface of the reinforcement,thus initiating the corrosion of the reinforcement.The volume expansion of the corrosion products leads to the cracking of the concrete protective layer and reduces the service life of the reinforced concrete structure.Chloride ions in cement-based materials are mainly combined by calcium silicate hydrate（C-S-H）and monosulfate hydrate（AFm）in cement hydration products,lowering the free chloride ion content.It is conducive to delaying the corrosion of steel bars and improving the durability of reinforced concrete structures.Therefore,to improve the chloride binding performance of cement-based materials and reduce the hazard of chloride ions on initiating steel corrosion,it is necessary to investigate the mechanism of C-S-H and AFm binding chloride ions and the mechanism of the influence of bound chloride ions on the steel initial corrosion process.In this work,the binding mechanism of C-S-H and AFm to chloride ions in a simulated concrete pore solution was investigated by synthesizing C-S-H and AFm.On this basis,the mechanism of the effect of C-S-H and AFm binding to chloride ions on the passivation and corrosion process of steel reinforcement was studied.In order to investigate the effect of synthetic C-S-H and AFm on the chloride binding properties of cement-based materials,further internal mixing of synthetic C-S-H and AFm to prepare cement paste was carried out to investigate the mechanism of the effect of synthetic C-S-H and AFm on chloride ion binding performance in cementitious materials.The main research contents and conclusions of this topic are as follows.（1）The chloride binding behavior of C-S-H/AFm was investigated in the simulated concrete pore solution by considering the effects of C-S-H/AFm doping amount and chloride ion concentration.The results showed that there was a correlation between the C-S-H binding behavior of chloride ions and the p H of the simulated solution,and the adsorption of chloride ions by C-S-H decreased with the increase of C-S-H doping in alkaline Na Cl solution.The adsorption of C-S-H on chloride ions at chloride ion concentrations of 0.1～1.0 mol/L was consistent with the Freundlich isotherm adsorption model.The binding amount of AFm to chloride ions increased with the increase of chloride ion concentration when the doping amount of AFm was constant.With the constant chloride ion concentration,the appropriate change of the amount of AFm could improve the binding capacity of AFm to chloride ions.The isothermal adsorption model for the binding of AFm to chloride ions was consistent with the Freundlich isotherm adsorption model for chloride ion concentrations of 0.1～0.5 mol/L and with the Langmuir isotherm adsorption model for chloride ion concentrations of 0.5～1.0 mol/L.（2）The effect of incorporating C-S-H/AFm in the simulated concrete pore solution on the initial corrosion process of steel reinforcement was investigated.The electrochemical methods were used to monitor the passivation and initial corrosion process of steel in the simulated solution.The mechanism of the effect of chloride ion binding on the initial corrosion process of steel was investigated.The results showed that the doping of C-S-H/AFm in the simulated solution increased the chloride threshold value for rebar corrosion,and the passivation film of rebar in the C-S-H/AFm doped group had a higher Fe²⁺content and a more stable passivation film.The higher p H of the simulated solution in the C-S-H group,the presence of silicates in the simulated solution and the adsorption of chloride ions by C-S-H delayed the development of the steel bars in the C-S-H group from the depassivation to the severe corrosion stage.The more vital ability of AFm to bind chloride ions per unit mass relative to C-S-H made the chloride threshold value for corrosion of steel in the AFm-doped group higher than that in the C-S-H group.However,AFm combined with chloride ions to generate Friedel’s salt was unstable under the interference of sulfate ions,and the bound chloride ions were re-released,making the reinforcing steel rust rapidly reach the severe corrosion stage after reaching the chloride threshold value.（3）Cement paste specimens were prepared by intermixing synthetic C-S-H/AFm.The specimens were analyzed for the content of bound chloride ions and phase assemblages at each age,and the microscopic morphological changes of the specimens were observed.The mechanism of the effect of internal blending of synthetic C-S-H/AFm on the chloride binding properties of cement-based materials was investigated.The results showed that the p H value of cement paste increased after the addition of synthetic C-S-H,no Friedel’s salt was generated in the early stage of cement hydration,and the larger particle size of synthetic C-S-H reduced the total specific surface area of C-S-H in cement paste.Hence,the addition of synthetic C-S-H reduced the chloride binding capacity of cement paste instead.The p H of the cement paste increased after the addition of synthetic AFm,the production of Friedel’s salt and AFt increased,and the production of C-S-H decreased,so the addition of AFm in the cement paste could effectively improve the chloride binding capacity due to the strong chloride binding capacity of AFm.However,the C-S-H generation in the AFm-doped cement paste specimens was reduced,and the chloride ion binding in the specimens was mainly dependent on AFm,which made the free chloride ion content of the cement paste in the AFm-doped group vary irregularly with age due to the instability of Friedel’s salt.