Effect Of Reducibility Sulfur In Slag On Corrosion Resistance Of Steel Bars

Ground granulated blast furnace slag(hereinafter referred to as slag)as a kind of concrete cementitious material has been widely used in reinforced concrete structures.When the half-cell potential method is used to detect the corrosion state of reinforcement in reinforced concrete mixed with slag,the measured potential is lower than the normal potential value,which indicates that the reinforcement has begun to corrode with high probability.However,after the structure is broken,it is found that the reinforcement does not corrode.As a widely used application of non-destructive detection method for corrosion state of steel bars in structures,half-cell potential method can lead to misjudgment of corrosion state of steel bars when detecting steel bars in structures with slag.At present,it is generally believed that the reducibility sulfur caused by the continuous hydration of slag leads to the decrease of measured potential.However,there is still a lack of in-depth research on the types of reducibility sulfur contained in slag,the release law of reducibility sulfur with the hydration of slag,the reasons for the reduction of steel bar potential caused by reducibility sulfur and its impact on the corrosion resistance of steel bars.Therefore,in-depth exploration of these issues is the focus of this study.Based on this,this paper studies the release law of reducibility sulfur with slag hydration by forming the paste test block with different slag content and testing the concentration of reducibility sulfur ion by pressing the pore liquid of the paste test block.At the same time,the hydration degree of slag is tested to establish the relationship between the hydration degree and ion concentration.The electrochemical performance and corrosion resistance of steel bars were studied in the simulated concrete pore solution mixed with reducibility sulfur.The composition and structure of the passive film of steel bars were characterized by microscopic test,and the effect of reducibility sulfur on the passivation and corrosion resistance of steel bars was explored.The main rules are as follows:(1)After the reducibility sulfur in slag enters into the slurry pore solution,95%of it exists in the form of S^2-and the rest is a small amount of S₂O₃^2-and SO₃^2-.For alkali activated slag system,the hydration rate of slag is mainly concentrated in the first 28 days,and the reductive sulfur in pore solution reaches the maximum value in the first 28 days,and then decreases slowly.After 28 days,the hydration rate of slag accounts for almost one sixth of the first 28 days;for cement slag system,the hydration rate of slag in the first28 days is slower than that of alkali activated slag group,and the hydration rate of slag from 28 days to 90 days accounts for the first 90 days the concentration of reducibility sulfur reaches the maximum at 90 days.In addition,the concentration of reducibility sulfur in pore solution of alkali activated group is much higher than that of cement slag group,even under the same hydration degree of slag.Although sealed curing does not affect the hydration degree of slag,the reductive sulfur concentration of pore solution in the experimental group is about 2 times higher than that in the standard curing group.(2)When S^2-participates in the normal passivation process of steel bars in alkaline solution,the normal passivation process of steel bars will be interfered,and normal iron oxide will not be generated,but iron sulfide will be generated.The main components are Fe S and Fe S₂,and the thickness is about 5nm?10nm.The surface of this film is very rough,and the bond with steel bars is not strong,and the resistance to Cl^-erosion is very low,so it cannot be used to treat steel bars effective protection.On the contrary,S₂O₃^2-does not interfere with the normal passivation process of steel bars in alkaline solution.Even if S₂O₃^2-exists,the composition of the passivation film of steel bars is still normal iron oxide.(3)When the steel bar is passivated in the normal alkali solution,the S^2-will still erode the iron oxide passivation film formed on the steel bar surface,change the main composition of the passivation film,and reduce the corrosion resistance of the steel bar to Cl^-.On the contrary,S₂O₃^2-has no effect on the composition of passive film and corrosion resistance of steel bars after normal passivation.(4)No matter whether the steel bar is passivated in alkali solution or not,the introduction of S^2-will reduce the corrosion potential of the steel bar in a moment,but S₂O₃^2-has no effect.The reason for this phenomenon is that S^2-consumes the dissolved oxygen in the solution,erodes the steel bar surface,and increases the corrosion current density of the steel bar,thus causing the cathodic polarization curve on the Tafel curve to move downward and the anodic polarization curve to move upward Moving to the right causes the intersection point of the tangent lines of the polarization curves of the anode and the cathode to move downward,so the corrosion potential decreases.