Study On Formation Of Passive Film And Depassivation Of Steel Bar In Alkali Slag System Based On Analysis Of Pore Solution

As a new type of binding material,alkali-activated materials have the advantages of low carbon and low energy consumption,and have wide application prospect.However,the internal solid and liquid phase environment of alkali-activated slag system is quite different from that of ordinary portland cementitious system.Therefore,the evolution of liquid environment in alkali activated slag system with time,the formation and failure mechanism of steel passivation film in alkali-activated slag system,and the threshold of chloride content in pore solution of steel depassivation still need further study.In this paper,the effects of activator type,alkali equivalent（7%,9%）,sodium silicate modulus（1.8,2.0）,blend of fly ash,Ca/Si,Al/Si on the strength,pH value,ion composition,zeta potential and the adsorption capacity of chloride ion of alkali-activated slag paste were studied.Open circuit potential method,electrochemical impedance spectroscopy（EIS）,x-ray photoelectron spectroscopy（XPS）and scanning electron microscopy（SEM）were used to study the formation process of passive film of steel bars in simulated pore solutions with different pH values（10.5,11.5,12.5,13.5）and the destruction process of passive film in chloride environment.The thickness and composition of the passive film were analyzed,and the depassivation thresholds of chloride content for the passive film in various simulated pore solutions were determined.The conclusions are as follows:（1）The pH value of C-（A）-S-H solution decreased with the prolongation of curing time;the cation concentration decreased gradually;zeta potential moves to the negative direction.The pH value of soaking solution increases with the increase of Ca/Si,and decreases with the decrease of Al/Si;the variation of cation concentration was the same;the decrease of Ca/Si and the increase of Al/Si lead to the zeta potential moves to the negative direction,the potential value of C-A-S-H is lower than that of C-S-H.（2）During the 28 day curing period,the pH value of the water glass-activated slag hole solution is between 12.8-13.4,the pH value of the sodium hydroxide-activated slag hole solution is between 13.5 and 13.8.The pH value of pore solution increases with the increase of alkali equivalent,and decreases slightly with the addition of fly ash.The cation concentration in pore solution decreased significantly with curing time.（3）The zeta potential of alkali-activated slag and ordinary portland cementitious system moves to the negative direction with curing time,the absolute values of zeta potential are as follows:water glass group>sodium hydroxide group>ordinary silicate group.The increase of alkali equivalent and the addition of fly ash will make zeta potential move to negative direction.（4）In the slurry dispersion,the slurry has a certain adsorption effect on the chloride ion mixed in the dispersion,and the adsorption of chloride ions will increase the absolute value of zeta potential of the dispersion.The chloride ion adsorption capacity is:water glass group>sodium hydroxide group>ordinary portland cementitious group.（5）The time and thickness of the passive film formation of the steel bars in the 6groups of simulated hole solutions are different,in the water glass 1 group with a pH of13.5,a passivation film with a thickness of 7nm can be formed in 3d;it takes 4 days to generate a stable passivation film in the OPC1 group with the same pH value;it takes5d,6d and 7d to generate the passivation film in the water glass 2 group,the sodium hydroxide group and the OPC2 group with a pH of 12.5,respectively.The thickness of the passivation film in the water glass 2 group and the OPC2 group are both 5nm;in the water glass 3 group with a pH of 11.5,a passivation film of 2.5nm can be formed after 8 days.It can be seen that as the pH value decreases,the time to generate the passivation film increases significantly,and the thickness of the passivation film decreases.（6）The steel bar passivation film is a double-layer structure,mainly composed of Fe,FeO,Fe₂O₃,FeOOH,as the pH value of the simulated pore solution increases,the ratio of the divalent iron ion compound in the passivation film increases.At the same pH value,the corrosion resistance of the steel bar passive film in the simulated pore solution of the water glass group is better than that of the sodium hydroxide group,and the ordinary silicate group is the worst.