Study On Synergistic Mechanisms Of Red Mud And Fly Ash On Eletrochemical Behavior Of Reinforcing Steels

With the proposal of the national strategy of"Carbon Emission Peak and Carbon Neutrality",it is urgent for concrete to reach a new level of"low-carbon".To achieve the goal of low-carbon concrete in China,it is a feasible approach to develop low-clinker cements and realize resource utilization of supplementary cementitious materials.However,on account for the low proportional ratio of cement clinkers in low-carbon concrete,the p H value of concrete pore solutions is relatively low,which would be detrimental to the passivity of reinforcing steels embedded in concrete.Therefore,the most essential consideration for the application of low-carbon concrete in reinforced concrete（RC）structures is its protection to embedded steels.Especially when exposed to marine environments,such as harbor wharves,undersea tunnels,and cross-sea bridges,RC structures are seriously subjected to chloride attack,high humidity,and high temperature.These harsh conditions often lead to the premature failure of RC structures within several years due to the corrosion of steels,consequentially cause the serious economic losses and potential safety hazards.In this context,with the implementation of national strategies of"Double Carbon Goal"and"Ocean Power",it is of scientific significance and social benefit to study the corrosion resistance of steels in low-carbon concrete.For the purpose of scientific design and reasonable preparation of the low-carbon and durable concrete suitable for marine chloride environments,a variety of extract solutions simulating the alkaline concrete pore environemnts were prepared,including ordinary Portland cement solution（OPC）,solution admixed with 20%fly ash（FA）,solution admixed with 20%red mud（RM）and solution admixed with 20%fly ash and 20%red mud（FA-RM）.Afterwards,the passivation behavior and chloride-induced corrosion behavior of Q235 steel,AISI 304 stainless steel and AISI 316 stainless steel were studied using electrochemical measurements such as electrochemical impedance spectroscopy,cyclic voltammetry,cyclic potentiodynamic polarization and Mott-Schottky curves.Moreover,microscopic characterization techniques such as X-ray photoelectron spectroscopy,atomic force microscopy and scanning electron microscopy were used to explore the influence of steel type and pore solutions of low-carbon concrete on the durability of reinforced concrete structures.Moreover,the synergistic mechanisms of red mud and fly ash on steel passivation and corrosion resistance to chlorides were proposed,and the summarized results are listed as follows:（1）After passivated in FA solution with a low p H value,a loose and unstable passive film enriched with Fe OOH and H₂O was formed on Q235 steel,thus resulting in a low corrosion resistance to chlorides.The favourable corrosion resistance of Q235 steel in RM solution was due to the high content of Fe（ox）in the passive film.Simultaneously,the negatively charged red mud particles not only represented competitive adsorption with Cl^-but also acted as the adsorption sites of Ca²⁺,thereby preventing the corrosion caused by chlorides.As for Q235 steel immersed in FA-RM solution,besides the superiority provided by the red mud,a moderate p H value was achieved by the addition of fly ash.These factors synthetically promoted the formation of a denser,Fe（ox）-rich and more protective passive film on Q235 steel,consequentially leading to the enhanced corrosion resistance in the presence of chlorides.（2）Similar to Q235 steel,304 stainless steel also exhibited high corrosion resistance to chlorides in FA-RM solution.Additionally,304 stainless steel exposed to FA-RM solution was promoted to form a denser and stable double-layer passive film,whick is enriched with Fe（ox+hyd）and Ni（ox+hyd）in the outer layer and Cr（ox+hyd）in the inner layer,thereby presenting a improved protective ability.（3）Being different from Q235 steel and 304 stainless steel,316 stainless steel formed a passive film with improved passivity and enhanced corrosion resistance to chlorides in FA solution,which was mainly related to the low p H value of FA solution,promoteing the formation of a highly dense passive film enriched with high content of Mo（ox）and low content of H₂O on 316 stainless steel.