Research On Protection Performance And Mechanism On Steel Bars Of Magnesium Phosphate Cement-based Materials

Magnesium phosphate cement（MPC）possesses unparalleled performance advantages,such as rapid setting and hardening,high early strength,good fluidity,good volume stability,and excellent bonding performance with the concrete substrate when used as a repair material.Therefore,it has broad application prospects.Reinforced concrete（RC）structure is widely used in main structure of civil engineering.Special attention should be paid to the protective effect on steel bars for the repair of damaged RC structures.MPC-based materials have been applied in the rapid repair and reinforcement of RC structures.However,there is little research on the protective effect on steel bars,especially for the protective effects of MPC-based materials on corroded steel bars in reinforced concrete.Hence,it is of great significance to unravel the protective performance of MPC-based materials on the steel bars and its protective mechanism,which is conducive to promoting the application of MPC-based materials in the repair and reinforcement of RC structures,as well as provides new insight into the durability of RC structures after repair and reinforcement using MPC,and is also favoring optimization and improvement of the performance of MPC-based materials.In view of this,this article mainly investigates the electrochemical behavior of steel bars in MPC-based materials through electrochemical testing compared with that in ordinary Portland cement（OPC）-and sulfoaluminate cement（SAC）-based materials.This paper focus on unraveling the influence of MPC matrix performance changes on the protective performance of steel bars and the evolution of its protective performance under harsh environmental conditions,and finally the protective mechanism of MPC-based materials on steel bars is illustrated.Firstly,based on the evaluation result of the electrochemical behavior of steel corrosion in the traditional OPC system,the applicability of relevant parameters in MPC-based materials was investigated.The results showed that the corrosion current density and polarization resistance are applicable in evaluation of the corrosion degree of steel bars.However,the potential index in OPC and SAC systems is not suitable for evaluation of the corrosion degree of steel bars in MPC due to the long passivation process of steel bars in MPC-based materials,resulting in more complicated changes in potential value.The passivation behavior of steel bars immersed in MPC simulated pore solution and MPC mortar were further investigated.The results showed that the passivation and film formation is a fast process in MPC simulated pore solution.The passivation film is composed of Fe₃O₄,Fe₂O₃,Fe OOH,Fe（OH）₂ and Fe PO₄.The existence of the Fe（OH）₂ is attributed to the slow conversion rate under low p H conditions,while the appearance of Fe PO₄ is ascribed to the participation of phosphate component in the film-forming reaction.However,the time for the passivation and stabilization of the steel bars in MPC mortar is much longer,even up to 28 d,which is related to phosphate dissolution near the steel bars and the difference in the degree of reaction of steel bars in MPC.A certain amount of phosphate participating in the reaction will continuously elevate the passivation degree of the steel bar.An M/P ratio of 3 could be considered as optimum,as it contributed to the highest passivation performance.This study unravels the long-term corrosion behavior of steel bars in MPC and the results indicate that compared with OPC,the protective property of MPC is more superior.Moreover,phosphate can also participate in the reaction and inhibit the development of corrosion after steel bars are corroded.In this part,the main factors,such as the surface state of the steel bars,M/P ratio,phosphate type,and age were changed to investigate the protective performance of MPC-based materials on steel bars.The results showed that increasing the surface activity of steel bars by polishing and reducing M/P ratio are both beneficial to improve the protective properties.According to the physical and mechanical properties of the matrix and the protective effect,the optimal M/P ratio for magnesium ammonium phosphate cement（MAPC）and magnesium potassium phosphate cement（MPPC）systems are different,which are 2.5～3.5 and 2.0～3.0,respectively.Relatively speaking,the protective effect of MAPC on steel bars is significantly superior to that of MPPC system.In the MPC-based materials of ultra-early age,the passivation of steel bars progressed gradually to a thermodynamic-steady state under accelerated corrosion conditions.However,an appropriate increase in the degree of reaction between the steel bar and the matrix is beneficial for its stability in a long-term corrosive environment.Herein,the unfavorable conditions that may occur during the repair and reinforcement of RC structures,including thin and ultra-thin protective layers,corroded steel bars,cracks in matrix,high chlorine environments,and galvanic corrosion,were simulated to investigate the long-term corrosion behavior of steel bars in MPC-based materials and the evolution of their protective properties.The results showed that MPC-based materials can provide sufficient and long-term protection for steel bars under harsh conditions.MPC-based materials exhibit efficient self-repairing ability for corroded steel bars and inhibit corrosion development of steel bars under high-chlorine environments.Among them,phosphate plays important roles in the passivation and corrosion inhibition of steel bars.The optimal M/P ratio is more conducive to the improvement of the protective performance of MPC-based materials.Finally,the protective mechanism of MPC-based materials for steel bars is elucidated by analyzing the electrochemical behavior of steel bars in MPC-based materials,combined with the theory of corrosion electrochemistry.We obtained these results as follows.Firstly,the internal environment of MPC is very similar to phosphating bath.The lower M/P ratio is beneficial to the passivation of steel bars in MPC-based materials.Secondly,according to the changes in strength of MPC matrix,MPC-based materials have excellent resistance to accelerated corrosion,and the dissolution of phosphate and reaction products are limited.A certain amount of dissolution is beneficial for corrosion protection of steel bars.Finally,the excellent protection performance of MPC-based materials for steel bars is related to the inhibition of Fe²⁺migration by the phosphate and Mg O components of MPC.The difference is that the former directly participates in the reaction with Fe²⁺,the latter functions via continuously hydrolyzing to provide OH^-that inhibits Fe²⁺migration for the local acidified area.In addition,the extremely high resistivity in MPC-based materials can inhibit the transfer of charges in the matrix,and thus affects the corrosion rate of steel bars.In summary,as a structural repair material,MPC-based materials exhibit excellent protective properties for steel bars,especially the self-repairing ability of corroded steel bars and the inhibition of subsequent corrosion in severe environments.It shows that MPC-based materials have obvious advantages in the repair and reinforcement of reinforced concrete structures.