Corrosion Mechanism Of Rebar In Potassium Magnesium Phosphate Cement

Magnesium phosphate cement（MPC）is a new type of cementitious material,which has strong chemical bonding feature by a through-solution acid-based reaction between dead burned magnesia and phosphate.Steel bar corrosion is a significant factor affecting the durability of reinforced concrete structure.Both carbonation and chloride salt attack will cause serious damage to the reinforced concrete structure due to the corrosion of steel bar.Recently,some experiements have revealed that the steel bar in MPC has better resistance to chloride attack than that in OPC.But the underlined mechanism is unclear.To explore the corrosion resistance mechanicam of steel in the MPC,the pore solution of MPC system was filtrated firstly.The Pourbaix diagrams of steel bar in both the simulated and various modified versions of MPC pore solutions were obtained by the thermodynamic modeling to investiagte the corrosion behavior of steel bars in the simulated pore solution in terms of electrochemcical experiment,which explained the mechanism of the test results.Then,the steel corrrsion behavior in carbonated MPC was studied by electrochemical test and thermodynamic modeling to fully understand the its long-term corrosion resistance.The corrroison mechanism of steel bars in MPC after carbonation was revealed through thermodynamic modeling.Finally,molecular dynamics（MD）simulation was used to futher explore the atomic-scal mechanism of corrosion of steel bars in the MPC.Pourbaix diagrams（Eh-p H diagram）of Fe-H₂O system under the action of anions,cations and aggressive ions were calculated by thermodynamic modeling according to the ion concentration in pore solution.The conclusion is that the formation of Fe₃（PO₄）₂?8H₂O,Fe₂O₃ and Mg Fe₂O₄ on the surface of the steel bar was likely to be an important factor for the corrosion resistance of the steel bar in the MPC system.Effects of phosphate concentration,p H values and temperature on corrosion behavior of steel bas were studied by electrochemical test.The following results could be obtained:（1）With the increase of the phosphate content,the corrosion resistance of the steel bar significantly increased.When the phosphate content reached 0.05mol/L,the corrosion resistance of the steel bar in the simulated pore solution was better than that of the steel bar in the OPC simulated pore solution.（2）The higher the p H value,the better the corrosion resistance of the rebar,and when the p H value was only 10.68,the steel bar had excellent corrosion resistance.According to thermodynamic calculations,the main reason was that at this p H value,PO₄^3-activity was very high,which was enough to maintain the stable generation of Fe₃（PO₄）₂?8H₂O.Compared with other iron oxides and hydroxides,the passive film and rust layer contained more Fe₂O₃,so it was easier to form Fe₃（PO₄）₂?8H₂O.The Fe₃（PO₄）₂?8H₂O film was still stable in the carbonated environment.Double-layer passive film structure were conformed,which the inner layer was the passive film containing iron oxide and hydroxide,and the outer layer was the phosphate layer containing Fe₃（PO₄）₂?8H₂O.（3）At 25-50℃,it was favorable for the formation of ferrous phosphate film layer,which made the passive film layer have more corrosion resistant.When the temperature was 50-75℃,the defects in the passive film had a major effect on the performance of the passive film.The pitting resistance of the steel bar quickly weakened.A quantitative relationship model of the influence of phosphate concentration and p H value on the critical chloride concentration was obtained.In the accelerated carbonation experiment of the reinforcing steel in MPC pastes,the corrosion behavior of the steel bar with different MPC proportion was investigated,such as M/P（molar ratio of Mg O to phosphate）,borax content（mass ratio of borax to Mg O）,W/C（mass ratio of water to（Mg O+phosphate））.At the same time,combined with thermodynamic calculations,the following research results were obtained.The pore structure of the MPC pastes differed greatly with different proportion before carbonation,which resulted in the large difference of resistance.After carbonation,the gap between the pastes resistance gradually decreased,because remained Mg O in the matrix was carbonized to Mg CO₃?Mg（OH）₂?3H₂O and Mg CO₃,which might fill the pores in the MPC.The protective effect of the matrix on the steel bar was better when the M/P value was 6 or 7,the amount of borax was 5%-8%,and the W/C ratio was about 0.16.In order to explain the microscopic corrosion mechanism of steel bars in MPC,the molecular dynamics simulation were applied to study the effect of factors(such as PO₄^3-content,OH^-content,temperature,phosphate species and various surfaces)on the diffusion behavior of ions in solution and interaction energy between solutions and Fe surfaces.Molecular dynamics simulation results showed that:（1）As the phosphate content increased,phosphate and Cl^-competed with each other on the surface of the steel bar,diffusion coefficient of the Cl^-decreaseed,the adsorption system became more stable,and the corrosion resistance of the steel bar increased.（2）When the solution contained more chloride ions,the interaction energy between the solution and the Fe matrix rapidly decreased at the ratio of phosphate/Cl^-was 1:1,which demonstrated that the corrosion resistance of the steel bar was significantly improved.（3）With the increase of OH^-content in the solution,the diffusion rate of Cl^-became smaller.On the contrary,the diffusion coefficient of PO₄^3-increased,which was benefical for the corrosion resistance of the rebars.The above results showd that the pore solution in MPC had excellent chemical protection on steel bars and the MPC pastes also had excellent physical protection on steel bars after carbonation.The mechanism research in this paper provides theoretical support and experimental basis for the use of MPC in structural engineering and marine environments,and provides references for subsequent research.