Research On Corrosion Of Steel Bar In Magnesium Phosphate Cement

Magnesium phosphate cement(MPC) have been extensively used as fast setting repair materials in civil and military engineering and stabilization of harmful wastes,which exhibit broad prospects in building materials. Compared with ordinary Portland cement(OPC),the MPC, as a new gas cementing material, have the characteristic of high liquidity, short setting time, early strength acquisition and adhesive properties, good volume stability, high abrasion and deicer-frost resistance as well as the property of chemically bonded ceramics. For its poor water resistance, the MPC have not yet been applied in the projects in water environments such as structural engineering and marine projects based on security reasons. However, it has been reported that there was no serious loss of strength of the MPC when exposed to water environments for a long time.Additionally, increasing researches focus on improvement of water resistance of the MPC and establishment of protection strategies, which provide its potential for application of structural engineering and marine projects.Corrosion of steel bar is a serious problem which occurs in structural engineering and marine projects. It has been reported that the MPC show a remarkable protection to steel bar and resistance to chloride corrosion compared to the OPC. However, currently there were few studies focusing on this subject, the results were still unclear and the methods needed to be updated. In this study, electrochemical techniques was employed to test and characterize the corrosion processes of steel bar under dry-wet cycle conditions in order to obtain its behavior and factors affecting corrosion of steel bar in MPC. This work aims for providing theoretical foundation for process improvement in technical application of steel materials and novel information for other relevant studies.There are a series of detection methods for corrosion behavior of steel in concrete,such as measurement of some parameters, physical techniques and electrochemical techniques. Among them, electrochemical techniques are more effective tools showing the mechanism of this electrochemical process.In this study, electrochemical test parameters and conditions were explored. The optimal electrolyte solution was screened via the investigations of a stable open circuit potential(OCP) configuration of steel electrode in different electrolyte solutions and specimens including the MPC and OPC as well as the value differences of AC impedance spectroscopy in the same specimen.We established an equivalent circuits model for the steel electrode, and numerical fittingand error analysis was carried out in our study. The results showed that all specimens could reach a stable corrosion potential in selected electrolyte solutions during the detection time(the potential did not vary by more than ±0.5 m V in 100s). We also found that it was prone to be stable if there was a high similarity between electrolyte and pore solution. Finally, saturated Ca(OH)2 solution was selected as electrolyte during the process of electrochemical test of the OPC and MPC system since the value of AC impedance spectroscopy varied in the different electrolyte solutions. In addition, the encode of equivalent circuits model for the electrode system was showed in this work,that was(Cc Rc)(Cb Rb)(Cdl(Rp Q)), which was in high precision revealed by fitting results and can be successfully employed.This work demonstrated that the MPC show a significant corrosion protection to steel bar compared to the OPC, which was revealed by the results of electrochemical corrosion test under the dry-wet cycle condition. Also, the protection mechanism was also characterized. In this study, corrosion behaviors under varies dry-wet cycle conditions in different MPC systems were investigated and the factors associated with MPC properties which could affect corrosion were explored. The results showed that steel bars in concrete are prone to corrosion constantly since a stable passive layer is hardly formed on steel surface. The MPC were more effective in terms of protecting steel against corrosion than the OPC, which was attributed to the stability of matrix and the property that strongly inhibiting charge and mass transfer. The effects of components in MPC system on protecting steel were investigated, that was cement paste > mortar > concrete. This work also indicated that there was a significant decline in anti-corrosion in the mixed solutions of Cl- and SO42- and the solutions with high level chlorine ion. With regard to MPC, the effects of protection were attenuating with increasing the water-cement ratio. Furthermore, it would provide high corrosion resistance when the cement retarder(8-10% by weight of total cement) and M/P=4 was applied.In this work, effect of corrosion behaviors of MPC with different factors were explored. Varies candidate factors were designed in the study, including different types of steels, components of matrix, construction processes and the amounts of admixture of concrete. The results showed that the high-grade steel was more remarkably resistant to corrosion than ordinary reinforcing steel. Surface treatment has significant effects on the corrosion of steel. The supplied steel bar without grinding demonstrated a more stable performance and excellent protection against corrosion, especially at the early stage ofdry-wet cycle conditions. In this study, NH4H2PO4 was replaced by KH2PO4 for phosphorus supply, and we found that steel bar of potassium based MPC show more stable. The results also showed that the construction process that concrete casting after coating with quality MPC paste in steel surface could effectively improve corrosion resistance at the early stage and was suitable for long-term corrosion prevention. The mineral admixtures in MPC did not show advantage of protection to steel bar Obviously,furthermore,the results of steel’s long-term corrosion and more detailed test will be investigated to determine its effect on the corrosion behavior of steel bars.