Multi-Scale Analysis On Bonding Properties And Investigation On Soulation Corrosion Resistance Of Magnesium Phosphate Cement

Concrete is easy to be damaged and cracked due to the influence of environment and load during its service cycle.In order to improve the durability of concrete structure,it is urgent to repair and strengthen the damaged concrete structure.At present,the research results of magnesium phosphate cement(MPC)show that MPC has excellent mechanical properties,outstanding bonding performance with concrete,no obvious drying shrinkage and rapid setting and hardening,and is suitable as a repair material.However,the understanding of magnesium phosphate cement repair and reinforcement and corrosion resistance performance is relatively lacking.In the field of numerical simulation,researchers generally only focus on the damage evolution of the overall performance of components or structures,and lack of research on the influence of microscopic crack initiation,distribution law and initial defects,especially on the influence of the inherent characteristics of the bond interface(interface defects,differences in mesoscale distribution near the interface and differences in material stiffness,etc.)on the bond performance.In this paper,the bonding characteristics and solution corrosion of magnesium phosphate cement suitable for repair and reinforcement are taken as the starting point,and the research method of MPC multi-phase and multi-scale bonding properties from micro,micro to macro levels is established.The deterioration behavior of modified MPC matrix under acid-alkali salt corrosion solution is considered,which has important practical significance for expanding the application of magnesium phosphate cement repair and reinforcement.The main research work and innovative achievements of this thesis are as follows.Ⅰ.Multi-scale research method of magnesium phosphate cement-concrete bonding properties.The causes of the macroscopic nonlinear mechanical behavior of magnesium phosphate cement are the composition of microscopic phase and the distribution of mesoscopic pores,and the causes of the nonlinearity of concrete are the nonlinear mortar and the distribution of mesoscopic aggregates.Due to the random failure of magnesium phosphate cement-concrete bonding specimens(failure of matrix,repair material or bonding interface),The mesoscopic characteristics of magnesium phosphate cement and concrete as well as the initial defects at the interface must be considered when performing numerical simulations.The specific work is as follows.1.Based on the microscopic analysis of magnesium phosphate cement,it is determined that the volume fraction of magnesium oxide,struvite-K and pore in MPC are 32.6%,59.1%and 7.6%,respectively.The elastic modulus of struvate-K/magnesia is 51.2MPa and Poisson ratio is 0.19.The MPC model of random distribution of pores is established and analyzed.The results show that the larger the porosity,the smaller the elastic modulus,the smaller the initial damage strain and the lower the peak stress.2.A three-point bending test is conducted to evaluate the effect of sand binder ratio on the repairing effect of magnesium phosphate cement-concrete.The test shows that the bending bond strength increases with the decrease of sand-binder ratio.Considering the mesoscale model and initial interface defects of the near-interface matrix material,a magnesium phosphate cement-concrete bond model is established by using Python language and ABAQUS numerical simulation.The results show that the random defect distribution has a great influence on the bending bond strength,and the lower the concrete strength,the lower the bond strength.Ⅱ.In the acid alkali salt corrosion solution environment magnesium phosphate cement deterioration behavior.Different from Portland cement,the strength of magnesium phosphate cement will shrink in aqueous solution.A large number of researchers have compared the specimens cured in air as the benchmark specimens with the specimens in corrosion solution,which will lead to the effect of aqueous solution and corrosion ion mixed together,and it is not easy to explain the deterioration behavior of MPC.It is necessary to specify two roles when evaluating the deterioration behavior of MPC in corrosive solution environments.The following related work has been carried out.1.The deterioration rule of MPC in aqueous solution is considered,and then the deterioration degree of MPC in acid,alkali and salt solution is evaluated based on the deterioration results of aqueous solution.After modification,the degree of solution erosion of magnesium phosphate cement is as follows from severe to slight: acid solution erosion,alkali solution erosion,sodium chloride solution erosion,and magnesium sulfate solution erosion.2.XRD and SEM-EDS are used to investigate the deterioration mechanism of MPC’s compressive strength after long-term acid-alkali salt solution erosion.Acid solution can inhibit the nucleation and crystallization of struvite-K,and struvite-K in alkali solution can be transformed into crystalline brucite,resulting in structural expansion and damage.The erosion results of salt solution are similar to those of aqueous solution.