Preparation And Self-healing Properties Of Force-chloride Ion Triggered Microcapsules/Cementitious Composites

The cracking of concrete and the corrosion of its internal reinforcement are the main factors that affect the durability of reinforced concrete structure.The microcapsule self-healing technology based on bionics provides a new solution for reinforced concrete structure to realize self-healing and improving the durability.However,the traditional microcapsules’drawbacks of low crack-triggering efficiency and the low contact rate between healing agent and curing agent can hardly be resolved and it is also helpless to deal with the corrosion of steel.In order to improve the durability of cementitious composites from the two aspects of mechanical properties and electrochemical properties,a force-chloride ion triggered microcapsule was designed and synthesized.The force-chloride ion triggered microcapsules with Pb SO₄ embedded on the wall material were successfully synthesized by a two-step interfacial polymerization method.The synthesis parameters that affecting the structure and properties of microcapsules were systematically studied to determine the optimal preparation process.The structure and properties of the microcapsules were characterized by Scanning Electron Microscopy（SEM）,Energy Dispersive Spectrometer（EDS）and Fourier Transform Infrared Spectrometer（FT-IR）,etc.By means of SEM,EDS,mechanical experiment,Nano indentation and Digital Speckle Correlation Method（DSCM）,the self-healing properties of cementitious composites were studied from the aspect of mechanical properties.The self-healing properties of cementitious composites were studied from the perspective of electrochemical properties by Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy,and the electrochemical model of the self-healing process was established.The results showed that the microcapsules synthesized by the optimal preparation process had good morphology,with an average particle size of 53.8μm.FT-IR and Thermogravimetric analysis（TGA）showed that the structure of the microcapsule with double-core and double-wall has been formed.SEM and EDS confirmed that Pb SO₄ was successfully embedded on the capsule wall,and the microcapsules had good thermal stability and force-chloride ion triggered performance.The results of the study on the mechanical self-healing properties of force-chloride ion triggered microcapsules/cementitious composites showed that highest strength recovery rate of 84%can be obtained by adding 3%of microcapsules,and it can be improved to 101%in the environment containing chloride ions.The microcapsules can survive in the cement mixing process and be evenly distributed in the matrix.The critical stress of inner core and outer core of double wall microcapsule is 9.2 MPa and 8.4 MPa respectively deduced by nanoindentation and dimensional analysis method.DSCM was used to track the deformation behavior of self-healing cementitious composites during loading procedure,obtaining the self-healing mechanism based on filling cracks,limiting cracks’development and healing cracks from the variation of stress-strain,distribution of strain field and eigenvalue St _C and St_s.The results of the study on the mechanical electrochemical self-healing properties of force-chloride ion triggered microcapsules/cementitious composites showed that adding microcapsules to cementitious composites can effectively reduce the corrosion current density of the internal reinforcement.9%microcapsules content can provide the highest healing efficiency of 40%after 28 days of soak in 3.5%sodium chloride solution,and the highest healing efficiency can be increased to 56%under the joint response of cracks and chloride ions.Based on the evolution and fitting of the equivalent circuit model,the electrochemical self-healing mechanism of microcapsules was clarified,which is based on improving the resistivity of cementitious composites and protecting the passive film of steel bars to delay the corrosion rate of steel bars in self-healing cementitious composites.