Self-healing Mechanism And Cracks/Chloride Ion Triggered Design Of Microcapsules Composite Cement-based Materials

In the service process of the cement-based materials,they are vulnerable to being affected by the load and the external environment,resulting in the crack damage.Corrosion medium or water entering the matrix through crack passage will cause corrosion of internal reinforcement and reduce the durability of cement matrix composites.Therefore,the study of self-healing cement-based materials based on microcapsules has attracted more and more attention.In this study,the inorganic materials were used as the core materials,and ethyl cellulose(EC)was used as the wall materials to synthesize microcapsules.Microcapsules were introduced into the cement matrix to prepare self-healing cement-based materials,and the self-healing cement-based materials were studied from two aspects of mechanical and anticorrosion properties.Microcapsules with core-shell structures were synthesized by spray rolling granulation.With the compressive strength and strength recovery rate of self-healing cement-based materials as the evaluation indexes,the core components of microcapsules were obtained by the orthogonal experiment.Combined with the single index method and comprehensive balance method,the core components of microcapsules were determined:clay curing agent was 10%,MgO expansive agent was 35%,microcrystalline cellulose was 6%,and bentonite was 49%.By adjusting the preparation process conditions of microcapsules,the technological parameters were determined as follows:the rotation speed of the round-pot granulator was 60 r/min,and the amount of EC solution was 40%of the solid material of microcapsules,and the concentration of EC solution was 4%.The structure of the microcapsules were tested and analyzed by the scanning electron microscope(SEM),image particle analysis system and x-ray diffractometer(XRD),which showed that the microcapsules had good sphericity.The thermogravimetric analysis(TG)showed that the microcapsules had good thermal stability.The yield stress of microcapsules was deduced to be 8.7 MPa by nanoindentation test and dimensionless equation.SEM and X ray energy spectrum(EDS)tests showed that the chloride ion trigger was successfully embedded in the capsule wall,and the microcapsules could realize the chloride ion response with a good response effect.The mechanical properties of self-healing cement-based materials show that when the particle size of microcapsules was between 0.2-0.4 mm and the dosage of microcapsules was 3%,the self-healing cement-based materials had high strength.Under the combined action of the cracks and chloride ions,the compressive strength recovery rate of self-healing cement-based materials containing 3%microcapsules is 110.3%,indicating that self-healing cement-based materials have certain mechanical self-healing properties.The results of XRD and SEM showed that the repairing products of microcapsules are mainly acicular ettringite crystals.The deformation behavior of self-healing cement-based materials before and after healing under the load was tested by the digital speckle correlation method,and the variation laws of stress-strain,gray coefficient eigenvalues and strain eigenvalues of self-healing cement-based materials under the load were obtained.The results show that microcapsules can play a role under the combined action of the cracks and chloride ions,which can repair cracks by filling cracks and restricting the development of cracks,the microcapsules can restore the mechanical properties of the cement-based materials by the way of"filling cracks-limiting the development of the crack-repairing the crack-healing cracks".The study on the anticorrosion properties of self-healing cement-based materials showed that after incorporation microcapsules,the open circuit potential of the internal reinforcements shifted positively to different degrees in the middle of corrosion,indicating that microcapsules had a specific protective effect on the steel bars in the self-healing cement-based materials.It can be seen from the electrokinetic potential polarization test that the corrosion current density of the intact sample at the initial stage of corrosion can be reduced to 0.13μA/cm~2,indicating that the addition of microcapsules can effectively delay the corrosion of steel bars in cement-based materials.The equivalent circuit was established based on the characteristics of electrochemical impedance spectroscopy(EIS),and the parameters of each component were fitted according to the equivalent circuit.The results show that the self-healing efficiency of the intact sample containing 9%microcapsules can reach 37.9%after soaking for 56 days.For the pre-damaged samples,the repair efficiency of the samples containing 3%microcapsules after soaking for 28 days can be increased to 41.1%.Therefore,when the content of microcapsules is 3%,the self-healing cement-based materials have the highest electrochemical repair efficiency while ensuring good mechanical properties.