Effect Of Crack Self-healing On Chloride Ion Transmission In Ultrahigh Performance Cementitious Materials

Ultra High Performance Cement Based Composites（UHPC）have excellent properties with high compressive strength,low permeability and good durability.Ultra-high performance cement-based composites have achieved a lot of research results in terms of mechanical properties.However,there is insufficient research on its permeability resistance,such as the transmission performance of harmful ions such as chloride ions in ultra-high-performance cement-based materials after cracking,especially for study of the self-healing on penetration resistance of cracks in wet environments.In this paper,CEMHYD3D model was used to analyze the self-healing degree of cracks in ultra-high performance cement-based composites with different mix ratios of different mineral admixtures.At the same time,the finite difference method was used to analyze the water and chloride ions in consideration of self-healing conditions.Performance Transmission in cement-based composites.The effect of crack morphology on the transmission of cracked ultra-high-performance cement-based composite materials was discussed,and the following results were achieved:First of all,this paper uses the CEMHYD3D hydration model to simulate the hydration process.In order to use the CEMHYD3D model to simulate the cement-based composites containing silica fume and fly ash,the correlation coefficient of the CEMHYD3D model needs to be modified.The cracks with different widths（10μm,30μm,and 50μm）were preFAbricated in a hydration microstructure model 28 days after hydration,and the degree of reaction of each substance and the microstructure in the crack after different rehydration times were obtained,and the position of each substance and degree of self-healing of cracks were analyzed.The addition of mineral admixtures has a minor effect on the self-healing of cracks.When the cracked cement-based material was in the saturated state for 5 months,the healing degree was 70%or more when the crack width was 10μm,the healing degree was 35%or more when the crack width was 30μm,and the healing degree was 20%or more when the crack width was 50μm.Subsequently,based on the degree of self-healing of cracks and the distribution of various substances in the cracks obtained by CEMHYD3D hydration model,the effect chloride ion and moisture transmission coefficient in the cracks of crack self-healing with different time,different crack widths（10μm,30μm,50μm）were obtained by the random walk method.The results showed that the effect of self-healing on the chloride ion coefficient:the diffusion coefficient of chlorine ion in the cracks in the early stage of self-healing in the ternary cement-based system was slower than that in the binary cement-based system.At the later stage,it gradually overtakes the rate of change of chloride ion diffusion coefficient in the binary cement-based system.When the self-healing is about 5 months,the diffusion coefficient of chloride ions in the cracks of the three-phase mixed cement-based composite system is slightly lower than that of the binary system and the pure cement system.For the initial cracks with a crack width of 10μm,the diffusion coefficient of chloride ions in the crack decreases rapidly.When the healing time is around 1000 h,the diffusion coefficient of chloride ions in the fractures decreases to the order of 10^-12 m²/s.As the width of the crack increases,the effect of self-healing on the transmission rate of chloride ions in the fracture decreases.Finally,the finite-difference method was used to determine the transport depth of chloride ion in cracked cement-based composite materials for a certain period of time.The influence of content of different mineral admixtures in the same water-binder ratio,different crack widths,different fracture lengths,different healing times,different cements and initial relative humidity were compared and analyzed.The results show that when the crack width is between 10μm and 50μm,with the increase of crack width,the chloride ion concentration in the crack increases,and the transmission depth along the crack direction has no obvious effect.The addition of fly ash will increase the chloride ion in the matrix.And the depth of transmission around the fracture,which is mainly due to the addition of fly ash increases the chloride ion diffusion coefficient in the matrix,and fly ash added to slow the healing rate of the initial healing,reducing the rate of change of the chloride ion transmission coefficient.The initial internal relative humidity has a significant influence on the chloride ion transport depth of cracking ultra-high performance cement-based materials.The lower the initial internal relative humidity,the greater the depth of chloride ion transport.The results show that when the crack width is between 10μm and 50μm,the chloride ion concentration in the crack increases with the increase of crack width,and the transmission depth along the crack direction has no obvious effect.The addition of fly ash will increase the chloride ion in the matrix.The initial internal relative humidity has a significant influence on the chloride ion transport depth of cracking ultra-high performance cement-based materials.The lower the initial internal relative humidity,the greater the depth of chloride ion transport.To verify the accuracy of the simulation results,the following tests were conducted:1)To verify the accuracy of the CEMHYD3D model in simulating the degree of crack healing,an X-ray microscopy test was used to analyze the degree of healing of different crack width cracks;2)To verify the finite difference method based on Fick’s law determines the depth of transport of chloride ions in cement-based materials and uses rapid electromigration（RCM）to determine the 28-d chloride diffusion coefficient of cement-based composites with the same water-binder ratio for different mineral admixtures,and to use nitric acid.The silver titration method determines the depth of transport of chloride ions soaked in NaCl solution for 5months in the natural diffusion method;3)The CsCl-enhanced X-CT method was used to determine the depth of moisture transport with different water-cement ratios（0.18,0.35）.The test results are in good agreement with the simulation results.