Investigation Of Self-healing Properties Of Cement-based Materials Based On Chelating Crystal And Its Influencing Factors

As the most important building material,concrete is widely used in various engineering construction fields.However,concrete is a kind of porous brittle material,which is easy to produce internal damage and cracks under the action of freeze-thaw,load and chemical erosion,resulting in the decrease of concrete durability.In order to increase the service life of buildings,the self-healing concrete has become a research hotspot.The chelating crystal self-healing concrete depends on the reaction of chelator and calcium ion,migrates in the concrete to forms crystalline products at the crack,so as to realize self-healing.Compared with microcapsule self-healing concrete and microbial self-healing concrete,chelating crystal self-healing concrete has the advantages of strong practicability,low cost and better application prospect.Many researches have been carried out on the self-healing of cracks by chelator,the influence of chelator on the hydration process and microstructure of cement,the migration ability and its long-term effectiveness of self-healing function,the external factors affecting the chelating crystallization reaction,and the self-healing ability of chelator on concrete in saline environment have not been systematically studied.of chelating crystal self-healing concrete.The in-depth study of these problems will promote the development and application of of chelating crystal self-healing concrete,which is of great significance.In this paper,the influence of chelator on the hydration process and microstructure of cement paste was studied by testing the hydration heat,chemical bound water and compressive strength of cement paste.The migration distance of chelator in mortar and its self-healing effect on cracks were studied by simulation experiment.Through the self-healing of prefabricated cracks of mortar with different ages,the long-term effect of self-healing function of chelator was studied.Then,the effects of CO₂and mineral admixtures on the self-healing ability of chelating crystal were studied by curing in high concentration CO₂environment and using different types of mineral admixtures to replace part of cement.Finally,the corrosion resistance and self-healing performance of concrete mixed with chelator and mineral admixture in different saline environments are studied.The main conclusions are as follows:（1）Chelator promotes the hydration of cement and improves its pore structure.When the content of chelator is 0.50%of cement mass,the hydration process and pore structure of cement paste are significantly improved.Compared with the ordinary cement paste,the cement pastes mixed with 0.50%chelator after curing for28 d,its chemically bound water content increased by 13.0%,and the proportion of harmful pores greater than 0.1μm decreased by 30.8%.TG-DTA and XRD tests further proved that the chelator promoted the hydration of C₃S and C₂S,and formed more hydrated products such as C-S-H gel in the pores of cement paste,which increased the density of cement matrix.（2）The migration distance of the chelator in the cement matrix is closely related to the self-healing effect of cracks.When the migration distance of chelator is within5 cm,the cracks with width of 0.30 mm can be repaired after curing for 7 d.When the migration distance of chelator is 8 cm,the crack with a width of 0.20 mm is still difficult to heal.SEM analysis shows that when the migration distance is small,a large number of calcite crystals are formed in the cracks,which has a good healing effect on the cracks.When the migration distance is too large,only loose calcite crystals can be produced in the cracks,and the self-healing ability is obviously weakened.（3）Chelator has excellent self-healing ability for the cracks of cement-based materials of different ages.After splitting and preparing cracks in ordinary mortar and mortar mixed with chelator at ages of 3 d,28 d,90 d and 180 d,the relative water permeability coefficients of ordinary mortar cured for 30 d are 16.7%,51.3%,57.1%,and 62.1%,while the relative permeability coefficient of mortar mixed with0.50%chelator after curing for 30 d are 0%,0%,5.3%and 9.3%.The mortar aged7 d and 90 d is pre-damage and then cured to 21 d and 180 d respectively,the compressive strength recovery rate of mortar mixed with 0.50%chelator is 96.9%and 99.5%,while the compressive strength recovery rate of ordinary mortar is89.9%and 82.4%,which proves that the chelator has long-term effect on self-healing ability of cement-based materials.（4）CO₂can promote the chelating crystal self-healing of cement-based material.Mortar mixed with chelator can repair 0.46 mm crack after soaking in water for 28 d in CO₂environment with a volume concentration of 3%,and the relative permeability coefficient and water absorption coefficient of the mortar mixed with chelator are also significantly reduced.This is due to the fact that high concentration of CO₂can provide more carbonate ions,which is conducive to calcium carbonate crystals in cracks.（5）Blast furnace slag（BFS）,fly ash and limestone powder can enhance the chelator crystal self-healing of cement-based material.Among them,the combination of chelator and BFS has the best self-healing effect on mortar cracks.After curing for56 d,the cracks with a width of 0.55 mm in the mortar mixed with chelator and BFS can be completely closed,the chloride ion diffusion coefficient is 73.7%lower than that of ordinary mortar,and the recovery rate of compressive strength is 63.7%higher than that of ordinary mortar.SEM observation shows that chelator and BFS can promote the formation of a large number of C-S-H and calcium carbonate crystals in cracks.（6）Self-healing concrete with chelator and BFS exhibits good corrosion resistance and self-healing ability in Na₂SO₄solution,Na Cl solution and Na₂SO₄+Na Cl solution.In saline environment,the porosity of ordinary concrete will increase and the compressive strength will decrease,the self-healing concrete can effectively reduce the influence of salt erosion on the pore structure and mechanical properties.After semi-immersion in sulfate solution for 120 d,the total porosity of self-healing concrete is 13.5%lower than that of ordinary concrete,and the compressive strength is 26.2%higher than that of ordinary concrete.and it can obviously promote the crack healing of concrete.After semi-immersion in sulfate solution for 56 d,the cracks with the width of 0.40～0.50 mm in self-healing concrete are completely closed.SEM-EDS test shows that Ca SO₄,AFt and Ca CO₃will be formed at the crack of self-healing concrete semi-immersion in sulfate solution.