Self-healing Of Cracks Based On Aggressive-ion-bonding Agent In Cement-based Materials In Sea Water

Concrete is a brittle material that easily cracks during construction and service processes.Cracks in concrete can accelerate invasion of aggressive ions in marine environments.As a result,cracks can extremely shorten the service life of concrete structures.It would be useful for concrete to have a self-healing capacity so that cracks can be healed autonomously once they appear.In order to significantly improve the durability of concrete structures in a marine environment by self-healing of cracks,it is necessary to promote the crack closure and simultaneously weaken the damage of aggressive ions in cracks from sea water.In this study,novel self-healing agents that can bind with aggressive ions in cracks and simultaneously form expanded reaction products to heal the cracks rapidly was developed.The promotion and the mechanism of self-healing of cracks by the developed self-healing agents were explored.The specific research contents and results in this thesis are decribed as follow:?1?Thermodynamic modeling was performed to design the compositions of the novel self-healing agent.It is well known that Cl^-,SO₄^2-,and Mg²⁺in concrete can be chemically bound in Friedel's salt,ettringite,brucite and hydrotalcite,repectively.From a thermodynamic point of view,if adequate concentrations of Ca²⁺,Al?OH?₄^-and OH^-are present in the crack solution,Friedel's salt,ettringite,brucite and hydrotalcite can be formed,which means that chemical binding of Cl^-,SO₄^2-,and Mg²⁺in the crack can take place.Simultaneously,the crack can be filled by these precipitates.When no aggressive ions precents in crack,transition products?Ca O?_x·?Al₂O₃?_y·?H₂O?_zwill be formed.By adjusting the chemical compositions of self-healing agent,the efficiency on aggressive-ions binding can be optimized.According to the thermodynamic results,aggressive ions in sea water can be chemically bound by Ca O-metakaolin agent and Ca O-Na Al O₂agent.Dosage of self-healing agents and its Ca/Al molar ratio have significant effects on the mineralogy of the phase assemblages.The removal of Cl^-in sea water after mixed with self-healing agents firstly increases and then decreases with Ca/Al,while the removal efficiency of Mg²⁺and SO₄^2-ions is hardly influenced by Ca/Al and approximates 100%.When the ratio between the mass of self-healing agent to the mass of sea water is 0.18,the removal efficiency of Cl^-in sea water reaches 80%for the Ca O-Na Al O₂agent with a Ca/Al of 5.In comparison,when the ratio between the mass of self-healing agent to the mass of sea water is 0.3,ion removal of Cl^-in sea water reaches 90%for the Ca O-metakaolin agent with a Ca/Al of 3.Besides,by adjusting the composition of self-healing agents,the volume expansion of self-healing agents can be optimized and the p H in crack solution can also increase to delay the destruction of passive film on steel bars.?2?The reaction kinetics of the developed self-healing agent with sea water was explored.When the ratio between the mass of self-healing agent to the mass of sea water is 0.2,for Ca O-Na Al O₂agent,the removal efficiency of Cl^-in sea water reaches 40%in 1 day and increases to 50%in 28 days,while for Ca O-metakaolin agent with a Ca/Al of 5,the removal efficiency of Cl^-in sea water increases slowly in early periods,also reaches 40%but in 7 days,and reaches 50%in 28 days.The removal efficiency of SO₄^2-and Mg²⁺can reach 100%in 12hours,which compared well with the thermodynamic results.It was found that Na Al O₂can rapidly dissolve in water and release Al?OH?₄^-and consequently,precipitates binding aggressive ions in synthetic water formed very fast.For the Ca O-metakaolin agent,portlandite mainly formed within the first 1 day,while the reaction of metakaolin occurred mainly after 3 days,resulting in the formation of Friedel's salt,SO₄-CO₃-AFm salt,hydrotalcite and C-S-H.Therefore,reactions between Ca O-Na Al O₂agent and sea water,particularly the binding efficiency of Cl^-,is faster than that between Ca O-metakaolin agent and sea water.However,the ultimate binding efficiency of Cl^-at 28 days by the agents with different compositions is about 50%?expect for the Ca O-metakaolin agent with a Ca/Al of 3?,which is lower than thermodynamic modeling results by 5?10%.?3?The promotion and mechanism of self-healing in sea water based on the developed self-healing agents weres deeply investigated.To avoid the reactions between the self-healing agents and water during the casting of cement paste,the self-healing agents were compressed as pills and coated with PMMA,called“artificial aggregates”.The surface of artificial aggregates was modified to improve the compatibility between the artificial aggregates and concrete paste so as to ensure the cracks can intersect them.The self-healing agents developed in this study can promote the closure of cracks effectively.For the specimens with 5%?by volume?Ca O-Na Al O₂agent with a Ca/Al of 5,or the Ca O-metakaolin agent with a Ca/Al of5,closure ratio of cracks can reach 80%after self-healing in sea water for 12 hours,about 5times that of the reference.Besides,the closure ratio of cracks in specimens with 5%?by volume?Ca O-Na Al O₂agent with a Ca/Al of 3 is 60%within the first 12 hours of self-healing.For specimens with Ca O-metakaolin agent with a Ca/Al of 3,the crack closure ratio also reaches 45%within 12 hours,which is evidently higher than that of the reference.According to the BSE/EDS results,the self-healing products tend to precipitate nearby the crack mouth and inside the broken artificial aggregates.After the specimens with self-healing agents curred in sea water for self-healing for 1 day,a 250-?m-thick layer of self-healing products was formed nearby the crack mouth.The thickness of the products layer reached 400?m after self-healing for 7 days.After that,the precipitation of self-healing products in the crack mouth slows down significantly.Besides,there is only a product layer less than 100?m thick formed on the crack surface inside the crack and thus crack can not be completely filled.As the precipitation of self-healing products on crack mouth growth thicker,Ca²⁺and OH^-can hardly penetrate through the products layer and Mg²⁺and CO₃^2-are not replenished from sea water outside the cracks.Therefore,the precipitation of self-healing products slows down.Friedel's salt and hydrotalcite were found inside the broken artificial aggregates,which indicates the effective chemical binding of aggressive ions in sea water.However,compared with the reaction products of self-healing agents directly mixed with sea water,the contents of the Friedel's salt and hydrotalcite in self-healing products formed in the broken artificial aggregates are much lower.During the self-healing process,Ca O in self-healing agents can hydrate into portlandite rapidly and provide adequate Ca²⁺and OH^-in the crack solution.Then Ca²⁺and OH^-migrate to the crack mouth.When meeting Mg²⁺and CO₃^2-in sea water,brucite,calcite and aragonite are precipitated nearby the crack mouth.Cl^-and SO₄^2-inside cracks are chemically bound by self-healing agents.The migration of Al?OH?₄^-from artificial aggregates to cracks is much slower than that of other ions.Therefore,the aggressive-ions-binding products are mainly formed inside the broken artificial aggregates.The further ingress of aggressive ions from sea water is hindered by the rapid crack closure due to self-healing.Thus,the contents of the Friedel's salt and hydrotalcite in self-healing products formed in the broken artificial aggregates decrease.