Self-healing Of Cracks Based On Ions Chelating Agent In Marine Concrete Structures:Efficiency And Mechanism

Concrete structures easily crack during the construction and service processes.In a marine environment,the service life of concrete structure can be significantly shortened since cracks facilitate the ingress of aggressive agents.The durability of concrete structures will be improved if they have the capacity of self-healing.However,it must be emphasized that in most of the reported studies so far,self-healing of cracks proceeded in tap water.The effect of external multi-ions in sea water on self-healing of cracks was not clear.Moreover,in order to significantly improve the durability of concrete structures in a marine environment by self-healing of cracks,it is necessary to promote self-healing of cracks so that cracks can be healed completely in a short time.In this study,self-healing of cracks based on ions chelating agents that can capture ions in sea water to promote the self-healing efficiency was investigated.The cooperation between ions chelating agents and the ions in sea water,and the mechanism of the promotion of self-healing by ions chelating agents were explored experimentally.The specific research contents and results in this thesis are described as follow:?1?The effect of external multi-ions in sea water on the evolution of autogenous self-healing of cracks in cement paste was investigated.It was found that Mg²⁺in sea water,rather than Cl^-and SO₄^2-,plays a primary role on the healing process.The percentage of Mg?OH?₂ in the reaction products of self-healing was more than 40%.Furthermore,wet-dry cycles with a wetting and drying period of 12 hours,respectively,can significantly increase the self-healing efficiency by promoting the formation of Mg?OH?₂ and CaCO₃.However,wet-dry cycles of a wetting period of 1 hour and a drying period of 23 hours will cause a low crack closure ratio.Moreover,it was found that the promotion effect of external ions and wet-dry cycles on self-healing was more obvious in the crack with an initial width of 400?m and 600?m than in the crack with an initial width of 150?m.This is due to the fact that the larger crack width facilitates the entrance of external ions during immersion process and evaporation of water during the drying process in wet-dry cycles.?2?The promotion and mechanism of self-healing in sea water based on ions chelating agents was deeply investigated.Under the condition of immersion in sea water,triethanolamine?TEA?can significantly increase the self-healing efficiency in cement paste.Immersed in sea water for 2 days for self-healing,cracks in a cement paste with 1.5 wt.%TEA?per dry cement weight?can be completely healed.It was also found that the main mineral formed in cracks during the healing process was Mg?OH?₂,accounting for 66%in the reaction products of self-healing.Experimental results showed that alkalinity of a solution can be increased significantly by TEA,and consequently the concentration of OH^-ions increases.As the concentration of Mg²⁺ions adjacent to the crack mouth is high as well,super saturation for deposition of Mg?OH?₂ reaches firstly in this local region.As a result,Mg?OH?₂ is formed on the surface cracks and the cracks can be blocked quickly.As self-healing proceeds,the CO₂ from the air dissolves in sea water,diffuses into the crack,and reacts with Ca²⁺leaching from the bulk matrix to form CaCO₃.As a result,the mass percentage of CaCO₃ in the reaction products of self-healing increases from 14%to 33%from the healing periods of 1day to 28 days.In addition,the addition of TEA in cement paste can decrease the Ca?OH?₂ crystal size and reduce its crystallinity,so that Ca?OH?₂ exposed to the crack surface increases and leaches from the matrix more easily to react with ions in sea water to form Mg?OH?₂.?3?The effect of ion chelating agent on the basic properties of cement-based materials were investigated.The yield stress of cement paste slurry with 1.5%TEA is increased significantly.Thus the fluidity of fresh cement paste was reduced,and the final setting time is prolonged.In cement paste with 1.5%TEA,the threshold pore diameter is increased,and the amount of large pores increases slightly.The strength of mortar with 1.5%TEA is reduced.However,when 0.05-0.1%slow-release superplasticizer is added together into the cement paste,the negative effects of TEA can be effectively mitigated,and the workability and mechanical properties of the cement paste can meet the national standard requirements.