Study On Self-healing Properties Of Cementitious Materials Based On Ion Chelating Agents

The inherent self-healing ability of concrete materials can actively repair the cracked position,which provides a new idea for solving the problems about concrete structure performance degradation and life shortening caused by cracks.However,the width of cracks that concrete itself can heal is limited.Therefore,optimization and exploration of self-healing materials are needed.Compared with concrete materials,concrete structures have higher requirements on the quantity,construction technology and cost of self-healing materials.Ionic complexing agents have the potential to improve the self-healing behavior of cement-based materials,which can be directly mixed with cement-based materials with low cost and convenient construction.Ionic complexing agents show excellent application potential for improving the self-healing properties of concrete structures.However,there are many kinds of ionic complexing agents with different complexing properties,which will directly affect their improving effect on self-healing properties.Meanwhile,further exploration of admixtures that can increase the number of healing reactants is needed,based on the limited number of healing reactants and the self-healing reaction of ionic complexes related to the number of available ions.In addition,the optimization of the self-healing properties of concrete materials is to endow the concrete structures with excellent self-healing properties.The main research work and results of this paper are as follows:(1)Effects of ionic complexing agents on hydration and basic properties of cement-based materials.In this paper,four ionic complexing agents are se lected:sodium hexametaphosphate,tetrasodium glutamic acid diacetate,tetrasodium ethylenediaminetetraacetate,and pentasodium diethylenetriaminepentaacetate.The complexing properties of ionic complexing agents were studied by testing the calcium complexing capacity and the stability of calcium complexes.And the NMR test revealed that a large amount of ionic complexing agent in a free state exists in the pore solution,which facilitates the progress of the complexation-substitution reaction.Through the studies of hydration performance,compressive strength,transport performance and pore structure,it was found that ionic complexing agent would hinder the growth of hydration product crystal nucleus,reduce the number of hydration product nucleation,thereby inhibiting early hydration,and reducing the basic performance in the early curing stage(3 d and 7 d).The ionic complexing agent with the larger the calcium complexing capacity and the smaller the stability of the calcium complex is more helpful to improve the hydration and basic performance in the later curing period(28 d and 56 d).Moreover,the effect of calcium complexation capacity on hydration and basic properties is higher than that of calcium complex stability.(2)Effects of ionic complexing agents on self-healing properties of cement-based materials.By studying the mechanical properties recovery,durability recovery and crack self-healing behavior,it is found that the ionic complexing agent is more beneficial to improve the self-healing performance of cement-based materials in the environment containing calcium ions.Ionic complexing agents have long-term effectiveness in improving self-healing behavior of crack.Moreover,the effect of calcium complexation capacity on self-healing performance is more significant than that of calcium complexation stability.The complexation reaction of the ionic complexing agent can proceed spontaneously.And the stability of calcium complex is lower than that of calcium hydroxide,making the lower activation energy required for the formation of calcium carbonate,which is beneficial to promote the formation of calcium carbonate.The smaller the stability of calcium complexes,the lower the activation energy required for the formation of calcium carbonate,while the greater the capacity of calcium complexation will in crease the number of reactants in the substitution reaction,which helps to improve the reaction rate of the formation of calcium carbonate.Therefore,the larger the calcium complexing capacity and the smaller the stability of the calcium complex,the more beneficial the ionic complexing agent is to improve the self-healing performance.(3)Effect of calcium formate on self-healing properties of cement-based materials.It was found by ion chromatography that the addition of calcium formate could significantly increase the concentration of calcium ions in the pore solution.The tensile test,capillary water absorption test and healing behavior of crack revealed that calcium formate could endow the cracked specimen with better self-healing properties.Meanwhile,the study of the microstructure of the self-healing products showed that the number of self-healing products at the cracks of the calcium formate-containing specimen increased,confirming the effectiveness of calcium formate to increase the number of healing reactants.Moreover,the addition of calcium formate promotes a tendency for the sel f-healing product to be more uniformly distributed and denser in microstructure.(4)Self-healing properties of cement-based materials based on ionic complexing agent and calcium formate.Starting from the improvement effect of ionic complexing agent on the healing reaction rate and calcium formate on the number of reactants,the coupling effect of the composite crystalline admixture based on the two and the curing environment on the self-healing performance of cement-based materials was explored.And the superposition effect coefficient Ki was introduced to quantitatively characterize the coupling effect of composite crystalline admixture and curing environment on the self-healing performance.Studies have shown that saturated calcium hydroxide solution and comp osite crystalline admixture have a synergistic effect on the improvement of self-healing behavior compared to water and air.Compared with the specimens containing only ionic complexing agent or calcium formate,the self-healing properties of the specimens containing the composite crystalline admixture were more significantly improved.The composite crystalline admixture promotes the formation of calcium carbonate in the healing process through the complexation-substitution reaction of the ionic complexing agent and the increasing effect of calcium formate on the number of reactants,and its mechanism of action is affected by the ionic conditions of the curing environment.In the presence of calcium ions in the environment,the self-healing performance improvement behavior of the composite crystalline admixture mainly depends on the ionic complexing agent.(5)Self-healing properties of RC/ECC composite beams based on ionic complexing agent and calcium formate.The effects of composite crystalline admixtures based on ionic complexing agents and calcium formate on the self-healing properties of RC/ECC composite beams were explored through accelerated corrosion tests.It is found that the composite crystalline admixture can improve the self-healing performance of concrete structures at the component level,and effectively improve the chloride ion penetration resistance and corrosion resistance of cracked components.Moreover,the evaluation parameter cr ack healing degree was proposed based on the crack healing area,and it was found that it was positively correlated with the self-healing performance of concrete members.The self-healing properties of concrete elements can be improved by adding self-healing materials with excellent crack healing.In addition,the necessity of research on the application of self-healing materials at the component level is revealed by comparing the research results at the material and component level.