Experimental Study On Self-healing Cementious Materilas Based On Admixtures And Ceramsite’s Built-in Carbonate

Concrete is one of the most widely used building material in the world.Due to its heterogeneity and low tensile strength,cracks are easily occur during concrete structure’s service.The existence of cracks can not only reduces the bearing capacity of structures,but also reduces material’s ability to resist the erosion of harmful substances.The failure of the material or the damage of the structure may happen if cracks are not repaired in time.In order to meet the needs of sustainable development of buildings,“self-healing” appeared on the basis of the traditional repair and maintenance method.This novel concept provides a new way for people to eliminate the durability problem.In this study,experimental investigations were conducted on the self-healing potential of concrete.Calcium sulfoaluminate based expansive agents(CSA),crystalline admixture(CA),and calcium hydrogen phosphate were used as additives of cementitious materials,while porous ceramsites served as the carrier for sodium carbonate solution.The quantification of the widths/areas of the cracks was performed to examine the feasibility of the approach and optimize mix proportions.Gas permeability tests were conducted to investigate the gas permeability change of pre-cracked samples.Subsequently,X-ray diffraction(XRD)and SEM techniques were employed to explore the related healing mechanism.After that,the test of the fluidity,strength,resistivity,resistance to chloride-ion penetration and the polarization resistance of steel were conducted on two preferred mixes and the control mix to investigate to the influence of this self-healing system on the performance of concrete.Finally,the encapsulation technology was used to upgrade the self-healing system.According to the present experimental investigation and the analysis of the experimental data,the following conclusions can be drawn:(1)Two methods were proposed for creating cracks for 40*40*160 mm prism and 50*50 mm circular samples,respectively.Built-in wire-mesh were used in both method.The damage model of samples with different reinforcement forms were analysed by the finite element method,the shape of the built-in wire-mesh was selected based on the result of simulation.The width of cracks made by the two methods is between 0 and 200 μm,which is suitable for the introduction of damage.(2)The impregnation of ceramsite by sodium carbonate combined with different additives was suggested as a new approach to improve the self-healing capacity of concrete.The research activity was aimed at assessing the crack closure and gas permeability,performing a microstructure analysis,and examining both the feasibility and efficiency of the proposed approach.After curing in still water for 28 days,the pre-cracked specimens with ceramsites containing sodium carbonate,and appropriate dosages of additives exhibited considerable efficiency in surface crack closure and healing in gas permeability.The obtained results suggest that mineral admixtures(CSA,CA,and Ca HPO4·2H2O)and the use of ceramsites as a sodium carbonate container are potential methods to design repeatable self-healing concrete.Additionally,the experimental results indicate that the assessment method for characterizing the change in cracks by measuring the crack area are suitable approaches for the self-healing evaluation.(3)The XRD result showed that the crack closure in all the groups is mainly due to the formation of Ca CO3.The SEM results indicated,in II-7 mix(contains both pre-adsorbed ceramsite and additives),the precipitation of calcite was found in deeper positions(at a depth of 10 mm from the crack mouth)when compared to that of the control mix.Based on the experimental results,a healing mechanism was also discussed.(4)After the addition of the self-healing system,the performance of concrete on the fluidity and strength both increased.The addition of the admixture will postpone the occurrence of the lowest resistivity and increase the resistivity of the matrix at the age of 7 days significantly.In the aspect of the permeability,the chloride diffusion coefficient D in the experimental group(II-5 and II-7)is 8.08 and 8.50 respectively,which are both lower than that of the control group(I-0),10.18.It was also found that the activity of the steel bar can be improved by the addition of the self-healing compound system.(5)In order to achieve the encapsulation of ceramsite,a method of coating ceramsite with epoxy resin and separating samples with silica powder was proposed.The quality of this method is evaluated and verified by the thermal stability test and ion diffusion test.The average mass loss of encapsulated samples was 2.73% after 3h in a 60℃ oven.After soaking 336h(14d)in deionized water,the amount of diffused chloride ion is 1.07% of its total content.Finally,in the self-healing test,the self-healing phenomena was still observed in samples with the new selfhealing admixture system.