Influence Of Cement And Supplementary Cementious Materials Properties On The Intrinsic Self-healing Ability Of Cement Based Materials

Cement based materials are the most used civil engineering materials and play an important role in national economic construction.However,cement-based materials are prone to produce micro-cracks.If not repaired in time,these micro-cracks will continue to extend under the action of load and/or environmental factors,which eventually lead to the destruction of the component and even the project.At present,the process of production and application in cement based materials,people tend to pursue high early strength and fast construction speed,it can easily lead to the irrational mineral composition of clinker,excessive grinding of cement particles and the repetition of supplementary cementitious materials.It affects the strength development,durability performance of concrete and weakens the cracks self-healing ability of concrete.A series of physicochemical processes of cement-based materials,such as continuous hydration and carbonation,achieve partial or complete filling of microcracks,which is called intrinsic self-healing.This paper aims to study the effects of cement and supplementary cementitious materials on the intrinsic self-healing of cement based materials,through the measurement of cracks opening width,changes in water permeability,improvement of ultrasonic transmission and compressive strength recovery.The regularity of intrinsic self-healing,the microstructure evolution,the composition and characteristics parameters of raw materials to improve the self-healing of concrete are obtained.The theoretical basis of improving the self-healing ability of cement based materials is provided.It can reduce the cost of concrete maintenance and extend the safe service life of concrete,because of the improvement of self-healing ability of cement-based materials.As a result,the resource consumption and environmental pollution can be reduced.The social benefits,economic benefits and ecological benefits achieve the coordination and unity.Specific conclusions are as follows:（1）The influence of mineral composition of clinker on the intrinsic self-healing was proved.The clinker with different mineral composition was prepared by controlling the three rate of value.With the increase of C₃S,the self-healing ability of cement-based materials is enhanced.Especially,the cracks of 7-day samples are completely closed and the recovery rate of ultrasonic transmission is more than 50%.The silicate minerals are conducive to the improvement of intrinsic self-healing.The self-healing ratios of 7-day samples are as high as70%and the strength recovery rates are more than 34%,while the strength recovery rates of28-day samples are less than 12.5%.The increase of C₃A content promoted the self-healing ability of 7-day samples.When C₃A content is 8%,the recovery rate based on ultrasonic pulse velocity,amplitude and frequency is above 50%.（2）The effect of cement particle size on intrinsic self-healing ability was studied.The recovery rate of ultrasonic waveform and frequency was more than 75%and the recovery rate of compressive strength was more than 40%.The optimal cement particle composition（cement particle content of 30-60μm>20%）improved the self-healing ability of 28-day cracked samples.The recovery rate of ultrasonic waveform and frequency was 60-80%and the recovery rate of compressive strength was about 20%.Gray correlation analysis showed that the cement particles in the range of 9.39-29.24μm were beneficial to the self-healing ability of the 7-day cracked samples,while the cement particles in the range of 29.24-59.46μm had the highest correlation with the self-healing ability of the 28-day cracked samples.（3）The supplementary cementitious materials enhance the intrinsic self-healing potential of cement-based materials.The addition of GGBS improves the potential of continuous hydration and enhances the intrinsic self-healing ability.When the dosage of GGBS is 40wt%of cementitious materious,the time of cracks closing is shortened,the water tightness is improved,the ultrasonic recovery rate is close to 80%and the strength recovery rate of 28-day crack samples is above 35%.The self-healing potential of GGBS is also affected by the particle size.The early strength of cement-based materials was significantly improved when the average particle size of GGBS is 3.48μm.The average particle size of 20.42μm can improve the intrinsic self-healing ability of cement-based materials,and the ultrasonic waveform recovery rate of 28-day cracked samples is more than 50%.The gray correlation analysis shows that the correlation degree between the content of GGBS particles and self-healing efficiency is highest in the range of 4.62-25.37μm,whose correlation coefficient is above0.9.The increase of fly ash content improves the self-healing ability of 28-day samples and the recovery rate of compressive strength is more than 30%.The addition of low calcium fly ash weakens the crack filling ability of the 210-day samples,the crack closure ability and ultrasonic transmission performance are gradually weakened,but the compressive strength recovery ability is slightly enhanced.（4）Six different self-healing environments are designed to simulate the service conditions of some concrete,which is used to verify the important role of liquid water in intrinsic self-healing.The influence of the healing environment on the self-sealing performance of the crack is evaluated by the change of the crack width and water permeability,while the filling effect of the crack is evaluated by the parameters of ultrasonic transmission.The sealing performance and the internal filling performance of cracks are consistent with the change of the healing environment.The self-healing performance from strong to weak is as follows:Static water,wet-dry cycles,humid air with 95%RH,flowing water,air with 60%RH and air with 30%RH.The static liquid water is conducive to the continuous hydration of unhydrated cement particles and the generation of Ca CO₃,which plays a significant role in the self-healing of cement-based materials.It is found that the continuous hydration products are mainly generated at the early stage of self-healing process,while the calcium carbonate plays a leading role in the later stage of self-healing process and covers the continuous hydration products.