Study On The Effect Of Metakaolin On Mechanical Properties And Damage Mechanism Of Recycled Concrete

With the rapid development of urbanization in China,a large number of waste concrete buildings have been demolished and rebuilt,resulting in hundreds of millions of tons of construction waste every year.The disposal of construction waste in stacking and landfill seriously pollutes the environment and causes a certain degree of waste of resources.In addition,the production of concrete will consume a large amount of natural aggregate,and the greenhouse gases emitted during cement burning seriously affect the air quality.In this paper,recycled coarse aggregate(RCA)is used to replace natural coarse aggregate(NCA)to produce recycled coarse aggregate concrete(RAC)to alleviate the environmental problems caused by demolition of buildings and reduce the dependence on natural aggregates.In order to improve the inherent low compressive strength and elastic modulus of RAC and reduce the amount of cement,metakaolin(MK)was used to replace part of the cement to prepare RAC.Uniaxial compression tests were carried out to explore the effects of MK replacement rate,curing age,sulfate dry-wet cycle and dynamic strain rate on the mechanical properties of RAC.Using nuclear magnetic resonance,acoustic emission,scanning electron microscopy and X-ray diffraction and other testing methods,the effect of MK on the microstructure of RAC under the above-mentioned different influencing factors was explored.The statistical damage constitutive model was used to investigate the mesoscopic damage evolution law of RAC during uniaxial compression,and to establish the relationship with the macroscopic mechanical properties.The main contents of this research work are as follows:(1)The uniaxial compression test of RAC with different MK substitution rates was carried out,and the optimal substitution rate of MK based on this test condition was 15%.At this time,the peak stress and elastic modulus of RAC were higher than other substitution rates.MK improved the mechanical properties of RAC through the filling effect and pozzolanic reaction,and strengthened the structurally weak parts of RAC.(2)Compared with the MK substitution rate of 0% and 15% respectively,uniaxial compression tests with different curing age,sulfate dry-wet cycle and dynamic strain rate were carried out.Research shows that: The peak stress and elastic modulus of RAC gradually increased with the extension of curing age,and the peak strain showed a decreasing trend.Under the action of sulfate dry-wet cycle,the mechanical properties of RAC increased at first and then decreased,and the specimen structure gradually appeared expansion cracking.RAC shows obvious strain rate effect,the strain rate increases and the mechanical properties improve.On the basis of the specimen without MK,RAC showed more excellent mechanical properties after adding MK.Through the mesoscopic test,it was found that MK improved the microstructure of the specimen,reduced the structural porosity,and inhibited the formation of calcium hydroxide and expansive products.The acoustic emission test effectively evaluates the energy release of the specimen in the failure process,and the peak stage of the ring count lags behind the peak stress and is located in the descending section of the stress-strain curve.(3)Based on the statistical damage theory,the microscopic damage evolution mechanism of RAC under uniaxial compression under different curing ages,sulfate dry-wet cycles and dynamic strain rates was revealed.The entire deformation and failure process of the specimen was divided into two stages: uniform damage and local damage.Two mesoscopic damage modes,fracture and yield,were considered,and the peak state and critical state are distinguished.The characteristic parameters show regular changes.The whole deformation and failure process were understood from the point of view of effective stress,and the mesoscopic mechanism that the peak stage of acoustic emission lags behind the peak value of stress-strain curve was explained.