Effect Of Different Nanomaterials On The Properties Of Recycled Concrete And Its Interfacial Transition Zone

The recycled aggregates prepared by the production of waste concrete have high crushing index and water absorption due to the adherence of old mortar,and there existing a large number of complex interfacial structures in recycled concrete,resulting in serious deterioration of the long-term performance of recycled concrete in terms of mechanical properties,durability and volume stability.At present,a lot of researches has been carried out to improve the performance of the interface transition zone of recycled concrete,but most of them take advantage of the macroscopic performance indexes of recycled aggregates and concrete to define the improvement effect,while little research has been reported on the quantitative improvement of microstructural performance of the interface transition zone by improvement methods.Therefore,it is necessary to combine the macroscopic performance of recycled concrete with the potential law of microstructural evolution of the interfacial transition zone,propose a new approach to accurately define the interfacial transition zone,and reveal the mechanism of the improvement method for the performance improvement of the interfacial transition zone of recycled concrete.Supported by the National Natural Science Foundation of China “Microstructure Regulation and Performance Evolution of High Performance Recycled Concrete(52078261)”,varieties of nanomaterials are firstly selected for the study,and the use of nano-silica is finally clarified as the modifying material by the dispersion and incorporation effects.The relationship between the input energy and the distribution of nano-silica particles clarifies the energy distribution law of ultrasonic dispersion and the optimum dispersion method of nano-materials is determined;based on the iconography analysis,the physical phase division and statistics of the interfacial transition zone are realized,and the width and pore distribution of the interfacial transition zone are quantified and analyzed according to the statistical method.By studying the changes in the composition,mechanical properties and microstructural characteristics of the interfacial transition zone of recycled concrete under carbonation,chloride ion attack and sulfate attack,the mechanism of nano-silica on the structural improvement and durability enhancement of the interfacial transition zone is clarified;based on the optimal dispersion conditions and dosing content,the preparation of recycled concrete modified with nano-silica(NS-RAC)was carried out.The mainconclusions of this paper are as follows.(1)The dispersion effect of nano silica in cement paste is the best and the performance improvement of cement mortar is the most obvious under the same adding method.Increasing the ultrasonic dispersion power can improve the nanosilica dispersion effect,but over long dispersion time leads to the re-agglomeration of nanomaterial particles,and according to the relationship between ultrasonic input energy and dispersion temperature and nanoparticle size distribution,there is no proportional relationship between the distribution of ultrasonic input energy.Meanwhile,increasing the dispersion temperature can reduce the agglomeration degree of nano-silica particles in the dispersion solution and effectively promote the hydration reaction of cement,but it is more prone to agglomerate after adding into the cement paste solution,which has a negative impact on the hardening performance of mortar;while decreasing the dispersion temperature can effectively alleviate the agglomeration of nano-silica particles in the cement solution,and enhance the mechanical properties of mortar significantly.Through the comprehensive analysis,the dispersion degree of nano-silica particles in cement paste solution is the best when the dispersion temperaturereaches about 10℃,and it has the optimum effect on the improvement of workability and mechanical properties of mortar.(2)Using image analysis technology,the phase type and distribution in mortar matrix at different distances from the boundary was statistically analyzed,and the width range of interface transition zone of recycled concrete was accurately defined by combining the phase variation coefficient with microhardness test technology.The width and hardness of interface transition zone were both improved by adding the NS to different degrees,and the improvement degree of aggregate-new paste interface was the greatest.At the curing age of 28 days,the width of interface transition zone between aggregate and old mortar with 1%,2% and 3% nano-silica content decreased by about 5μm,10 μm and 15 μm respectively,and the microhardness increased by about 3MPa,10 MPa and 13 MPa,respectively,compared to the control group.The incorporation of NS effectively reduced the proportion of harmful pores in the interfacial transition zone and refined the pore size,and this improvement effect becomes more and more obvious with the increase of the adding amount and curing age.By comparative analysis of the improvement effects of the pore content,mechanical properties,and width of the concrete interface transition zone,it is determined that the optimal content of nano silica is about 2% of the total mass of the cementitious material..(3)Combining the performance changes of NS-RAC under different environmental effects,the microscopic analysis techniques such as microhardness and SEM were used to analyze the microstructural damage laws of carbonation,sulfate attack and chloride ion attack on the concrete interface transition zone,and to objectively evaluate the degradation degree of environmental effects on the performance of multiple interface transition zone of NS-RAC through the changes of physical phase composition,hardness value and pore content.The effect of carbonation can reduce the pore content to a certain extent and improve the structural compactness of the interface transition zone of recycled concrete,and the pozzolanic effect and microfilling effect of nano-silica can further enhance this effect;sulfate erosion and chloride salt erosion both have the greatest damage to the performance of the new interface transition zone of the aggregate,and the incorporation of nano-silica effectively reduces the damage degree to the properties of the interface transition zone due to salt erosion,such as mechanical properties and pore content,thus delaying the deterioration ratio of the properties of the interface transition zone caused by corrosive salt erosion.(4)Based on the optimal dispersion conditions of ultrasonic waves and the improvement effect of microstructural properties of recycled concrete,the preparation of nano-materials modified OPC-RAC was carried out using well-dispersed nano-silica.The workability,mechanical properties and durability of nano-silica modified OPC-RAC were analyzed and found that the recycled concrete with 100% recycled aggregate replacement at 2% nano-silica content still obtained same-level mechanical and durability performance comparable to that of natural aggregate concrete.A linear optimization method was used to fit the relationship between the compressive strength and the amount of cementitious materials of nano-silica modified OPC-RAC and the cement-water ratio to accurately calculate the reference mix proportion ratio of recycled concrete.The results showed that the amount of cement can be effectively reduced by the incorporation of nano-silica at the same strength level,but it causes a sharp increase in the cost of concrete construction.(5)Based on the optimal dispersion conditions of nanomaterials,different kinds of nanomaterials were used to modify the properties of HBSAC-based materials.From the analysis of performance tests such as cement setting time and mortar strength,it was found that NT had the best effect on the performance improvement of HBSAC-based materials.Based on the quantitative analysis method of the interfacial transition zone in the previous work,the performance evolution law of different doping amounts of NT on the width of the HBSAC-RAC interfacial transition zone and the distribution of the multi-harmful pore content was studied,and the optimal doping amount of NT was determined.Based on the optimum admixture amount and admixture effect,the nano-modified HBSAC-RAC was prepared using NT.The analysis of the mechanical properties and key durability performance of concrete concluded that HBSAC-RAC with no more than 50% substitution of recycled aggregates at 2% NT admixture still obtained better durability performance than natural aggregate concrete.