Study On Properties Of Recycled Concrete Reinforced With Nano Metakaolin

With the service life of concrete buildings,a large number of cement-based solid wastes(waste concrete,etc.)will appear in the process of demolition and construction.In order to solve the problem of waste concrete,nano metakaolin was added into recycled aggregate concrete to form nano metakaolin recycled concrete.And to reveal the influence of nano metakaolin on the mechanical and durability of recycled concrete and its mechanism,the mechanical and durability properties of recycled concrete were tested.The cube compressive strength test was carried out on it,and the influence of nano metakaolin replacing ordinary Portland cement on compressive strength of recycled concrete was further studied.The internal microstructure of recycled concrete was observed by scanning electron microscope.BET multi-point method and BJH method were used to test the specific surface area and total pore volume of recycled concrete.The characteristics of pore size distribution were analyzed also.The flexural strength and splitting tensile strength of recycled concrete with nano metakaolin were also tested to explore the influence of nano metakaolin on the compressive strength,flexural strength and splitting tensile strength of recycled concrete,and revealed the relationship between compressive strength,flexural strength and splitting tensile strength.The quick freezing method was used to explore the effect of different nano metakaolin content and different replacement rate of recycled aggregate on the frost resistance.Based on the test and damage mechanics,the damage model of nano metakaolin recycled concrete was proposed and constructed.The conclusions are as follows:(1)When the content of nano metakaolin remains unchanged,the compressive strength,flexural strength and splitting tensile strength of recycled concrete gradually decreased with the increase of aggregate replacement rate.Adding nano metakaolin could improve the cube compressive strength,flexural strength and splitting tensile strength of recycled concrete with different replacement rates.(2)When the replacement rate of aggregate was fixed,the 28 d compressive strength increases with the increase of nano metakaolin content,but the increasing trend becomed slow.When the replacement rate of aggregate was 30% and 50%,and the content of nano metakaolin was 5%,the compressive strength reached the maximum,and the increase rate was 17.0% and 8.0% respectively.With the increased(3)of nano metakaolin content,the compressive strength increased most obviously when the substitution rate is 100%.When the content of nano metakaolin is7%,the 28 d compressive strength increased by 27.1%.(4)When the micropore volume was zero,the total pore volume and specific surface area of the nano-recycled concrete had the same trend,which could be considered as directly related but not linear.There were pores in the recycled concrete with pore sizes of 3.5-4 nm mainly.When nano metakaolin was added to the recycled concrete,the small size effect of it could fill part of the mesopores and macropores and also reduced the total pore volume of the recycled concrete.Moreover,its strong volcano ash effect could participate in the secondary hydration reaction to generate a large number of C-S-H gels,which would enhance the compactness and further enhance the compressive strength.After the proper amount of nano metakaolin was incorporated into the recycled concrete with 30% replacement of recycled aggregate,it could not only fill the pores and cracks in the recycled concrete,but also fill the pores in ordinary concrete.Nanoparticles could fully exert their own special properties,making the strength of recycled concrete optimal and exceed ordinary concrete.(5)When the content of nano metakaolin was 5%,the flexural strength of recycled concrete reaches the peak value when the replacement rate of recycled aggregate was 30%,50% and 100%.Adding appropriate amount of nano metakaolin can improve the flexural strength of recycled concrete and the strength of interface transition zone of recycled concrete.When excessive nano metakaolin is added,the development of strength will be restrained due to agglomeration effect,and the flexural strength will increase negatively.When nano metakaolin is 3% and aggregate replacement rate is 30%,the brittleness of recycled concrete is the smallest and the toughness is the strongest(6)When the replacement rate of recycled aggregate is 30% and 50%,the splitting tensile strength,tension compression ratio and ductility of recycled concrete increase with the increase of nano metakaolin content;when the nano metakaolin content reaches 5%,the ductility reaches the best.When the replacement ratio of aggregate is 30% and the content of nano metakaolin is 5%,the splitting tensile strength and ductility are the highest.(7)When the content of nano metakaolin is 5% and the replacement rate of aggregate is 30% and 50% respectively,the specimens show better frost resistance during the freeze-thaw cycle.Based on damage mechanics,the established cumulative damage power function model had high accuracy and could better reflect the freeze-thaw damage change law of nano recycled concrete during the freeze-thaw process.