Study On Strengthening Recycled Coarse Aggregate By Microbial Carbon Fixation

As the largest amount of artificial materials,concrete will not be replaced in the foreseeable future.However,the traditional concrete industry is facing three unprecedented crises:the resource crisis caused by the shortage of natural aggregate,the environmental crisis caused by the accumulation of waste concrete,and the carbon emission crisis caused by cement clinker calcination.For the sustainable development of the construction industry,recycled aggregate prepared from waste concrete has been used in practical engineering.However,the recycled aggregate produced by simple crushing process has the properties of low strength,high water absorption,large dispersion and poor quality,which can not fundamentally solve the problem of shortage of natural aggregate resources for structural concrete,and the waste powder produced in the crushing process is difficult to use due to poor quality.In this paper,the process of preparing recycled coarse aggregate with waste powder was explored,the microbial carbon fixation technology and mechanism of strengthening recycled coarse aggregate were studied,the curing conditions of recycled coarse aggregate were optimized,and the workability,mechanical properties,durability,self-healing performance and interface transition zone characteristics of recycled coarse aggregate concrete were tested and systematically studied.The economic and ecological benefits of the recycled coarse aggregate prepared in this study were analyzed.The main achievements obtained include the following four aspects:(1)The foundational formulation,shaping method and curing conditions of recycled coarse aggregate with waste sand as the core and waste powder and cement as the shell raw materials were obtained:the maximum particle size of waste powder was 300μm,the mass ratio of cement to waste powder was 0.16,and the water-solid ratio was 0.16;The disc granulation method was adopted,and the diameter,inclination angle and rotation speed of the disc were 400 mm,45°and 35 r/min respectively;After granulation 24 hours,the recycled coarse aggregate with particle size of 5 mm～19 mm could be prepared by using 20%carbon dioxide,curing at 70%relative humidity and 20℃for 24 hours,which crushing index,apparent density,water absorption,powder content and ruggedness were 31%,2520 kg/m~3,9.7%,1.2%,and 6.0%respectively.(2)The microbial carbon fixation technology was invented to strengthen the recycled coarse aggregate:1.0%carbon fixation microorganism was added to the shell raw materials,and the activated microorganism liquid was sprayed to in the granulation process.After shaping 24 hours,the concentration of 95%carbon dioxide was used to curing recycled coarse aggregate 24 hours at 70%～80%relative humidity and 20℃.Finally,10%silicone emulsion was used to soak 18 h and then dried.The crushing index,apparent density,water absorption,powder content and ruggedness of recycled coarse aggregate were 11%,2620kg/m~3,2.8%,0.5%and 4.4%respectively,which met the performance requirements of class I recycled coarse aggregate in GB/T 25177-2010.(3)Activated microorganism can promote the dissolution of calcium ions in waste powder,and also was conducive to the growth and germination of carbon fixation microorganism;Carbon fixation microorganism could accelerate the capture of carbon dioxide,and precipitate calcium carbonate from calcium ion mineralization deposition,which was beneficial to dense structure and improve properties of recycled coarse aggregate.Compared with no microorganism,calcite crystals with denser density and more complete crystal growth were formed on the surface of recycled coarse aggregate after adding microorganism.The micro porosity of recycled coarse aggregate was reduced from 20.98%to 13.88%,and the macro porosity was reduced from 1.36%to 0.78%.The production cost of recycled coarse aggregate was about 75.58 yuan/t,and the fixed amount of carbon dioxide could reach 154.28 kg/t,which has significant ecological benefits and social values.(4)Preparation of concrete with recycled coarse aggregate,with the increase of replacement rate of recycled coarse aggregate,the workability and mechanical properties of concrete increased first and then decreased.When the replacement rate of recycled coarse aggregate was 50%,the mechanical properties of recycled coarse aggregate concrete and ordinary concrete of 28 days were 40.5 MPa and 39.7 MPa respectively,and the splitting tensile strength were 3.16 MPa and 3.29 MPa respectively;Recycled coarse aggregate had no obvious effect on concrete durability;Recycled coarse aggregate concrete showed good self-healing performance,the crack area repair rate of 28 days was 83.5%,while the crack area repair rate of ordinary concrete was only 18.1%.The interface transition zone width of recycled coarse aggregate concrete was 41.7μm and Vickers hardness was 62.