Study On Water Absorption And Action Mechanism Of Reclaimed Aggregate Modified By Microorganisms

Under the background of the rapid development of urbanization,the reconstruction of the old city and the construction of the new city are carried out simultaneously.When the old buildings are demolished,a large amount of construction waste is generated,which brings negative impact to the environment.The recycling of the waste concrete after crushing treatment as the recycled aggregate can effectively reduce the environmental pollution and land waste caused by construction waste piling.Effectively alleviates environmental and economic pressure on local governments.However,some micro-cracks will inevitably occur in the concrete crushing process of recycled aggregate,which leads to the problems of low density,high water absorption and poor interface adhesion with cement-based materials of recycled aggregate compared with natural aggregate.The mechanical properties and durability of recycled concrete are also reduced compared with ordinary concrete.In this study,the reclaimed aggregate was modified by microbial mineralization deposition,and the modified reclaimed aggregate was mixed into reclaimed concrete.The influence of microbial modification on the properties of reclaimed aggregate and reclaimed concrete was studied,and the mechanism of action was revealed by microscopic testing methods such as SEM,EDS,XRD,FT-IR,DSC,X-CT and BET.The main work and conclusions are as follows:（1）Firstly,the optimal carbonization curing method of modified reclaimed aggregate was selected,and the performance of reclaimed aggregate treated by different carbonization methods was compared.The influence of different CO₂ concentration and carbonization pressure on the performance of reclaimed aggregate was studied.The results showed that the water absorption of reclaimed aggregate modified with 20%CO₂ was the best at 25±2℃and a standard atmospheric pressure（101KPa）.Compared with untreated reclaimed aggregate,the water absorption of reclaimed aggregate was reduced by 35.6%,and the weight gain rate and apparent density increased by 0.26%and 2.65%,respectively.When CO₂ concentration is 99.9%and carbonization pressure is 0.1-0.4MPa,the water absorption rate of reclaimed aggregate decreases first and then increases with the increase of carbonization pressure,and reaches the lowest value at 0.3MPa CO₂.In this case,the water absorption rate of reclaimed aggregate decreases by 21.4%and the apparent density increases by 1.89%compared with that of untreated reclaimed aggregate.The weight gain rate was 0.64%.Based on the results of optimal carbonization test conditions,the modification effects of different types and concentrations of calcium sources on reclaimed aggregate were studied.The test results showed that 0.1mol/L calcium acetate solution had the most significant effect on the water absorption performance of reclaimed aggregate.According to the results,the difference of influence of 0.1mol/L calcium acetate solution on the performance of reclaimed aggregate under different carbonation systems was further studied.The results showed that 0.1mol/L calcium acetate solution had the best effect on the water absorption of reclaimed aggregate at25±2℃,a standard atmospheric pressure and 20%CO₂ concentration.Compared with untreated reclaimed aggregate,the water absorption decreased by 39.4%and the apparent density increased by 4.55%.（2）The modification of reclaimed aggregate was studied by microbial mineralization,and the influences of four factors,including different treatment technology,concentration of bacterial solution,carbonization curing system and additional calcium source,on the performance of reclaimed aggregate were compared.The test results show that:The regenerated aggregate was soaked with 3%bacterial solution and 0.1mol/L calcium acetate was added at the same time.After 20%CO₂ carbonization curing,the water absorption of the regenerated aggregate improved best.Compared with the untreated reclaimed aggregate,the water absorption rate decreased from 9.08%to 4.23%,and the apparent density increased by2.65%.Compared with the reclaimed aggregate that only underwent 20%CO₂ carbonization curing,the water absorption rate decreased by 27.7%,and the apparent density had no significant difference.Compared with the natural aggregate,the water absorption rate was3.03%,and the apparent density was 20.8%lower than that of the natural aggregate.（3）Microscopic testing methods such as SEM,EDS,XRD and DSC were used to further reveal the mechanism of water absorption of reclaimed aggregate modified by microorganisms.It has been found that the microbial modification method can effectively improve the microscopic properties of reclaimed aggregate.Microbial modification method can effectively improve the microscopic properties of reclaimed aggregate.Bacillus KW reacts with CO2 to generate microbial mineralized products through its own metabolism,and fills micro-cracks and pores through the mineralized products to improve the surface structural compactness of reclaimed aggregate,improve the pore structure of reclaimed aggregate,and block the access of external water into the cracks.Thus effectively improve the water absorption performance of recycled aggregate.（4）Natural aggregate,untreated recycled aggregate and modified recycled aggregate were used to prepare concrete specimens,and the influences of different aggregates on the working performance,mechanical properties and durability of recycled concrete were studied.The test results show that,compared with the recycled concrete mixed with untreated recycled aggregate,the collapse degree and compressive strength of the reclaimed concrete modified by microorganisms increase by 41mm,the compressive strength increases by 32.0%,the carbonation resistance and freezing resistance are also significantly improved,and the structure of the transition zone of the concrete interface is more compact.