Properties Modification Of Recycled Concrete Aggregate Based On Microbial Mineralization And Its Mechanism

In recent years,with the continuous deepening of China’s new-type urbanization,construction waste has increased accordingly.With the development of urbanization,the demand for aggregate resources is still growing.Therefore,the recycled aggregate made from construction waste has a good application prospect as a sustainable renewable resource.However,due to the irregular particle shape of recycled aggregate and the existence of a large number of old mortar and micro-cracks on its surface,its porosity,water absorption and crushing index are poor compared with natural aggregate,which makes it difficult to be used as a conventional construction resource in large-scale engineering construction.In this paper,microbial mineralized deposition technology was used to modify the recycled aggregate to improve the quality of the recycled aggregate.Firstly,the optimal mineralization reaction conditions of microorganisms in the solution were explored from the aspects of bacterial concentration,temperature,pH value and calcium ion concentration.On this basis,the quality improvement effect of recycled aggregate under different microbial mineralization modification processes（modification processes A,B,C and D）was studied,and the optimal modification process was obtained.Based on water absorption,crushing index and apparent density,the microbial mineralization effect of recycled fine aggregate（04.75mm）and recycled coarse aggregate（5-10 mm,10-20 mm）under the optimal modification process was verified.Secondly,the recycled aggregate modified by microbial mineralization is respectively made into recycled mortar and recycled concrete,and the changes of working performance,mechanical properties and durability of recycled mortar and recycled concrete are studied,and the properties of recycled mortar and concrete made by natural aggregate and recycled aggregate before modification are compared and analyzed.The properties of recycled mortar and recycled concrete modified by microbial mineralization were investigated.Finally,the microscopic properties of recycled aggregate,recycled mortar and concrete after microbial mineralization modification were explored by means of microscopic testing,and the mechanism of microbial mineralization modification of recycled aggregate was revealed.The main conclusions are as follows:（1）When the pH value is 9.5,the bacterial concentration is 108 cells/mL,the calcium concentration is 0.5 mol/L and the temperature is 30℃,the mineralization ability of microorganisms is the best.The recycled aggregate is greatly affected by different modification processes,and modification process D has the best modification effect on the recycled aggregate with three different particle sizes.The water absorption rate of recycled fine aggregate（0-4.75mm）decreased by 25.7%,the crushing index decreased by 4.2%,and the apparent density increased by 2.6%.Water absorption of recycled coarse aggregate（5-10 mm）decreased by 15.5%and apparent density increased by 2.8%.Water absorption of recycled coarse aggregate（10-20 mm）decreased by 20.6%,crushing index decreased by 18.1%,and apparent density increased by 2.5%.（2）Compared with recycled mortar and recycled concrete made of unmodified recycled aggregate,the fluidity of recycled mortar made of recycled aggregate modified by microorganism mineralization and slump of recycled concrete are improved in terms of working performance.In terms of mechanical properties,the 28d compressive strength of recycled mortar and recycled concrete made of recycled aggregate modified by microbial mineralization increased by 7.0%and 10.5%,respectively.In terms of durability,the capillary water absorption coefficient of recycled mortar and recycled concrete made of recycled aggregate modified by microbial mineralization decreases,and the chloride penetration resistance increases.（3）SEM-EDS and XRD analysis results show that dense mineralized deposition products are formed on the surface of the recycled aggregate after microbial mineralization modification,and the deposited products are calcite type calcium carbonate.At the same time,bacterial imprint is found on the surface of calcium carbonate crystal,indicating that microorganisms are involved in the deposition process of calcium carbonate.The results of metallographic microscope showed that the distribution of sediment layer on the surface of the recycled aggregate was continuous and uniform.MIP test results showed that the porosity of the recycled aggregate decreased after microbial mineralization.The results showed that the calcium carbonate formed by microbial mineralization effectively filled the pores and microcracks on the surface of the recycled aggregate,thus reducing the water absorption rate and crushing index of the recycled aggregate,and improving the apparent density.The microstructure of interface transition zone between recycled mortar and concrete is improved after microbial mineralization modification,so as to improve the macroscopic properties of recycled mortar and recycled concrete.