Research On Physical And Mechanical Properties Of Construction And Demolition Waste Recycled Aggregates Modified By Microbial Mineralization Technology

The demolition,reconstruction and expansion of infrastructure have produced a huge amount of construction and demolition waste(CDW).One of its main reuse methods is to make recycled aggregate,while many micro-cracks and pores exist in recycled aggregate,which leads to worse physical and mechanical properties and lower product performance than natural aggregate.This paper mainly studies the main influencing factors of microbial mineralization modification reaction,discusses the method of modification,evaluates the changes of physical and mechanical properties,and explores the application of modified recycled aggregate in subgrade filler.The main conclusions of this paper are as follows:(1)The optimal reaction conditions and chemical reaction parameters of urease were determined.Urease activity is positively correlated with the number of bacteria and temperature.When pH value is 8,urease activity reaches the maximum.The increase of urease activity can improve the rate and efficiency of microbial mineralization reaction.The increase of substrate concentration can improve the rate of microbial mineralization reaction,while too high concentration will reduce the reaction efficiency.Calcium acetate aqueous solution is alkaline,and thus it can improve the microbial mineralization reaction rate by about 10%.However,considering the high price of calcium acetate,calcium chloride was finally selected as the calcium source.(2)Based on the optimum reaction conditions,the recycled aggregate was modified by soaking method and spraying method respectively.The water absorption was used to evaluate the best modification method,and the changes of physical and mechanical parameters such as apparent density,water absorption characteristics,bonding characteristics,hardness,shear strength and crushing value were studied.The modification effect of soaking method is the best when the substrate concentration is 0.5 mol/L,while the modification effect of spraying method gets the peak when the substrate concentration is 1.0 mol/L.Comparing the best modification effect of the two methods,the spraying method is better than the soaking method under each gradation.With the increase of spraying times,the apparent density and internal friction angle of recycled aggregate increases gradually,and the water absorption,abscission rate and crush value decrease continuously.The microbial mineralization modification has a slight increase in the hardness of the parts with the Leeb hardness less than 300 HL.(3)Through mercury intrusion porosimetry(MIP),BET test,scanning electron microscope(SEM)and X-ray diffraction(XRD)test,the changes of pore characteristics,microstructure and mineral composition before and after modification were studied,and the modification mechanism was discussed.MIP and BET results show that the pore size of coarse aggregate and medium aggregate is mainly concentrated in 10-200 nm,and that of fine aggregate is mainly concentrated in 4-100 nm.The small pore volume was greatly reduced after modification,indicating that the pores were filled and clogged.SEM test shows that the modified calcite fills the pores and cracks on the recycled aggregate and wraps the fine particles on the surface,so as to improve its physical and mechanical properties.XRD test shows that the content of calcium carbonate increases obviously after the modification of recycled aggregate,and the main crystalline form of calcium carbonate is calcite.(4)Based on the above research,the kinetic model of microbial mineralization modified recycled aggregate was finally established considering the effects of pore filling and clogging,and the model was verified combined with the experimental results.The established microbial mineralization kinetic model can calculate the pore distribution characteristics and porosity changes of recycled aggregate under different modification methods and treatment times.The comparison between the calculation results and the experimental data shows that the model has good accuracy.(5)The modified recycled aggregate was applied to the subgrade filler to study the variation law of the bearing ratio of recycled aggregate under different mix proportions.Because the single grade aggregate is easy to be broken in the heavy compaction test,the final measured California bearing ratio(CBR)is smaller than that of the composite formed by one grade recycled aggregate and soil.When the dry weight proportions of coarse aggregate,medium aggregate,fine aggregate and soil are 28.00%,28.00%,14.00%and 30.00%respectively,the CBR of the composite formed by the three-grade recycled aggregate and soil is the largest,which is 101.81%.After modification,the bearing ratio is 123.40%with an increase ratio of about 21.20%.