Study On The Modification Of Recycled Aggregate By Microbially Mineral Precipitation And Its Mechanism

With the rapid development of urban construction of our country,construction waste emissions increased year by year,recycled aggregate which prepared by waste concrete is a new type of green building materials,which can reduce construction waste emissions,but also to protect the natural resources.Because the traditional recycled concrete aggregate has many shortcomings,such as granular edges and corners,rough surface and a certain number of hardened cement mortar,which can not be widely spread and applied.Therefore,it is urgent to improve the quality of recycled aggregate.In this paper,the basic properties of recycled aggregate are improved by using microbial mineralization calcium carbonate precipitation technology(MICP)to produce calcium carbonate crystals on the Surface and interior of recycled aggregate,and then can improve the mechanical properties and other properties of recycled mortar.Supported by the National Natural Science Foundation of China(51578342),this paper has carried out the following research works.Firstly,the mineralization effect of aerobic bacteria DSM8715 and DSM6307 was studied,and the optimal strain was DSM8715.The conditions of mineralization activity of DSM8715 in different concentration of bacteria,pH value,temperature,concentration of calcium ion and calcium source was studied.And its optimal mineralization conditions is 108 /mL,10,10g/L,room temperature,calcium hydroxide,accordingly.And the crystal morphology of the newly formed sediment observed by micro scanning electron microscope(SEM)and X-ray diffraction(XRD)was calcite type calcium carbonate,and the content of calcium carbonate in sediment was 86% by thermogravimetric analysis(TGA).Secondly,The text researches the mineralization effects of different types of aggregate such as brick,waste mortar,waste concrete,recycled aggregate,natural stone,found that mineralization efficiency is optimal for waste mortar,followed by recycled aggregate,brick,waste concrete,natural stone.There was no significant change in the concentration of bacteria after soaking,and the calcium carbonate produced by bacterial mineralization can be found on the aggregate surface through the microscopic scanning electron microscope.Compared aggregate basic properties before and after the treatment of microbial mineralization at particle size of 5~10mm and 10~20mm of recycled coarse aggregate,and the recycled fine aggregate size of less than 4.75 mm,the water rate after the treatment of microbial mineralization decreased by 13.3%,10.3%,21.7% respectively,and the apparent density were increased by 1.2%,0.7%,7.1%,respectively,and crushing index of recycled coarse aggregate size at 10-20 mm and particle size less than 4.75 mm of recycled fine aggregate were decreased by 15%,8.7%,respectively.Finally,the change of working properties,mechanical properties and water absorption rate of BRM,RM and M are showed at water cement ratio of 0.35,0.5.BRM is a kind of recycled mortar which prepared by recycled fine aggregate under MICP technology.RM is another kind of recycled mortar which prepared by untreated recycled fine aggregate.M is a common mortar which prepared by standard sand.As a whole,the properties of recycled mortar is much less than that of ordinary mortar due to the shortcoming of the recycled aggregate.but the properties of the BRM is improved after the microbial mineralization.In the working properties,when the water cement ratio is 0.35,the consistency and mobility of BRM increase by 7.69% and 1.5% compared with RM,respectively.when the water cement ratio is 0.5,compared with RM,the BRM consistency and mobility increase by 2.86% and 2.22%,respectively.In mechanical properties,when the water cement ratio is 0.35,the compressive strength of BRM at 3d,7d,14 d,,28 d,56d increase by 14.8%,3.6%,6.9%,6.8%,11.4% compared with RM,respectively.When the water cement ratio is 0.5,the compressive strength of BRM increase by 14.7%,21.8%,0.75%,10.8%,5.4% respectively compared with RM,respectively.In terms of water absorption,compared with RM,The water absorption of BRM decrease by 48% at water cement ratio of 0.35.When the water cement ratio was 0.5,the water absorption of BRM decrease by 15% compared with RM.Through the micro scanning electron microscope,BRM was found calcium carbonate crystal internally which was produced by the mineralization of bacteria.