Study On The Modification Technology Of Recycled Coarse Aggregate Based On Mineralization Deposition

The use of construction and demolition waste(C&DW)to prepare recycled aggregate can not only effectively alleviate the problem of depletion of natural aggregate sources,but also solve the problem of landfill and environmental pollution.However,the old mortar attached to recycled aggregates and micro-cracks inside leads to higher water absorption and low crushing value.The mechanical properties and durability performance of recycled aggregate concrete has been widely reported to be inferior to those of natural aggregate concrete,which largely limits high-grade application of recycled aggregate concrete in pratical engineering.At present,some researchers have tried to use Microbially Induced Carbonate Precipitation(MICP)to strengthen the recycled aggregates,but there are still problems of by-products threatening the environment,low modification efficiency and long modification time.In view of the above deficiencies,this thesis adopted Microbially Induced Carbonate Precipitation(MICP)method and Enzyme Induced Carbonate Precipitation(EICP)method to modify recycled coarse aggregate,preferred the modification process and investigated the effects of the two preferred modification processes on the performance of recycled aggregate concrete.The main contents and conclusions of this thesis can be summarized as follows:(1)Chloride ions were removed from the microbial modification process by replacing the components of the medium.The influences of aggregate type on the modification effect were investigated.The influences of the introduction method of bacterial solution and precipitation medium on the modification effect were tested.NH-forms by-products were reduced by using permutite.Finally,an efficient microbial modificaditon process was selected.It was found that it is feasible to use chlorine-free medium instead of chlorine-containing medium.The improvement effect of water absorption of three coarse aggregates showed MA>BA>GA.The preferred microibial modification process was I-S by 4 times.By adding permutite to the medium,the NH-forms by-products could be effectively reduced.In addition,the mechanism of MICP modified recycled coarse aggregate was explained in terms of bacterial adhesion,mineralization reaction,crack and pore filling.(2)The influences of aggregate type on the modification effect were investigated.The influences of the introduction method of urease solution and calcium sourceson the modification effect were tested.NH-forms by-products were reduced by using permutite.Finally,an efficient enzyme modificaditon process was selected.It was found that the improvement effect of water absorption of three coarse aggregates showed MA>BA>GA.The preferred enzyme modification process was MS twice.By adding permutite to the mineralization solution,the NH-forms by-products could be significantly reduced.In addition,the mechanism of EICP modified recycled coarse aggregate was explained in terms of mineralization reaction,crack and pore filling.(3)The mineralization deposition modification processes preferred in Chapter 2 and Chapter 3 were used to modified mortar aggregate,red brick aggregate and concrete aggregate respectively.After casting modified aggregates into concrete,the mechanical and durability properties,corrosion of reinforcement inside concrete and microscopic properties were tested.It was found that both modification processes improved the mechanical properties and durability performance of three types of recycled aggregate concrete,and enzyme modification process had a better effect.The resistance to reinforcement corrosion were enhanced after modification.By testing the microhardness values of the interfacial transition zone,it is found that both modification processes significantly improved the mechanical properties of the interfacial transition zone.Finally,the reasons for the improved performance of modified recycled aggregate concrete were explained in terms of aggregate,microscopic properties of concrete and macroscopic properties of concrete.