Study On The Modification Of Polypropylene Fiber Surface By Microbial-induced Calcite Precipitation

In order to overcome the shortcomings of easy cracking and poor toughness of concrete,fiber-reinforced concrete(FRC)is often prepared by adding fibers to cementitious materials.Among these fibers,Polypropylene(pp)fibers have received extensive attention due to their chemical stability and low cost.However,the poor interfacial bonding property between fiber and matrix remarkably limits the efficiency of pp fiber in the fiber-reinforced cementitious due to its low surface free energy and smooth surface structures.In order to improve the interfacial bonding strength between fiber and matrix,the pp fiber surface needs to be modified to improve the performance of FRC.In this thesis,microbial-induced calcite precipitation(MICP)technology is used for the modification of pp fiber surface.The fiber-matrix interfacial bonding strength was improved by forming a biological Ca CO₃film on the surface of pp fiber,and ultimately leading to the improvement on the performance of FRC.In this thesis,the selected microorganisms used for mineralization,namely urease bacteria and carbonic anhydrase bacteria,were firstly studied,including bacterial activation,bacterial growth and reproduction,strain preservation,,analyzed the growth and reproduction of bacteria and changes of enzyme activity during the cultivation process,and discussed the influence of environment on bacterial growth and reproduction;Finally,through the analysis and comparison of the mineralization mechanism and mineralization products of these two MICP methods,urease bacteria was selected for the subsequent pp fiber modification experiments.To compare with MICP modification,this thesis studied the effects of physical and chemical modification methods on pp fibers,including sanding and acid-base soaking.The study found that scratches were formed on the surface of the pp fiber by the physical modification method,and with the increasement of particle size of the sandpaper,the more and deeper scratches were formed on the pp fiber surface;the corrosion pits were formed on the fiber surface by the chemical modification method,and with the increase of treatment time,the more and deeper corrosion pits were formed on the pp fiber surface.After exploring the process of MICP modification of pp fibers,the rotor rotation method was determined;and the influencing factors of MICP modification of PP fibers were studied,including bacterial concentration,reaction solution concentration,bacterial solution-reaction solution ratio,p H,temperature,speed.The study found that these factors could affect the process of pp fiber modification to a certain extent,but generally the modification time would not exceed 48h.Microstructure analysis revealed that after 24h of MICP modification,a biological Ca CO₃ film was formed on the surface of the pp fiber,with a thickness of about 10-20?m;and the calcium carbonate formed is calcite,with a spherical shape mainly,the crystal size is consistent with the film thickness.Pull-out tests indicated that the peak load and pull-out energy of MICP modified fiber was improved by 103%and 63%,compared with unmodified fiber.The three-point bending tests showed that MICP modification could improve the post-cracking resistance and energy absorption ability of fiber-reinforced mortar by 194%.Finally,the interfacial bonding property of the modified fiber-mortar and the bending toughness of the modified fiber reinforced mortar(MFRM)were studied.The results showed that the interfacial bonding strength of 28d was improved by 149%compared with 7d,and the embedding depth of the fiber had no obvious effect on the interfacial bonding strength;in addition,with the increase of the matrix strength,the interfacial bond strength between fiber and matrix was decreased slightly.The bending tests indicated that,when the age was 28d,the length of the modified fiber increased from10mm to 30mm,the post-cracking resistance and energy absorption energy of MFRM was increased by 568%and 381%,respectively.When the fiber length was 20 mm,the fiber content was increased from 0.5%to 1.5%,the post-cracking resistance and energy absorption ability of MFRM was only increased by 34%and 22%,respectively;when the fiber length is 30mm,the post-cracking resistance and energy absorption ability of MFRM was increased by 303%and 283%,respectively.