Research On Microbial Repairing Agent And Self-healing Concrete With Internal Carbon Source

As the major building material,concrete is still the most used artificial material.However,due to its brittleness and complex service environment,it is often inevitable to crack.Once cracks are formed,they will become a fast channel for water and corrosive substances to invade,which will accelerate the damage and failure of concrete,reduce the durability of concrete,and even cause structural failure.Therefore,repairing the tiny cracks of concrete in time has become the crux to increasing the service lifespan of concrete.The microbial self-healing concrete has attracted much attention due to its green,harmless,and sustainable properties.Nowadays,the microbial components used in concrete are highly air-dependent,relying on oxygen in the air for activation or capturing carbon dioxide from the air to produce carbonate ions.In this paper,a low-dependent microbial repairing agent has been developed to improve the self-healing ability of concrete and enhance the resistance of the repaired crack area to water and aggressive ions.In this paper,we first screened and domesticated a compound microbial repairing agent that can rapidly generate mineralized ions in a facultative environment.After optimizing the formula of the repairing agent,the adaptability of this repairing agent to saturated calcium hydroxide solution and cracks was verified.The repairing agent granules were prepared by forming a core-shell structure of low-alkali cement and repairing agent,and the microbial self-healing concrete was prepared by replacing the sand with the same quality of the granules.Besides,the crack repairing performances were systematically evaluated from macro to micro multi-scale,and the basic properties of concrete were also analyzed.Afterward,the evolution of bacterial flora in the cracks and the corresponding enzymatic hydrolysis mechanisms have been revealed.More importantly,this article clarified the sources of carbonate and calcium ions in the repairing products.Finally,based on the analysis of the characteristics of the repairing products,the repairing mechanism of this type of microbial self-healing concrete was explored from the perspectives of microbial-induced nucleation and biological tackifiers.The main content and research conclusions of this article have been listed as follows:(1)Development and adaptability verification of low air-dependent microbial repairing agentAfter screening and domesticating low dependent microbial flora composed of facultative strain M and strain A;the morphology,reproduction characteristics,and tolerant morph of bacteria M and A were carefully studied.The domesticated microorganism M could grow and proliferate normally in an alkaline environment(12<p H<13).Both strain M and strain A have stress-resistant forms.When the nutrient environment is unfavorable,strain M will form ascospores,and strain A will form spores.The initial inoculation amount of the compound bacterial colony in the crack should reach 10~8 cells/m L,and the optimal ratio of the two bacteria in the compound bacteria was A:M=5～10.The new microbial repairing agent could adapt to the supersaturated calcium hydroxide and the real crack area.The two types of microorganisms are compatible with each other and jointly promote the reproduction of the flora,and the ratio of the two microorganisms in the crack fluctuates around the design value.At the same time,the addition of compound microbial bacteria powder,compound substrate,nitrogen source,biological thickener,and calcium source were determined.There are two types of microbial repairing agents,AM type and AM-G type,according to whether they contained a bio-tackifier or not.This microbial repairing agent exhibited good mineralization ability in a sealed simulated environment and could produce organic-inorganic composite products.The mineralization product of 1g AM-type functional component was about 1.008 g,of which the crystal phase was 0.952 g;the mineralization product of 1g AM-G functional component was about 1.195 g,of which the crystal phase was 0.903 g;the product content and composition were excellent.(2)Research on preparation and performances of microbial self-healing concreteThe assembly of microbial repairing agent was achieved through the core-shell structure,which significantly reduced the negative impact on the concrete.The prepared core-shell granules were mixed into concrete in a way of replacing sand with the same quality,and cracks with a width of about 0.5 mm were prepared by the splitting method,then a dry-wet cycle maintenance was carried out to repair cracks.Compared with other studies,the self-healing performances of concrete have been significantly improved;the area repairing ratio was higher than 95%and the average repairing depths exceeded 30 mm.The repairing products were distributed on the whole section,thus the ultrasonic pulse speed could increase to 3.0 km/s.AM-G type repairing agent could form organic-inorganic composite products with better structure due to the introduction of biological tackifiers,which further improved the resistance of the fractures to water and aggressive ions.Supplementing the substrate in time during the maintenance process could form more products and fill the cracks more densely;supplementing the substrate could also promote the repair of repeated cracks.Compared with ordinary concrete,there were only slight fluctuations in the setting time,apparent density,slump,and expansion of fresh concrete.The microbial repairing agent had no impact on the mechanical properties of hardened concrete.The new interface transition zone between the introduced granules and the concrete would not become a new weakness,and the ability of self-healing concrete to resist chloride ion and sulfate attacks was not impaired.(3)The mechanism of microbial self-healing concreteFirstly,the amount of available functional components of the repairing agent in the fracture area was calculated,and the mechanism of the optimal ratio of two microorganisms in the flora was explained from the perspective of enzyme catalysis.The source of carbonate ions in the fracture zone was clarified through the analysis of the simulation reaction,and the distribution of carbonate in the fracture zone was simulated by the Comsol software.The results showed that the carbonates in the fracture zone originated from the substrate in the repairing agent and carbon dioxide from the air.This type of repairing agent has increased the available carbonate ions in the crack by nearly 30 times.The addition of calcium sources had no adverse effect on the hydration products.During the entire repair process,the number of calcium ions that could be dissolved in the crack area was greater than the amount of carbonate supplied by the microbial repairing agent.Thus,the production rate of carbonate has become the crux of the repairing performances of this microbial self-healing concrete and timely supplementation of nutrients could effectively improve the repairing effect.The distribution of repairing products in the section had the characteristics of multiple aggregation centers,and the simulation results were consistent with the experimental results.Moreover,the phase,morphology,composition,and adhesion of the repairing products fully highlighted the important roles of the flora and the bio-tackifier in the repairing process.The results showed that both negatively charged bacterial surface and extracellular polymer substances(EPS)had nucleating power.Meanwhile,the addition of bio-tackifier could improve the utilization ratio of calcium ions and increase adhesion of repairing products through intermolecular hydrogen bonds.Finally,the bioactivity of the repairing products also verified the ability of concrete to heal itself multiple times.