| The relatively low tensile strength leads to the formation of cracks easily in the concrete during the service process.Cracks provide a convenient channel for the corrosive media into the concrete,which seriously affects the durability of concrete structures.Therefore,repair methods of cracks have always been the focus of academics and engineers.In recent years,researchers have been inspired by microbial induced calcium carbonate precipitations in nature and a new bacteria-based self-healing method of concrete cracks has been proposed.The method has rapidly gained the attention of researchers both at home and abroad because of its environmental friendliness and wide application prospects.Based on the microbial mineralization technology,the optimal composition of microbial self-healing agent was determined by the selection of microorganisms and substrates and the microbial self-healing cement-based materials were developed in this paper.The self-healing efficiency of cracks was fully evaluated from macroscopic to microscopic scales and the effects of crack width,cracking time,curing way,microbial self-healing agent dosage and adding method on the self-healing efficiency of cracks were clarified.The effects of microbial self-healing agents on the workability,setting time,hydration heat,strength and durability of cement-based materials were studied.The reasonable and feasible solutions were proposed for the possible adverse effects.The microstructure characteristics of bacteria-based self-healing cement-based materials were made clear by a variety of modern microscopic analysis techniques and the mechanism of cracks self-healing and microstructure formation was revealed by numerical simulation.The main conclusions and innovations of this paper were listed as follows five aspects.(1)Inspired by the unique "vegetative body-spore-vegetative body" life cycle history of bacillus,the dormant spores were prepared and the substrate with good performance and suitable cost was screened.On this basis,the optimal composition of microbial self-healing agent was determined and the microbial self-healing cement-based materials were prepared.The self-healing efficiency of cracks of bacteria-based self-healing cement-based materials was evaluated by the area repair ratio,the resistance to water permeating repair ratio and the resistance to chloride ion permeating repair ratio.The effects of crack width,cracking time,curing way,microbial self-healing agent dosage and adding method on the self-healing efficiency of cracks of bacteria-based self-healing cement-based materials were studied.The results showed that the addition of microbial self-healing agents could significantly improve the self-healing ability of cement-based materials,thus effectively hindering the transmission process of water and chloride ions into the cement-based materials.The cracks were difficult to be healed with the increase of crack width and cracking time.The water curing was the best way for crack self-healing.The effect of microbial self-healing agent dosage on the self-healing efficiency of cracks was related to crack width.When the microbial self-healing agent was added by carrier immobilization,the long-term self-healing effect of cracks was better compared to adding directly.(2)The effects of microbial self-healing agents on the workability,setting time,hydration heat and strength of cement-based materials were systematically studied.The results showed that the addition of microbial self-healing agents RB,JB and NB could improve the fluidity of cement mortars.The addition of microbial self-healing agent RB resulted in the shortening of the initial setting time,the prolongation of the final setting time and the delaying of the cement hydration heat peak.The addition of the self-healing agents JB and NB resulted in shortening of the initial setting time and final setting time and the cement hydration heat peak ahead.The effects of microbial self-healing agents on setting time and hydration heat of cement were mainly caused by the substrate and the effect of microorganism was small.The addition of microbial self-healing agent RB would affect the early strength of cement mortar,but the late strength could reach or exceed the reference.The addition of microbial self-healing agent NB had a certain negative impact on the development of strength.Considering the test results of the workability,setting time,hydration heat and strength,the dosage of microbial self-healing agents RB,JB and NB should be controlled within 2%of cement mass.(3)The effects of microbial self-healing agents RB,JB and NB on the carbonation resistance,chloride ion permeating resistance,steel bars protection and anti-freeze-thaw cycle properties of cement-based materials were systematically studied.The results showed that the carbonization resistance property of uncracked cement mortars with microbial self-healing agents RB and JB was better than the reference group,while the addition of microbial self-healing agent NB had some negative influences on the carbonization resistance property of uncracked cement mortar.After the cracking of the cement mortar,the chloride ion permeating resistance and steel bars protection properties were significantly reduced.However,the cement-based materials with microbial self-healing agents RB,JB and NB had better crack self-healing effect and the chloride ion permeating resistance and steel bars protection properties were improved obviously compared with the reference group when the crack was healed.Meanwhile,the adverse effects of microbial self-healing agent NB on the strength,carbonization resistance and steel bars protection properties of cement mortars were effectively solved by adjusting the water-cement ratio and the addition of defoaming agent.Thereby the optimization design method of self-healing cement-based materials was put forward.(4)X-ray diffraction(XRD),differential scanning calorimetry-thermogravimetric analysis(DSC-TG),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),mercury intrusion porosimetry analysis(MIP)and high resolution X-ray computed tomography(XCT)were adopted to reveal the microstructure characteristics of self-healing cement-based materials matrix and crack position.The results showed that the addition of microbial self-healing agent RB,JB and NB did not affect the composition and morphology of hydration products of cement paste.However,the addition of microbial self-healing agents RB,JB and NB changed the pore structure of the cement-based material and the volume of pores in the range of 100?1000 nm in cement paste with microbial self-healing agents was increased.The closer packed products forming in the crack of specimens with the microbial self-healing agents is calcite with large crystal grains and many printing holes having similar shape with bacteria body on the surface of crystal could be found.High-resolution XCT could be used to characterize the self-healing efficiency of internal cracks.The content of calcium carbonate in the crack gradually reduced with increase of the depth and crack was mainly filled in the depth of 1-2 mm from the surface of crack.(5)The mechanism of microbial induced calcium carbonate precipitations in water solution was proved by studying the role of oxygen,pH value and substrate-in detail.On this basis,the mechanism of crack self-healing and microstructure formation were revealed by focusing on the survival capacity,germination,growth,enzymatic hydrolysis and crystallization characteristics of bacteria in cement-based materials and the transmission process of the oxygen and calcium ions in the crack zone.The results showed that the alkali resistance ability of spores was significantly increased compared with vegetative cell and the spores could survive for about 40 days when directly added to the cement-based materials.Once the cement-based material is cracked,the spores could germinate and grow as the pH value of the crack zone decreased,then exhibit the activity of inducing calcium carbonate precipitations.Meanwhile,oxygen,calcium ions and substrates could be transferred to the crack zone,thereby promoting the deposition process of calcium carbonate.The numerical simulation results showed that the diffusion of oxygen in the cracks was slow and the oxygen concentration in the cracks decreased significantly with the increase of crack depth.Therefore,the microbial induced calcium carbonate precipitations were mainly deposited on the surface of crack due to the high oxygen concentration on the crack surface. |
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