Preparation Of Microcapsules Containing Toluene-di-isocyanate And Their Influence On The Self-healing Capability Of Concrete

Cement concrete is widely used in civil engineering because of its abundant raw materials,low cost and convenient forming.However,due to the high brittleness and low tensile strength of concrete,it is highly susceptible to produce cracks during the service process,which leads to the great reduction of its mechanical properties and impermeability,and seriously affects the durability of concrete.The self-healing function of concrete is the main approach to improve its durability.In the existing concrete self-healing technology,the microcapsule self-healing technology has the advantages of strong environmental adaptability and fast healing speed.Nevertheless,most of the existing self-healing microcapsules were made of thermosetting materials with high strength.When the cracks appear in concrete,the shell of the microcapsules is difficult to break.At the same time,the healing agent(core)contained in the microcapsules also had the disadvantages of high viscosity,poor fluidity and the needed to react with the curing agent to repair,which limited their application.Therefore,it is of great significance for the development and application of self-healing concrete to develop a new type of the microcapsule with appropriate strength of the shell,easy to rupture under the stress of the crack tip,good liquidity of the healing agent and water(moisture)as the curing agent.In order to achieve the above purpose,self-healing microcapsules were prepared through melt condensation method using thermoplastic materials(paraffin,PE wax)as shell material and toluene-di-isocyanate(TDI)as core material.The effects of preparation parameters,shell material types and core-shell material ratio on the particle size,morphology,and core fraction of microcapsules were studied.On this basis,the effects of the type and content of microcapsules on the mechanical properties,impermeability,self-healing performance and durability of the mortar and concrete were studied,and the effects of environmental conditions(temperature and humidity)on the damage self-healing and crack self-healing ability of cement-based materials with microcapsules were discussed.The main conclusions are as follows:1.The paraffin coated TDI microcapsules(MC1)were prepared by melt condensation method.The particle size distribution,mechanical properties,sealing property,morphology and composition of the microcapsules were characterized by laser particle size analyzer,nano indentation tester,scanning electron microscope and Fourier transform infrared spectrometer.When the preparation temperature was 75,the agitation rate was 600 rpm,and the paraffin/TDI mass ratio was 1:2,the microcapsules were spherical with smooth surface,the shell thickness was about 1/10of the diameter,the core fraction was 66.5%,the particle size was between 30?m?300?m,the elastic modulus was 0.48 GPA,the hardness was 4.06 MPa,and the weight loss rate was 13.5%in 60 d.In the FTIR of microcapsules,there was an asymmetric stretching vibration peak(2270 cm^-1)of TDI,which proved that TDI has been successfully encapsulated into paraffin.2.Using paraffin,PE wax and nano-Si O₂ as the shell material coated TDI to prepare the paraffin microcapsules(MC2)and nano-Si O₂/paraffin/PE wax composite shell microcapsule(MC3).Compared with MC1,MC2 and MC3 had higher core fraction,larger particle size,better mechanical properties and sealing performance.The particle size distribution of MC3 was mainly concentrated in 400?m?600?m,the core fraction was 72.6%,the elastic modulus was 1.87 GPa,the hardness was61.67 MPa,and the weight loss rate was only 2.6%within 60 d.MC3 was regular spherical with rough surface,and the shell thickness was about 1/20 of the diameter.3.Microcapsules had certain influence on the mechanical properties,impermeability and microstructure of the mortar.Compared with the control sample(AM0),when the content of microcapsule was 3%of the weight cement,the compressive strength of the mortar with paraffin microcapsuels(AM1)increased,the chloride diffusion coefficient and the proportion of pores whose diameter larger than0.1?m decreased.However,the compressive strength of the mortar with paraffin/PE wax composite shell microcapsules(AM2)and the mortar with nano-Si O₂/paraffin/PE wax composite shell microcapsules(AM3)decreased,the chloride diffusion coefficient and the proportion of pores whose diameter larger than0.1?m increased.4.When the preload was 60%f_c0,the compressive strength reserved ratios of AM0,AM1,AM2 and AM3 were 37.1%,77.2%,78.1%and 82.1%respectively after 48h self-healing in the air.The cracks in AM1,AM2 and AM3 with initial width of0.1-0.28 mm,0.1-0.34 mm and 0.4-0.48 mm could be completely self-healed after 4 h.This showed that microcapsules had a good self-healing effect on the internal damage and surface cracks of cement-based materials.5.The environmental conditions(temperature and humidity)had great influence on the self-healing properties of cement-based materials with microcapsules.When the temperature rose from 10?to 50?,the self-healing ability of the mortar contained microcapsules raised with the increase of temperature.However,when the temperature arrived 60?,the shell material of MC1 became soft and hard to break due to the temperature close to the paraffin melting point,and the self-healing ability of AM1 decreased,while that of AM2 and AM3 remained unchanged.The self-healing ability of the mortar containing microcapsules increased with the rising of humidity.6.Compared with control concrete,the mass loss and mechanical properties of the concrete with microcapsules during the freeze-thaw cycle and the dry-wet cycle were significantly reduced,and the self-healing ability was enhanced.After 100freeze-thaw cycles or 150 dry-wet cycles,the proportion of pores whose diameter was larger than 0.1?m of concrete decreased significantly after 7 d of self-healing,and compressive strength reserved ratios and chloride diffusion coefficient recovery rates were higher.SEM observation showed that there were healing products in the pores of the concrete with microcapsules after self-healing,and the compactness had been greatly improved.The experimental results of freeze-thaw cycle and sulfate dry-wet cycle showed that the microcapsules significantly improved the self-healing ability and durability of concrete.