Research On Rooted Organic-inorganic Composite Coatings And Its Protective Performance For Concrete In Marine Engineering

With the implementation of major national policies in China such as"Marine Strategy"and"One Belt,One Road",the infrastructure serving in marine gathered.Concrete is the marine structure material which is most widely used in the world.The environmental effects such as seawater immersion,wind wave and tide easily caused the corrosion and damage of concrete structure,and the durability problems were increasingly prominent.One of the most practical method to improve the durability of concrete is to coat protective coatings on the surface with simple operation and low cost.As the limited protective performance of the single coating,the researches on concrete protective coatings in marine were mainly focused on the method of organic-inorganic composite to optimize the performance of the coatings at present.However,no detailed study was made on the bonding between organic polymers and cement-based materials and the dispersion mechanism of nanomaterials in organic polymers.In this paper,a permeable epoxy with high adhesion was prepared,and kaolinite nanosheets were prepared by intercalation and exfoliation using kaolin as the basic material.Based on the materials,a rooted organic-inorganic composite coating was designed and composed which can penetrate into the internal pores of cement-based materials and form a spreading structure on the surface.The designed coatings aimed at solving the damage of the organic part after aging and the worse adhesion between composite coatings and cement-based materials.On this basis,the mechanism of inorganic nanosheets on improving the penetration,adhesion,and barrier protection performance of organic epoxy was revealed.The specific research contents and conclusions of this study can be summarized as follows:（1）With the optimized design of the composition,the epoxy resin was modified with the mixed solution of acetone and furfural carbonyl compound as a diluent,and a permeable epoxy with good workability and adhesion was prepared.The best carbonyl compound content was70%,and the viscosity of the permeable epoxy with the content was 5.6 m Pa·s,the solid content was 86.8%,the penetration rate reached 0.913×10^-4g²/s,and wetting performance was relatively best.The permeable epoxy with low viscosity and high permeability bonded on the surface of the mortar,and also penetrated into the interior of the mortar to fill the pores,the maximum pull-off strength was 4.34 MPa which was 120%stronger than the ordinary epoxy.Compared with other diluents,carbonyl compounds can react with amine curing agents to form schiff bases,thus reducing the volatilization of diluents and promoting the cross-linking and curing of the epoxy.（2）With the optimization of process,the kaolinite nanosheets were prepared by intercalating and exfoliating the kaolinite after acid activation with ball milling and ultrasound,and dimethyl sulfoxide was used as an organic intercalating agent.The kaolinite nanosheets have high surface area and low stacking thickness.The specific surface area was increased from14.03 m²/g to 92.59 m²/g compared with the original kaolinite,and the thickness was reduced from 126.1 nm to 1.19 nm.The inserted organic solvent can buffer the damage of kaolinite structure during the mechanical treatment.The nanosheets with lower intercalation rate are easily damaged by mechanical force in the process of intercalation and exfoliation,while nanosheets with high intercalation rate can maintain the integrity of layer structure.Acidification can increase the precipitation of alumina and reduce the ineraction force between the layers.Ball milling can provide extrusion force and shear force,which effectively inserts the intercalation agent into the interlayer of kaolinite.Ultrasound can provide the energy required for exfoliation and improve the degree of exfoliation after the intercalation.Excessive acid and mechanical treatment will reduce the integrity of the kaolinite nanosheets.（3）Rooted organic-inorganic composite coatings were prepared by the kaolinite nanosheets with optimized properties and the permeable epoxy with optimized ratio,and the effects of nanosheets on improving the basic properties and barrier performance of composite coatings was studied.Compared with the original kaolinite,the nanosheets had good stability and suspension in the permeable epoxy.With less influence on the workability,the nanosheets had better compatibility with the permeable epoxy than the original particles.The uniformity and high temperature stability of the rooted organic-inorganic composite coatings were improved,and the addition of nanosheets can also improve penetration,adhesion and barrier protection performance of the rooted organic-inorganic composite coatings.When the nanosheet dosage was 1%,the pull-off strength of the composite coating was 5.68 MPa which was about 23.6%higher than that of the permeable epoxy without nanosheets,the maximum oxygen transmission rate was 4.438 cc/m² day,the gas and ultraviolet barrier performance of the composite coating were greatly enhanced compared to the pure permeable epoxy.（4）The effect of nanosheets on the penetration and adhesion of the organic epoxy was investigated.The addition of nanosheets enhanced the chemical force between the infiltrated part of the permeable epoxy and cement substrate,and also strengthened the mechanical locking force between the composite coatings and the substrate after curing,thereby improving the penetration and bonding properties of the composite coating.The dispersion of kaolinite nanosheets in the composite coating was statistically analyzed.The barrier protection performance of the composite coating was mainly affected by the volume fraction,the aspect ratio,the degree of exfoliation,and the arrangement angle of the nanosheets in the permeable epoxy.Based on the permeation and transport mechanism of the medium in the composite coatings,the random arrangement model of the nanosheets with the permeation direction of the medium was optimized.The relative permeability of the composite coatings was obtained by combining the statistics of parameters.The predicted trend of the relative permeability of the composite coatings was consistent with the actual test results.The nanosheets with high aspect ratio,completely exfoliated and arranged perpendicular to the medium penetration direction,can effectively reduce the relative permeability of the composite coatings and improve the barrier performance at low volume fractions.（5）The effects of the rooted organic-inorganic composite coatings on the durability of concrete under simulated marine and natural exposure conditions were investigated.The results showed that the concrete structure protected by the composite coatings still had high compressive strength under the long-term action of marine corrosion factors such as sulfate erosion,carbonization,scour simulation under 100 freeze-thaw cycles,and 360-day dry-wet cycles,the depth of carbonation and water absorption after simulated flushing were significantly reduced compared to uncoated blanks.The electric flux of the concrete with composite coatings was lower than that of the pure permeable epoxy resin.After 360 days of natural exposure test,the minimum loss rate of the tensile strength of the composite coating was about 4.87%,which was decreased about 60%than the pure permeable epoxy resin coating.Therefore,the rooted organic-inorganic composite coating prepared in this paper can effectively improve the durability of concrete structures in the marine environment and has great application potential.