Preparation Of Nano-modified Foam Concrete And Its Stability And Enhancement Mechanism

Foam concrete has been used widely in practical engineering due to its multifunctional properties,such as light weight,thermal insulation,energy and sound absorption,good fluidity,etc.However,the instability of the foam will lead to an uneven pore size distribution of the foam concrete and decrease the strength.Moreover,unstable foam will cause the collapse phenomenon when the density of foam concrete is lower.At the same time,the study of durability of foam concrete used in the subgrade replacement project is limited.Most of the problems of durability are closely related to water transport behavior.Therefore,the study of foam stability and water transfer behavior can make up for the existing deficiencies.It is of great significance for promoting the application of foam concrete and the development of foam concrete industry,as well as guiding practical engineering.In this dissertation,cocoamidopropyl betaine(CAB),nanosilica(NS)and hydroxypropyl methylcellulose(HPMC)were used to prepare a foam with good stability,which is suitable for preparing foam concrete.Base on that,the effects of nano-particle stabilized foams on the strength,stability and pore structure of foam concrete were studied.The corresponding stability mechanisms of modified foams and foam concrete were investigated.In addition,the water transfer behavior,transfer mechamism of foam concrete and improvement measures were studied.The relationship between pore structure and water absorption peoperties had been established based on the results.Finally,the application of nano-enhanced foam concrete in the protective and thermal insulation composite material was explored.The bond strength,dynamic impact strength,and thermal conductivity of the composite were tested.Based on the above experimental results,the following conclusions can be drawn:(1)The NS,HPMC,and CAB have significant synergistic effects in stabilizing the foam.After adding NS and HPMC into the CAB solution,the viscosity of the mixed solution is increased and the strength of foam film is enhanced.Also,NS can block Plateau channels and nodes,thus the stability of preform foam can be significantly improved.At the same time,the bubble size is refined and the distribution become more uniform.The higher content of NS,the effect become more obvious.The cations of the hydrophilic group of the CAB bind to the oxyanion of the NS surface through the Coulomb force,and the hydrophobic group and the HPMC dissolved in the liquid film are combined by van der Waals force,so that allow the three componets to play a synergistical role in the increasement of the foam stability.(2)HPMC/NS stabilized foam can solve the problems of collapse and uneven density of foam concrete.At the same time,its compressive strength and pore structure were better than the control sample.The nano-silica adsorbed on the surface of bubbles can be hydrated with calcium hydroxide.As a result,C-S-H is formed in the cell wall,making the inside of the cell wall become denser.At the same time,Nano-particles coated on the bubler surface would increase the surface area of bubbles contact with the slurry and increase the frictional resistance of the bubbles when moving in the slurry,thus the pore structure is improved and the mechanical properties are enhanced.(3)The sorptivity of foam concrete increases gradually with the increase of density,while the soaking water absorption rate shows the opposite trend.The one-dimensional water absorption is dominant by large capillary pore in the range of 100 nm~10 ?m,while the water soaking depends on large pores in the 10 ?m~3 mm range.Using the technique of X-CT enhanced with CsCl can successfully achieve the visualization of moisture transport through foam concrete,as well as the moisture distribution.The tracking of the change of the macrohole gray value in the two-dimensional image can verify the conclusion that the macro-hole does not participate in the one-dimensional water absorption process but participates in the process of soaking water absorption.The gray histogram of the 2D image obtained by XCT can be used to quantify the water uptake height of the foam concrete at different times.Foam concrete mixed with water-repellent agent can effectively improve its water-absorption performance.According to the results,silicones have the best effect of improving water repellency,followed by methyl cellulose and inorganic salts.(4)The drying shrinkage of foam concrete were much higher than the UHPC,so in the preparation of protective heat-insulating composite materials,foam concrete should be poured first and cured for 7 days,following by the pouring of UHPC.The bond strength in tensile increased with the increase of steel fiber and the density of foam concrete.When the density of FC is lower,the interfacial failure is mainly happened in the part of foam concrete.With the increase of the density,the fracture pattern changed from the failure of foam concrete to the adhesive failure.(5)PP fiber reinforced foam concrete(PFC)can increase the bond strength of composites,and its bond strength in shear was between 2.87-4.40 MPa.It showed an upward trend with the increase of steel fiber.However,it tended to increase first and then decrease with the increase of the density of the PFC.With the increase of the density,the fracture pattern changed from the cohesive failure to the adhesive failure.At the same time,PFC can absorb impact energy and reduce the thermal conductivity of composite effectively.With the increase of thickness and density of PFC,the energy absorption effect becomed more obvious.The thermal conductivity of composites tended to decrease inversely with the thickness of PFC.Also,the thermal conductivity decreased to about 0.338~0.438 W/m·k,which was a decrease of 75.0%~77.8% compare to the UHPC.There are no obvious defects,large holes and cracks at the interface of composites,which is in well-bonded condition.