| With the advancement of urbanization,the engineering structure is developing towards the direction of high strength,durability and functionality,which requires to further improve the performance of civil engineering materials.Nano materials have unique physical and chemical properties due to their small size effect,surface effect and quantum tunneling effect,which provide a new way for the development of multifunctional cementitious composites.Nano TiO2(NT)is one kind of nano materials which has excellent mechanical property,electromagnetic shielding/absorption properties,antibacterial property,strong redox property,and acid-base corrosion resistance.It is expected to develop multifunctional cementitious composites with excellent structural properties and functional/intelligent properties by combining cementitious materials with NT.However,the effects and modification mechanisms of different types of NT on the mechanical property,electromagnetic shielding property and electromagnetic absorption property of cementitious composites are still unclear.The dispersion of NT in cementitious composites still needs to be further improved to enhance its composite efficiency.The antibacterial property of NT is expected to reduce the microbial erosion of cementitious composites serviced in sewage and seawater,and has a potential application prospect in improving the corrosion resistance of cementitious composites against sewage and seawater.Therefore,this thesis aims to explore the effect and modification mechanisms of different types of NT on the mechanical property,electromagnetic shielding property,electromagnetic absorption property,anti-sewage erosion property and anti-seawater erosion property of NT filled cementitious composites.The main research contents and conclusions are as follows:(1)The effects of NT without surface treatment on the mechanical property of cementitious composites were studied.The compactness of NT filled cementitious composites was calculated by particle gradation model,and the reinforcing mechanisms of NT without surface treatment on cementitious composites were discussed by microscopic test method.The results prove that:NT can improve the flexural strength,compressive strength and the ratio of flexural strength to compressive strength of cementitious composites.The enhancement effects of rutile crystal phase NT are better than that of anatase crystal phase NT.For NT with the same crystal phase,the smaller the particle size is,the better enhancement effect of NT to the mechanical property of cementitious composites will be.However,when the content of NT exceeds 2.32 vol.%,the agglomeration of NT will affect its enhancement effect on the mechanical property of cementitious composites.The reinforcing mechanisms of NT on cementitious composites mainly include filling effect,cracking resistance effect and self-curing effect.(2)In order to improve the dispersion and composite efficiency of NT in cementitious composites,NT coated with silicon oxide(NSCT)filled cementitious composites were prepared.The mechanical property of NSCT filled cementitious composites were studied,and the reinforcing mechanisms of NSCT on mechanical property of cementitious composites were systematically studied from the aspects of hydration rate,properties of hydration products and microstructure of cementitious composites.The results prove that:The mechanical property of cementitious composites can be improved significantly by NSCT.At 28 days,the flexural strength and the ratio of flexural strength to compressive strength of cementitious composites can be increased by 87.00%/6.69 and 69.56%,respectively.The Ti-O-Si chemical bond on the surface of NSCT makes it negatively charged,which can improve its dispersion performance in cementitious composites through charge repulsion.The thin layer of silicon oxide coated on NT surface can react with calcium hydroxide to increase the binding force of NT and hydration products,thus greatly improving the composite efficiency of NT and cementitious materials.In addition,NSCT can improve the polymerization degree and chain length of[SiO4]4-in C-S-H gel,refine the size of calcium hydroxide,reduce the orientation of calcium hydroxide,and improve the microstructure of cementitious composites.(3)The effects of different types of NT on electromagnetic wave shielding and absorption properties of cementitious composites were studied.The modification mechanisms of NT's crystal phase,particle size,special structure and surface treatment on the electromagnetic wave shielding and absorption properties of cementitious composites were analyzed through the electromagnetic parameters of cementitious composites.The results prove that:NT can increase the proportion of reflection loss in transmission loss,absorption loss and reflection loss,thus improving the electromagnetic wave shielding property of cementitious composites.The effects of NT's crystal phase,particle size,special structure and surface treatment on electromagnetic wave shielding property of cementitious composites are not significant.NT can increase the polarization loss of cementitious composites under the action of electric field,so as to improve the electromagnetic wave absorption property of cementitious composites.The reflectivity of NT filled cementitious composites can reach-29.29 dB,which is 242.97%higher than cementitious composites without NT.The surface treatment can improve the dielectric loss and dispersion of NT,and lead to interfacial polarization and multiple scattering,which is beneficial to improve the electromagnetic wave absorption property of cementitious composites.(4)NT filled cementitious composites were placed into high concentrated sewage,and the sewage corrosion resistance of NT filled cementitious composites was studied from three aspects of physical,biological and chemical erosion resistance against sewage,respectively.The results prove that:Biological acids produced by microbial metabolism in sewage can react with cementitious composites to form ettringite and gypsum,reduce the pH of cementitious composites,decompose the C-S-H gel into thaumasite without gel ability,and reduce the bonding force between cement paste and aggregate,eventually leading to strength degradation,mass loss,apparent density decrease and surface roughness increase.The inhibition rate and killing rate of NT filled cementitious composites to the surface microorganisms can reach 80.93%and 37.35%,which can reduce the erosion of cementitious composites by microorganisms in sewage effectively,and improve the sewage corrosion resistance of cementitious composites.(5)NT filled cementitious composites were placed into Marine environment,and the seawater erosion resistance of NT filled cementitious composites was studied from the three aspects of seawater physical,biological and chemical erosion resistance.The results prove that:The erosion coefficients of seawater to the flexural strength and compressive strength of NT filled cementitious composites are within the range of-0.035?0.046 and-0.012?0.074,respectively.Moreover,NT can induce the conversion of interlayer water and gel water within C-S-H gel,promote the secondary fracture of C-S-H,and enable the cementitious composites to have self-healing ability in seawater.The inhibition and killing rates of NT filled cementitious composites are 96.81%and 76.98%,respectively.The surface biofilm thickness of cementitious composites is reduced by 29.66 m/49.13%due to the addition of NT.In addition,NT can reduce the chloride ion permeability of cementitious composites by 81.00%,promote the decalcification of C-S-H gel,and increase the degree of polymerization and chain length of C-S-H gel.Meanwhile,NT fillers can improve the interfacial transition zone between cement paste and aggregate,refine the internal pore structure of cementitious composites,and eventually improve the seawater corrosion resistance of cementitious composites. |
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