| Cement-based materials are the most widely used construction materials in the world.Their production and use have a huge impact on resources,energy demand and environment.At the same time,the high-performance and multi-functional cement-based materials have become an effective approach to achieve the sustainable development due to the large(huge)and complicated engineering infrastructures,the extremes of the service environment,multi-factor coupling and the continuous expansion of the application fields.As the leading technology of the new industrial revolution,nanotechnology has penetrated into the field of civil engineering and provided new impetus for developing the high performance and multi-functional cement-based materials.In this thesis,nano-core effect in cement-based materials with nano fillers was proposed.Based on nano-core effect,the electrostatic self-assembled nano fillers were exployed to overcome the dispersion issues caused by the thermodynamic agglomeration of nano-fillers in cement-based materials.And then the influence and underlaying mechanisms of different electrostatic self-assembly unit of the electrostatic self-assembled nano fillers on the mechanical,electrical,piezoresistive,electromagnetic wave shielding and absorption properties of cement-based materials were investigated.Specially,the percolation characteristics,equivalent circuit models and piezoresistive model of cement-based materials with electrostatic self-assembled fillers were studied.The main research contents and results and conclusions of this thesis are as follows:(1)In order to understand modification mechanisms of nano fillers to the properties of cement-based materials and thereby control the performance of cement-based materials with nano fillers,the phase,microstructure,hydration properties,and macroscopic properties of cement-based materials with different nano-filler were analyzed,and then nano-core effect was proposed to describe the modification mechanisms of nano fillers to the performances of cement-based materials.Core effect is based on the physical and chemical properties of the nano fillers and includes adsorption effect(participation in hydration reactions,optimization of hydration product structure,control of hydration process,and adsorption invasive ions),reduction of primary cracks,function effect,filling effect and bonding effect.Nano-core effect is closely related to the intrinsic properties of the nano fillers,the raw materials and mix proportion of the cement-based materials,and the dispersion of the nano fillers.The behaviors of cement-based material with nano fillers are controlled by nano-core effect zone,i.e.,nano-core-shell element.(2)In order to overcome the agglomerate of carbon nanotube(CNT)caused by direct addition and generate synergistic modification of hybrid fillers in cement-based materials,electrostatic self-assembly short and large-diamiter CNT/small size nano carbon black(NCB)composite fillers(S-CNT/CNB)were selected to fabricate multi-functional cement-based materials.The electrical,piezoresistive and electromagnetic wave absorption properties of cement-based materials with S-CNT/NCB were studied.Cement-based materials with S-CNT/NCB have a low percolation threshold,which is 1.52vol.%.This is due to that the S-CNT/NCB fillers are uniformly dispersed in the cement matrix and have synergistic effects of CNT's long-range conduction and NCB's short-range conduction,thus making them easier to form rich conductive networks in the matrix.The established equivalent circuit can accurately describe the AC impedance characteristics of cement-based materials with S-CNT/NCB.The maximum resistivity change rate,stress sensitivity,and strain sensitivity of cement-based materials with S-CNT/NCB are-22.18%,2.69%/MPa,and 704,respectively.The strain sensitivity of cement-based materials with S-CNT/NCB is 352 times of convetional metal strain gauges.The high sensitivity of the composite is due to that the significant change in the conductive networks results from distance change between the assembly fillers and itself under loading.Cement-based materials with S-CNT/NCB have excellent electromagnetic wave absorption performance.The electromagnetic wave reflectivity can reach-23.08dB,and the bandwidth less than-10dB is 1.44GHz.(3)Electrostatic self-assembly long and small-diameter CNT/big size NCB composite fillers(L-CNT/NCB)were used to develop multi-functional cement-based materials.The effect of S-CNT/NCB and L-CNT/NCB on electrical and piezoresistive properties of cement-based materials was particularly compared.In addition,the influence of water content on the piezoresistive property,and the force-electric constitutive relations of cement-based materials with L-CNT/NCB were studied.The volume of L-CNT/NCB electrostatic self-assembly unit is smaller than that of S-CNT/NCB.When the filler content is the same,the number of nano-core-shell units in cement-based materials with L-CNT/NCB is more.Therefore,cement-based materials with L-CNT/NCB have a lower percolation threshold(1.41vol.%)and get the best piezoresistive property with a lower filler content.The piezoresistive property of cement-based materials with L-CNT/NCB decreased significantly with decreasing water content.At lower water content,the contact resistance between the fillers decreased and the distances between the fillers shortened due to the shrinkage of the composite material,which leads to an increase in the conductive paths under no loading.At the same time,the tunneling effect became weaker with lower water content resulting to less conductive paths under loading.The built piezoresistivity model based on the effective medium equation and tunneling theory can effectively describe the force-electricity relationship of cement-based materials with L-CNT/NCB.(4)Electrostatic self-assembled CNT/TiO2 composite fillers composed of micron-sized TiO2 and CNT were used to fabricate multi-functional cement-based materials in order to overcome the adverse effects of NCB in the above two types of electrostatic self-assembled fillers on the workability and mechanical properties of cement-based materials.Their mechanical,electrical,piezoresistive,thermo-sensitive and electromagnetic wave absorption properties were studied.The results show that the cement-based materials with CNT/TiO2 have excellent mechanical properties.28d flexural strength of the composite can reach 9.9MPa,which is 17.9%higher than the control cement-based materials.The percolation threshold zone of cement-based materials with CNT/TiO2 ranges from 0.15vol.%to 0.45vol.%.The maximum fractional change in electrical resistivity,stress sensitivity and strain sensitivity of cement-based materials with CNT/TiO2 under ultimate load can reach-84.09%,1.04%/MPa and 217,respectively.Thermo-sensitive property of cement-based materials with CNT/TiO2 decreased with the increase of filler content.This is due to that the conductivity mechanism changes from ionic conduction to electronic conduction as the filler content increases.Electronic conduction is less affected by temperature compared with ion conduction.Cement-based materials with CNT/TiO2 have good electromagnetic wave absorption performance.The electromagnetic wave reflectivity can reach-32.01 dB and a bandwidth of less than-10dB,and a bandwidth of less than-15dB were 2.16GHz and 0.72GHz,respectively. |
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