| Carbon nanotubes(CNTs)have been considered the super fiber in the future due to their special properties:light, high performance, high heat resistance and high aspect ratio. CNTs can improve cement composite’s mechanical properties and durability, give cement composites thermal and electrical conductivity, electromagnetic shielding and piezoelectric properties and provides a new development space for newly structure and function composite. Therefore, the study of CNTs on the properties of cement composites’microstructure and macro-mechanical, is of great significance to the development of new cement composites.Closely abounding the macro-mechanical properties and micro-structure evolution, this paper was started from the dispersion method of CNTs and the quantitative method for analyzing the dispersion of CNTs in Cement Composite Materials. Hydration products, anhydrated cement particles,ITZS and pore structure were qualitatively and quantitatively characterized and analyzed by art-of-state testing technology on nano/micro scales. Then the relation between microstructure and mechanical properties,such as strength,toughness,wear resistance, elastic modulus,etc, of carbon nanotubes enhanced cement based composites was been established. Furthermore the piezoresistive mechanism of CNTs/cement composite was studied according to its piezoresistive and microstructure properties. The mainly study conclusions and innovation results of this article were list as following five points.(1) Determination of dispersion method and the quantitative method for analyzing the dispersion of CNTsDifferent dispersion methods were studied, then the more effective dispersion system was determined according TEM images:Firstly, the dispersant was added to water, magnetic dispersed for 2-3min, and the dispersant:CNTs ratio of 2:1 was fixed. Then, CNTs were added to the dispersant solution and ultrasonic dispersed for 30min to insure homogeneously. The most effective dispersant used in this study is Polyvinyl pyrrolidone (PVP).(2) Revelation of micro-structure evolution law and mechanism of CNTs/cement compositesFirstly the micro structure morphology of ITZS was qualitatively analyzed by SEM, the pore structure of ITZS was quantitatively characterized by BSEM and image processing software.Then hydration products were analyzed by SEM and spectrum analysis. Furthermore the microporous and macro mesoscopic pore structure were characterized integratedly by MIP and X-CT. The results showed that mineral admixtures, water/cement ratio and the dispersion of CNTs could affect the microstructure of CNTs/cement composites. Well dispersed CNTs could act as a nano-nucleus and improve the microstructure of cement composites.(3) Establishment of the relation between mechanical properties and microstructureFirstly the effects of CNTs on macro mechanical properties, such as strength,toughness,wear resistance, etc., of carbon nanotubes enhanced cement based composites were studied. The results showed that the dispersion of CNTs, water/cement ratio and minal admixtures could affect properties of CNTs/cement composites significantly. The well dispersed CNTs could improve the cement composites’ mechanical properties. The compressive strength of the cement mortar with 0.5% CNTs increased up to 49.4%, while the flexural strength increased up to 40.8%. The CMOD of cement motars with silica fume at CNTs amount of 0.2% and 0.5% was increased by 210% and 230%, respectively. However, the CMOD of cement motars without silica fume at CNTs amount of 0.2% and 0.5% was increased by 29% and 49%, respectively. Furthermore, the wear value of cement motars with silica fume decreased from 9.0 kg/m2 to 3.1 kg/m2, while the wear value of cement motars without silica fume decreased from 10.1 kg/m2 to 4.5kg/m2 with an increase of CNTs addition from 0.0% to 0.2%, respectively. However, compared to the excellent properties of CNTs, actual enhanced effects appears not ideal enough. The mainly possible were list as following tow points. First the addition of CNTs is lower or the agglomeration of CNTs in the cement mortars with a higher addition. Secondly the enhancement effects at nanoscale could not be expressed at macroscale.Then the relation between microstructure and mechanical properties of carbon nanotubes enhanced cement based composites was been established.Furthermore, combination nano-fibers with micro-fibers need to be further investigated to solve the cracking problem of cement composites.(4) Revelation of the mechanical behavior of CNTs/cement at the nanoscaleThe nano-mechanical properties of L C-S-H, H C-S-H, CH, etc., were studied by nano-indentation mechanical testing system. The results showed that CNTs changed the hydration products, filled in the nanometer pore of C-S-H, reduced the porosity of cement paste, improved the microstructure of C-S-H and increased the elastic modulus and indentation hardness of C-S-H. The enhancement mechanism was further explained at the nanoscale.(5) Analysis of piezoresistive evolution law and mechanism of CNTs/cement compositesFirstly the effects of CNTs on the volume resistance of cement composites were researched. Then the change of resistivity was discussed under recycle load. Furthermore the microstructure and piezoresistive properties were studied. The experimental results showed that CNTs dispersed well in the matrix, the microstructure of composites was improved and had no obvious defects with an increase of CNTs addition from 0.0% to 0.5%. The spacing between CNTs was bigger in matrix and macroscopic tunnel conductive effect plays a dominant role. When the content of CNTs was increasing,part of CNTs overlapped with each other and formed a conductive path, then the percolation conductive effect began to play a leading role, and the piezoresistive property of composites reduced. Furthermore, more defects appeared in the matrix and macro mechanical properties degradated due to the aggregation of CNTs when the content of CNTs was too high.The CNTs/cement composites had better piezoresistive characteristics when CNTs distributed in the cement matrix uniformly. |
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