Study On The Mechanical Properties Of Carbon Nanotubes And Carbon Fiber Modified Cement Based Materials

Cementitious composites are the most commonly used materials in construction and building filed.Such composites,however,exhibit extremely brittle failure and low tensile capacity,which are generally regarded as serious shortcomings that not only impose constrain in structural design,but also affect the long term durability of structures.To overcome the aforementioned disadvantages,in present study,Multi-Walled Carbon nanotubes(MWCNTs),Carbon Fiber(CF)and carbon fiber grafted carbon nanotubes(CF-CNTs)were applied to improve the strength and ductility of cementitious materials at micro-and nano-scale,simultaneously.Through optimizing the parameters for dispersion of MWCNTs-OH,alignment of CF and synthesis of CF-CNTs,in current research,CNTs modified cement based materials,aligned CF reinforced cement composites and aligned CF-CNTs modified cement composites were fabricated.By combining the results of macro-measurements(I.E.Visual observation and Macro-mechanical test)and multiple micro-characterization tests(I.E.UV-Vis,SEM,FT-IR,XRD,TGA,Nanoindentation),the dispersing mechanisms of the 5 different kinds of superplasticizers(TNWDIS,APEO,I-C-PCE,?-C-PCE,Silane-PCE)were analyzed.Meanwhile,the micro-structure,interfacial characteristics and reinforcing mechanisms of MWCNTs-OH,CF and CF-CNTs in cement based materials were investigated.Experimental results demonstrate that,APEO shows the best dispersibility among the 5 different kinds of superplasticizers with an optimal concentration of0.2mg/ml.Additionally,in the mechanical study of MWCNTs-OH modified cement paste,the flexural and compressive strengths of the experimental samples were found to improve as the MWCNTs-OH dosages increased when the usage of MWCNTs-OH were below 0.5wt.%;In the nanoindentation measurement,the significance of strict data selection was verified and the results further confirmed that the improvement of mechanical properties of cement by MWCNTs-OH was the results of physical enhancement and hydration promotion effect.It is also found that CF bridged andrestricted the expansion of the internal cracks,which resulted in multiple cracking and reducing the size of crevices,as a consequence,the flexural strength of CF modified cement composites were remarkably improved.Besides,the optimal conditions for the synthesis of CF-CNTs in this study are: CF oxidized via a mixture with a volume ratio of 98% concentrated sulfuric acid and 70% concentrated nitric acid to be 3:1,mass ratio of CF-COOH and 2?m MWCNTs-OH is 5:1,process of combining stirring with ultrasonication.Moreover,MWCNTs-OH that grafted on the surface of CF were found to be capable of significantly modifying the roughness of CF which leads to stronger bonding strength in the Interfacial Transition Zone,(ITZ).In addition,the nano-pore filling and bridging effects of MWCNTs-OH made the ITZ of the composites denser and stronger,thus dramatically improved the load transfer efficiency in the ITZ and the capability of composites to resist the external load.Furthermore,two distinct failure forms of MWCNTs,namely,pulling-out and detachment were observed in the micro-structure of composites.The above mentioned results illustrate that,suitable utilization of CNTs and CF can not only give full play to their excellent mechanical properties,which directly enhance the strength of cement composites,but also contribute to strengthen the weak interfacial bonding between CF and cement matrix,and improve the low utilization rate of CNTs in the cement matrix.