Experimental Study On The Ability Of Anti-carbonation And Chloride Corrosion Of Cement Mortar By CNTs

As a nano-scale fiber material, the carbon nanotube(CNTs) has excellent mechanical and electrical properties. Theoretically, CNTs are highly suitable for producing high performance, multi-functional and smart cement-based composite materials. By now, the effect of CNTs on the properties of cement-based composites has become one of the hot-spots in this research field. A few experimental results have been obtained for CNTs with respect to strengthening and toughening cement-based composite and improving the material's electrical performances. On the one hand, the cement-based composite with nano-fibers can efficiently constrain the crack propagating by the fibers' bridging effect, consume the energy of the crack opening by pulling-out effect, and improve the compactness of composite materials by nanometer filling effect. On the other hand, mixing CNTs into a cement-based material may improve electrical conductivity of the cement-matrix, and make it obtain pressure sensitivity property. However, there are few researchers focusing on the effect of CNTs on the durability of cement-based composites, especially about the carbonation and chlorine attack for the matrix. Whereas, considering the service life and security of the composite, researches on the durability are dispensable.Therefore, through an artificially accelerated carbonation test and a chlorine attack test, this paper studies the effect of CNTs with different additional concentrations on the durability of cement mortar. Firstly, using UV-Vis and SEM investigates the standing time of CNTs dispersed into water. The result found CNTs can be dispersed into the surfactants(PVP) with an ultra-sonic approach, which addresses the dispersion problem for consequent tests. Then, by measuring the compressive strength and bending strength of the CNTs cement mortar, it revealed that samples mixed with CNTs presented the improvement of compressive strength and bending strength, as well as the improvements increase with increasing the addition of CNTs. In the SEM images, CNTs serve as effect of bridging, filling and pulling-out, and those improve the compressive strength and bending strength of the composite.This thesis carried out the accelerated carbonation test. The test results show that the carbonation depth decrease in the same time duration of carbonation through comparing with cement mortar. In the analysis of XRD and MIP, the experiment found that the addition of carbon nanotubes not only can increase the hydration products generation, but also can slightly reduce the porosity of cement mortar and improve the structure of the pore, which can explain the reason that carbonation depth get decrease after the incorporation of CNTs.Meanwhile, the study explored the effect of reinforcement corrosion rate in cement mortar which added in CNTs through the gravimetric method, the linear polarization method, Anode-Ladder method. The test results show that the addition of CNTs accelerate the reinforcement corrosion rate in cement mortar, and which is increasing with the higher content of carbon nanotubes. In the analysis of the specimen resistance test and related research results in abroad, we speculated there are three reasons cause of this result, the first is the addition of CNTs which improve the conductivity of the composite, speed up the chloride ion penetration in the composite material. The second are CNTs, rebar and cement matrix can constitute a galvanic cell which can accelerate the corrosion rate in the reinforced. The last one is that it has the possibility of galvanic corrosion because of existing the potential difference between carbon nanotubes and rebar, and this can also accelerate the corrosion rate of rebar.