Abstract
Cement pastes and mortars based on ordinary Portland cement containing nitrogen-doped multiwalled carbon nanotubes (MWCNT-Nx) or oxygen-functionalized multiwalled carbon nanotubes (MWCNT-Ox) are investigated. To incorporate MWCNTs into the cementitious matrix, the as-produced carpets are dispersed over periods of 1 and 2 hours in distilled water at pH levels of 1 and 7. The cement pastes are prepared by adding 0.1 wt% of MWCNTs to cement powder, followed by characterization with SEM and X-ray diffraction (XRD) at an early age (first hours of hydration). The mortars are mechanically characterized during the hydration process for a period of 28 days. SEM characterization of cement pastes revealed that the carbon nanotubes are well incorporated in the cementitious matrix, with the hydrated cement grains interconnected by long carbon nanotubes. XRD characterizations demonstrated that, during the hydration of cement pastes, different peaks emerged that were associated with ettringite, hydrated calcium silicate, and calcium hydroxide, among other structures. Results of the compressive strength measurements for mortars simultaneously mixed with MWCNT-Nx and MWCNT-Ox reached an increment of approximately 30% in compressive strength. In addition, density functional theory calculations were performed in nitrogen-doped and oxygen-functionalized carbon nanotubes interacting with a cement grain.
Highlights
The potential ability of carbon nanotubes (CNTs) to improve the mechanical properties of hydraulic concrete opens a novel route for the generation of new building materials [1,2,3,4,5,6,7,8,9,10]
The dark spots inside the MWCNTNx correspond to Fe particles, which are responsible for nanotube formation during the chemical vapor deposition process [22, 23]
The effect of mixing carbon nanotubes looks like an algebraic adding of each separated effect of each nanotube type; this could be explored with different quantity ratio and different type of nanostructures in order to understand the overlap effect. This improvement with respect to the other mortars is clearly appreciated for pH = 1 (see Figures 12(a) and 12(b)). These results suggest that multiwalled carbon nanotubes (MWCNTs)-Ox and MWCNT-Nx interact via nitrogen doping and the different functional groups attached to their surfaces, promoting the formation of networks based on interconnected carbon nanotubes; these new structures could play a crucial role in the improvement of the compressive strength
Summary
The potential ability of carbon nanotubes (CNTs) to improve the mechanical properties of hydraulic concrete opens a novel route for the generation of new building materials [1,2,3,4,5,6,7,8,9,10]. The concrete that is currently used is a heavy material exhibiting an average mass density of 2.3 tons/m3 with a time of hardness of approximately 28 days. The improvement of concrete materials presents several challenges, such as (a) an increase of the mechanical properties ( compression resistance and tension resistance and higher elasticity modulus), (b) a reduction of the volumetric mass density, and (c) a reduction of the hydration time. Different nanostructures (nanoparticles, carbon nanotubes, among others) have been incorporated into concrete, demonstrating the possibility of improving its mechanical properties [18,19,20,21,22]. The present study considers mixtures of nitrogen-doped or oxygen-functionalized carbon nanotubes with ordinary Portland cement and investigates the texture changes and mechanical properties of cement pastes during the hydration process
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