Mechanical properties and microstructure of multi-walled carbon nanotube-reinforced cementitious composites under the early-age freezing conditions

This research investigated mechanical properties and microstructure of multi-walled carbon nanotube (MWCNT) reinforced cementitious composites. Different cement mixes containing MWCNT-to-cement weight fractions of 0, 0.05, 0.1 and 0.2 wt% were prepared. After demolding, the samples were immediately cured in the deep freezer (−15 °C) for 7 days and then stored in the standard curing room (+20 °C) until the testing age. The strength results showed that the addition of MWCNTs can effectively enhance flexural and compressive strengths. With the incorporation of 0.05 wt% MWCNTs, the early flexural and compressive strength increased by 70.7% and 25.6%, respectively. And the late flexural and compressive strength were also improved by the addition of MWCNTs, with the maximum enhancement reaching 16.9% and 11.6% respectively. Mercury intrusion porosimetry (MIP) results showed that MWCNTs can reduce the total porosity and improve pore size distribution, especially a reduction in the number of mesopores. Thermogravimetric (TG) analysis and scanning electron microscopy (SEM) observations indicated that MWCNTs can accelerate the hydration process through the nucleating effect. Due to these effects, MWCNTs can effectively enhance the mechanical properties of cement pastes suffered from early-age freezing. Moreover, the crack bridging, filling, and bonding between MWCNTs and cement matrix are also observed in SEM images, which have a positive influence on the mechanical properties of cement pastes.