This paper aims to investigate the freeze-thaw resistance of cement-paste containing with different diameter and content of multiwalled carbon nanotubes (MWCNTs) in terms of mechanical properties and microstructure. Three groups of cement-paste with MWCNTs diameter of 10-20 nm, 20-40 nm and 40-60 nm were prepared by incorporating 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt% MWCNTs by cement weight. Experimental results showed that the addition of 0.1 wt% MWCNTs with diameters of 10–20 nm effectively improved the freeze-thaw resistance of cement-paste. In addition, microstructural analysis of these cement-paste specimens showed that MWCNTs addition increased the materials’ initial porosity, but a proper amount of MWCNTs decreased the porosity of cement-paste after freeze-thaw cycles and effectively improved the pore size distribution. MWCNTs 10–20 nm in diameter were found to be the optimal size for improving the microstructure of these MWCNTs cement-paste
