Issue 18

G. Ferro et alii, Frattura ed Integrità Strutturale, 18 (2011) 34-44; DOI: 10.3221/IGF-ESIS.18.04 38 after rupture of the cement matrix, the nanotubes, when visible, are rather clean and quite not recovered by cement hydration products. Property MWCNTs Average diameter [nm] 40-80 Length (average) [  m] 400-1000 Carbon Purity [% w/w] > 92 Metal Oxide (impurity) [% w/w] < 6 Table 1 : Features of the MWCNTs dispersed in the cement Figure 5 : Mortar containing 5 wt% MWCNTs with respect to cement (cylinder is 4 cm in height and has a diameter of 2 cm). Components Reference mortar Mortars + MWCNTs Cement: CEM II A-LL (EN 197/1 standard) Water to cement ratio (w/c): 0.50 Sand: normalized (EN 196-1 standard) Superplasticizer (wt% with respect to cement) 1.1 1.1 2 5 10 VMA (wt% / cement) 0.5 0.5 0.5 0.5 1 MWCNT (wt% / cement) 0 0.5, 0.75, 1, 1.25, 1.5 and 1.75 2 5 10 Table 2 : Mix-design of the prepared samples. According to the mix-design of the different mortars reported in Tab. 2, prisms of 40×40×160 mm 3 having 0.5 wt% of MWCNTs, as well as without carbon nanotubes, were also prepared for flexural tests. The results revealed that the addition of CNTs led to a significant increase of the mechanical strength, whatever the curing time, as reported in Tab. 3.