Issue 41

X. Wu et alii, Frattura ed Integrità Strutturale, 41 (2017) 388-395; DOI: 10.3221/IGF-ESIS.41.50 393 nano-tubes was 0.05% was the best (72.44MPa), suggesting an improvement of 13.01% after 28 days of maintenance. The improvement of the compressive and bending strength was obvious in the early stage, which indicated carbon nano-tubes could effectively improve the strength of cement in the early stage. With the changes of carbon nano-tubes amount, the compressive and bending strength of the test specimens showed similar tendencies, i.e., increasing first and then decreasing, but the fluctuation of the bending strength was more obvious. According to the results after 7, 14 and 21 days of maintenance, the improvement of the compressive and bending strength was better when the mixing amount of carbon nano-tubes was 0.10%. Figure 6 : The variation of the bending strength of carbon nano-tubes cement-based composite materials test specimens along with the change of age. Figure 7 : The variation of the compressive strength of carbon nano-tubes cement-based composite materials test specimens along with the change of age. Analysis using scanning electron microscope To further verify the mechanism of carbon nano-tubes in enhancing the mechanical properties of cement-based composite materials, the cross surface of the product was analyzed using a scanning electron microscope (SEM); the results are shown in Fig. 8. As shown in Fig. 8, carbon nano-tubes play a bridging role in the cement base. Because of the bridging effect, carbon nano-tubes could bear certain external force and consume external stress damages, which could help inhibiting the further growth of cracks and enhance the strength and toughness of the composite materials. With the growth of crack size or the increase of tensile stress, carbon nano-tubes could inhibit the expansion of cracks and shoulder most of external force. If local fracture occurred, a long and thin carbon tube would form at the fracture site of the cement material, and