Issue 41

X. Wu et alii, Frattura ed Integrità Strutturale, 41 (2017) 388-395; DOI: 10.3221/IGF-ESIS.41.50 391 Property characterization Effects of carbon nano-tubes on the bending strength of cement-based composite materials were investigated by performing a bending strength test with a DKZ-5000 antifracture test machine based on method of testing cements-determination of strength GB/T 17671-1999 which was formulated according to ISO 679:1989 (cement testing method - strength determination) whose main content is completely consistent with ISO 769; but some points of GB/T 17671-1999 were modified according to the actual condition of China, and the comprehensive strength testing results based on GB/T 17671- 1999 are equal to those based on ISO 679:1989. Effects of carbon nano-tubes on the compressive strength of cement-based composite materials were investigated by performing a compressive strength test with a WHY microprocessor controlled fully automatic press machine. The morphology of carbon nano-tubes cement-based composite materials was characterized using a SU-70 thermal field emission scanning electron microscope, and the mechanism of interaction between carbon nano-tubes and cement was initially explored. R ESULT DISCUSSION Mechanical properties he testing results of the mechanical properties of carbon nano-tubes cement-based composite materials after seven days of maintenance are shown in Fig. 3. Fig. 3 suggests that, the bending and compressive strength of carbon nano- tubes cement-based composite materials was significantly improved compared to the blank group, 10% mostly. When the mixing amount of carbon nano-tubes was 0.10 wt% that of cement, the bending strength improved for 25.36%, i.e., 6.96 MPa, and the compressive strength improved for 21.8%, i.e., 40.94 MPa. The variation tendencies of bending and compressive strength of the test specimens were the same, increasing at first and decreasing after reaching the maximum value. It could be concluded that a small amount of carbon nano-tubes could produce a great influence on the mechanical properties of cement-based composite materials, but the changes in the strength were little when the mixing amount of carbon nano-tubes exceeded a certain value, i.e., excessive carbon nano-tubes might reduce the compressive and bending strength instead of improving the strength. Figure 3 : Mechanical properties of cement-based composite materials test specimens which were mixed with different amount of carbon nano-tubes after seven days of maintenance. The testing results of the mechanical properties of carbon nano-tubes cement-based composite materials after 14 days of maintenance are shown in Fig. 4. Fig. 4 demonstrates there were changes in the variation tendencies of the bending and compressive strength of the test specimens; the bending strength fluctuated greatly with the increase of carbon nano-tubes amount, increasing first and then decreasing. The optimal values of the bending and compressive strength of cement-based composite materials were different. When the mixing amount of carbon nano-tubes was 0.10% that of cement, the bending strength reached the maximum value, 8.14 MPa, suggesting an improvement of 15.67%. When the mixing amount of carbon nano-tubes was 0.15%, the compressive strength reached the maximum value, 58.27 MPa, suggesting an improvement of 17.59%. T