Issue 45

P. Jinchang et alii, Frattura ed Integrità Strutturale, 45 (2018) 156-163; DOI: 10.3221/IGF-ESIS.45.13 161 test specimens. But when the water-cement ratio increased from 0.45 to 0.5, the strength of the test specimens which were not mixed with graphene oxide significantly declined, and the bending and compressive strength changed more gently. When the water-cement ratio was 0.5, the growth rates of the bending and compressive strength of the test specimens which were not mixed with graphene oxide were maximum, 73.09% and 72.35% respectively. On one hand, residual water evaporated during setting and hardening of cement paste, leading to the generation of large pores; on the other hand, graphene oxide was prone to fill in the micro-pores of the cement paste to improve the structure under a high water cement ratio. But graphene oxide as a nanometer material cannot fill in the large pores left after evaporation of water; therefore, the overall decrease tendency of the strength did not change with the variation of the water cement ratio. Effects of graphene oxide on the microstructure of graphene oxide reinforced cement-based composite material Fig. 5 exhibits the SEM images of the test specimens under different mixing amount of graphene oxide (0, 0.01%, 0.03% and 0.05%). The calcium silicate hydrate (CSH) gel and calcium hydroxide (CH) grew in the test specimens which were not mixed with graphene oxide. There were many pores in the hardened cement paste, and the insufficient ettringite inside the pores led to loose connection. But a large amount of ettringite scattering on the surface failed to play the roles of filling and connection, which was not beneficial to the structure of cement mortar. With the addition of graphene oxide, tiny CSH gels in a size of 1 μm distributed on the surface of cement particles like a roll leaf (Fig. 5(b)). When the content of graphene oxide was 0.03%, the structure of the hardened cement paste was significantly improved, no isolated hydration products were observed, and CSH structure in the shape of roll leaf which became more even and compact covered on the surface of other crystals and cement particles (Fig. 5(c)). When the content of graphene oxide was 0.05%, a compact and even structure which was composed of various hydrates was observed, but large pores and cracks appeared in the surrounding and flocculent CSH gel scattered on the surface (Fig. 5(d)). (a) The content of graphene oxide was 0. (b) The content of graphene oxide was 0.01%. (c) The content of graphene oxide was 0.03%. (d) The content of graphene oxide was 0.05%. Figure 5 : The test specimens under different mixing amount of graphene oxide under scanning electron microscope.