Issue 45

P. Jinchang et alii, Frattura ed Integrità Strutturale, 45 (2018) 156-163; DOI: 10.3221/IGF-ESIS.45.13 157 and workability [7, 8]. The cement composites have a noticeable feature of relatively high compressive strength and low tensile and flexural strength, which belong to brittle materials. New carbon materials such as carbon fibers and carbon nanotubes were used to enhance the strength of cement composites or provide the cement composites with improved thermal performance. Nevertheless, the reinforcing materials such as carbon fibers and carbon nanotubes only play a physical role in the cement composites and take no participation in the hydration and microstructural modification of the cement, especially the pore structure and crystalline structure of cement paste. The dispersion of carbon fibers and carbon nanotubes in the cement matrix is also challenging because of the hydrophobic surface of these reinforcing materials [9-11]. Therefore, it is urgent to find a new material which cannot only disperse uniformly in the aqueous system of hydrated cement, but also improve the toughness of hardened cement paste by microstructural modifications. Graphene oxide is an intermediate product in graphene preparation, which has many advantages as a reinforcing material such as excellent mechanical, electrical and thermal properties [12-14]. It is easy to disperse uniformly in cements, which is beneficial to the reinforcing effect. But the study on modifying traditional cement with graphene oxide is just getting started. Some scholars have investigated the modification of cement based composite material with graphene oxide. Cao et al. [9] added the modified graphene into cement and found that the addition of graphene could produce promotion and template effects on the formation of hydrate crystal product to improve the strength and tenacity of cement-based materials. Babak et al. [15] studied the mechanical properties of graphene oxide reinforced cement based composite material and found that the tensile strength had an improvement of 48% when the weight proportion of graphene oxide was 1.5%. In the study of Liang et al. [16], graphite oxide was dispersed to water via ultrasound and blended with polyvinyl alcohol solution. Then polyvinyl alcohol/graphene oxide composite was prepared by volatilizing solvent via solution casting. When 0.7wt% of graphene oxide was added, the tensile strength and Young's modulus of the composite was improved76% and 62% respectively compared to pure polyving alcohol, and moreover the thermal decomposition of the composite was also improved. In this study, graphene oxide was prepared using oxidation reduction to study the effects of different mixing amount of graphene oxide and water cement ratio on the mechanical properties (bending and compressive strength) and microstructure of cement based composite material and analyze the variation rules. This work aims to figure out the influencing mechanism of the mixing amount of graphene oxide and water cement ratio on graphene oxide reinforced cement based composite material to lay a basis for the application of graphene oxide in cement based material. E XPERIMENTAL SCHEME Experimental materials aterials included graphite powder (granularity ≤ 30 μm, Sinopharm Chemical Reagent Co., Ltd., China), concentrated sulfuric acid (H 2 SO 4 , 98% mass fraction), potassium permanganate (KMnO 4 , mass fraction ≥ 99.5%), concentrated phosphoric acid (HPO 3 , mass fraction ≥ 85%), hydrogen peroxide (H 2 O 2 , 30% mass fraction), polycarboxylate superplasticizer, ordinary portland cement P.O.42.5 and ISO standard sand. Preparation of graphene oxide 1 g of graphite powder and 46 mL of concentrated sulfuric acid were added into a conical flask. The temperature was kept below 3 °C. Then 6 g of KMnO 4 was slowly added during stirring for 2 h of reaction at 15 °C and 10 h (20 h/30 h/40 h) of reaction at 35 °C; the reactant turned to be green. Then 100 mL of deionized water was slowly added for 30 min of reaction at 80 °C. After the addition of 15 mL of 30% hydrogen peroxide, the reactant turned to be golden yellow; the reaction continued for 30 min. Next centrifugation and washing with deionized water were performed until there was no  2 4 so in the washing liquid. The value of pH was adjusted to 7.0. It was processed by ultrasonic wave under 500 W for 30 min to obtain graphene oxide dispersion liquid after the addition of polycarboxylate superplasticizer. Its mass fraction was controlled at 0.2%. Representation of graphene oxide The X-Ray Diffraction (XRD) of graphite and graphite oxide were detected using D/max 2200 PC X-ray diffractometer (Rigaku, Japn). Cu Kα was X-ray source. The test was performed under 40 kV and 40 mA, and the angle was between 2° and 70°. IRPrestige-21 Fourier transform infrared spectrometer was used to represent the molecular structure and functional group of graphene oxide and identify the category of the functional group. Specimens used for Fourier Transform Infrared Spectroscopy (FTIR) was dried using a dryer and ground into powder. KBr pellet pressing method was used. M