Mechanical and rheological properties of graphene oxide reinforced cementitious composites

As a major component of concrete, Portland cement has become the most consumed manmade substance. The massive production process of Portland cement not only causes depletion of natural resources but also raises severe environmental issues. One of the best approaches for reducing the consumption of cement is to improve its mechanical properties, so that it can be used more efficiently. Graphene oxide (GO) is an emerging nano-scale reinforcement with a large surface area, two-dimensional geometry and high mechanical properties. Although its superior physical reinforcing capability is well established in ceramic matrix and polymeric matrix, only very few research studies have been done on the effect of GO in reinforcing cementitious matrix. In this thesis, we investigate the use of GO to reinforce cement paste from nano-scale. Mechanical test results show that the addition of small proportions of GO leads to a significant enhancement of mechanical performance. In particular, 0.02 wt% GO with an average size of 520 nm increases the compressive strength by 46.3%, tensile strength by 53.3% and fracture toughness by 21.9%. In order to understand the reinforcing mechanisms, the influences of GO on hydration and pore structure are studied. By tracing the evolution of the rheological properties of fresh GO-cement system using various rheology tests, it is found that the addition of GO greatly shortens the induction period and the time approaching initial set. The degree of hydration of the GO-cement composite was studied using a thermogravimetric analysis (TGA), which shows that GO additives greatly enhance the degree of hydration of cement paste. The pore structure of the GO-cement systems are investigated using mercury intrusion porosimetry (MIP) tests, which show that the pore structure of cement paste is greatly refined and densified with addition of GO. These test results indicate that the high reinforcing effect was attributed to two reinforcing mechanisms: (1) GO is a well-established nano-size reinforcement, so it can physically reinforce the initial water-filled space in cement paste, and (2) it accelerates the hydration process and improves the degree of hydration, which leads to refinement and densification ofthe initial water filled space. A full exploration of the reinforcing potential of GO is, however, hindered due to deterioration of workability accompanied with the incorporation of GO. It is found that even a small proportion of GO additives significantly decreases the mini-slump diameter and increases the yield stress and viscosity of cement paste. The degraded workability may be attributed to (1) the absorption of water on the surface of GO sheets that leads to reduction of free water available for the lubrication of cement particles, and (2) the formation of GO intermesh. Future researchers in this area are recommended to solve the workability issue first. Given the fact that GO can be readily produced from naturally abundant graphite, the potential of large scale application of GO in cement and concrete is vast. There will be more research works required to fully understand the behaviours of the GO-cement composites and the GO-concrete composites if the use of GO is finally expanded to concrete.Awards: Vice-Chancellor’s Commendation for Masters Thesis Excellence in 2013