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Recent years have witnessed a growing research interest in graphene-reinforced alumina matrix composites (Al2O3-G). In this paper, to better achieve the dispersion of graphene in composites, a ball milling method for adding raw materials step by step, called stepwise feeding ball milling, was proposed. The Al2O3-1.0 wt % graphene composites were prepared by this stepwise feeding ball milling and hot pressing. Then, the effects of sintering temperature and sintering pressure on the microstructure and mechanical properties of composites were studied. Results showed that the bending strength, fracture toughness and Vickers hardness of composites increased firstly and then decreased with increasing sintering temperature. The mechanical properties of composites were all at their maximum with the sintering temperature of 1550 °C. For example, the bending strength of composites reached 754.20 MPa, which was much bigger than 478.03 MPa at 1500 °C and 364.01 MPa at 1600 °C. Analysis suggested that the strength of composites was mainly related to the grain size, microflaw size and porosity. [ABSTRACT FROM AUTHOR]

Considering that it is difficult for traditional materials to simultaneously meet the requirements for filling grouting of water-filled karst caves and subsequent shield tunneling, an environmentally friendly and controllable new underwater cementitious filling material (NUC-FM) is developed, with abandoned shield mud as the basic raw material. Through laboratory tests, the mechanical property parameters of NUC-FM are tested, and its micromechanism is analyzed. The research results show that there is excellent synergistic interactions among shield mud, cement, flocculant, fly ash and other raw materials. The NUC-FM grouting filling material with superior performance can be prepared when the water binder ratio is between 0.45 and 0.6 and the water consumption is between 270 and 310 kg/m3. It has the characteristics of non-dispersion underwater and moderate consolidated body strength. The compressive strength of the NUC-FM consolidated body samples under each mix proportion is much higher than 0.5 MPa, which meets the technical strength requirements of a construction site, and the microstructure shows that there is an obvious dense and stable block structure inside. The cost of the NUC-FM prepared with an optimized mix proportion is only 34.57 dollars/m3, which is far lower than the market purchase price of concrete and cement mortar. It can be predicted that the NUC-FM is an ideal filling grouting material for water-filled karst caves in shield tunnels in water-rich karst areas. [ABSTRACT FROM AUTHOR]