Improving mechanical properties of ordinary Portland cement pastes utilising carbon nanotubes : fabrication, characterization and modelling

Carbon nanotubes (CNTs) are the strongest fibres that have been made, and have been used as reinforcing additives in ordinary Portland cement (OPC) since 2005. One major aim of incorporating CNTs in OPC paste has been to improve the mechanical properties of cement, thereby reducing the consumption of cement in construction and limiting its environmental impact. However, the reported reinforcing effect of CNTs has so far shown large discrepancies due to the limited understanding of the dispersion of CNTs and reinforcing mechanism in OPC paste. This PhD study aims to improve theoretical understanding of the composite and to develop applicable fabrication methods that can produce stable and significant increases in the mechanical properties of CNT-OPC paste. To achieve this aim, three main tasks are defined as: (1) investigating the dispersion and agglomeration of CNTs in water and alkaline environment; (2) studying the reinforcing mechanism of CNTs; (3) optimizing the fabrication of CNT-OPC paste composite. Experimental, theoretical and numerical approaches are adopted to accomplish these tasks. Experimental techniques such as the use of scanning electron, transmission electron and optical microscopy, UV-vis spectrometry, centrifugation and compressive, flexural and fractural tests are employed to investigate the dispersion and reinforcing effects of CNTs as well as to optimize the fabrication protocol. In the theoretical part of this study, molecular mechanics/dynamics (MM/MD) simulation and theoretical development are conducted to develop models to study CNT dispersion and predict the reinforcing effect of CNTs. Regarding task (1), MM simulation suggests that variation in the molecular characteristics of surfactants generates different interaction energies with the CNT surface, thereby altering their packing morphologies on the CNT surface. Surfactants with long chain-like and planar like structures interact more effectively with CNTs compared with short and linear molecules. Surfactant-dispersed CNTs are found to be in a semi-stable state in a calcium based alkaline environment. A theoretical model is developed to simulate the agglomeration of the CNTs in such an environment. This model suggests that CNTs prefer first to form parallel bundles with a few tubes, before growth into large 3D agglomerates occurs. In terms of task (2), the reinforcing mechanism of CNTs is investigated by developing a micro-mechanical crack bridging model. This model is based on length distribution of the CNTs, which is found to follow a log-normal distribution. The length distribution of the CNTs is heavily affected by the ultrasonication process used in the fabrication of the CNT-OPC paste which promotes the dispersion of CNTs while scissoring them into shorter ones. This crack bridging model is used to predict the optimum ultrasonication energy for reinforcing purposes. In task (3), the prediction is further verified by mechanical testing results, where the optimum ultrasonication energy, 75 J/ml, observed in the tests matches the prediction of the model. By using this optimal energy, the increment in fracture energy and flexural strength (notched beam) is doubled compared with low (25J/ml) or very high (400J/ml) ultrasonication energy. The mechanical test results also show that the reinforcing effect is almost proportional to the concentration of the dispersed CNTs, which again matches the model’s assumption. Further experimental investigation of the maximum concentration of dispersed CNTs suggests a limit of 0.264 wt % (in water with cement-compatible surfactants), above which significant agglomeration will occur. After incorporation into fresh OPC paste, ~60 wt % of the CNTs stays dispersed for 4-16 hours and ~35 % is adsorbed by cement grains within 5 minutes. This PhD thesis enhances understanding of CNT-OPC paste and addresses some future research direction of nanoscale particle reinforced cementitious materials.