Microstructure and durability of Portland cement-carbon nanotube composites

The incorporation of carbon nanotubes (CNTs), fibres with diameters less than 100 nanometres that exhibit a tensile strength in excess of ten times greater than steel, into Portland cement (OPC) is a relatively novel, yet promising, development for next-generation construction materials exhibiting enhanced strength and ductility, even multifunctionality. When added to Portland cement, creating a Portland cement-CNT nanocomposite material (OPC-CNT), CNTs promote the nucleation of the principal cement hydration product calcium silicate hydrate during the hydration process, altering the composition of the material and a strong CNT-matrix bond. Mechanical performance improvements to OPC-CNT over OPC are impressive, with reported average increases of 45%, 35%, 43% and 27% to elastic modulus, compressive, flexural and tensile strengths respectively, with the addition of as little as 0.05% CNTs to the mix (by mass of OPC powder). However, the results are highly variable between studies, principally due to the formation of poorly-dispersed CNT agglomerations, arising from challenges in dispersing individual CNTs within OPC paste. The effects of these CNT nucleation and dispersion phenomena upon the microstructural and pore network characteristics of OPC-CNT are poorly understood. This research contributes to the development of OPC-CNT by addressing this paucity of understanding on the microstructure and fluid transport properties of the nanocomposite, features that are important for the durability of cementitious materials. Additionally, a CNT dispersion technique to fabricate OPC-CNT pastes, employing a cement superplasticiser, together with occupational health and safety protocols for handling CNTs and OPC-CNT, were developed as a part of this research. Experimental characterisation of the effects of CNT dose (up to 0.25% by weight of OPC powder), dispersion method (with or without added superplasticiser) and dispersion quality (well- and poorly-dispersed CNTs) upon the microstructure and associated capillary pore network of OPC-CNT was conducted using 2D and 3D microscopy techniques, in conjunction with porosimetry. Further, the effects of CNTs upon the early-age hydration behaviour of the material was monitored using isothermal calorimetry, and the fluid transport behaviour of OPC-CNT was assessed using water sorptivity, chloride diffusivity and water permeability tests. Findings demonstrate that the introduction of CNTs significantly alters the composition and spatial distribution of hydration products and pores within the nanocomposite material, increasing the proportion of higher-density hydration products, and forming a more discontinuous pore network. These effects are largely correlated with CNT dose, and contribute to significant improvements to the water sorptivity resistance of the nanocomposite material. The results further show that, unlike for mechanical performance, poorly- and well-dispersed CNTs have a similar beneficial impact upon the microstructure of OPC-CNT, forming a discontinuous pore network, improving the measured fluid transport resistance of the nanocomposite. The results of this research, together with the processes developed to fabricate and handle OPC-CNT materials, provide a valuable contribution in further development of CNT-reinforced cement composite materials.