Characterisation of the pore structure of portland cement-carbon nanotube composites compared with cement pastes

Portland cement is one of the most widely-used construction materials in the world, but it has a poor tensile strength and strain capacity, limiting its utility for a broad range of structural applications. In order to meet the demand for increasingly efficient construction materials that are stronger, more durable, economic and environmentally sustainable, there has been a great deal of recent research interest in promising composites that incorporate nanomaterials, including carbon nanotubes (CNTs), into the cementitious matrix. The majority of research into Portland cement-carbon nanotube composites (OPC-CNT) to date has focussed upon the mechanical reinforcing effect of the CNTs within the cementitious matrix. In some cases, experimental results have exhibited improvements in excess of 50%, but they are highly variable, most likely due to the difficulty in obtaining adequate dispersion of CNTs within the cement matrix. Other important properties of OPC-CNT composites, such as the porosity and pore size distribution, have received little attention in the literature. Our results show that the addition of CNTs to cement can have a significant influence upon the pore size distribution and total pore volume of the composite, particularly in the medium capillary porosity range. However, questions remain, including: by how much does varying the percentage of CNTs alter the porosity; do well-dispersed CNTs alter the pore structure in a different way to poorly-dispersed CNTs within the OPC-CNT; and how does the pore structure of OPC-CNT develop with the cement hydration? This paper presents results from a comparative study of the meso-scale pore size distributions in pure OPC and OPC-CNT, using nitrogen adsorption porosimetry. This research investigates the influences that CNTs have upon the pore size distribution of OPC-CNT composites. The results of this study are being used to support understanding of the mechanical properties and transport/durability properties of OPC-CNT composites.