Spectroscopic Studies of the Influence of Multi-Walled Carbon Nanotubes on the Hydration of Tricalcium Silicate and Microstructure of Calcium Silicate Hydrate Phases

Abstract

Carbon Nanotubes (CNTs) were discovered by Ijima in 1991. They have remarkable properties such as high surface area and flexural strength, which is twenty times higher than steel. Additives with such qualities are required in concrete, mortar and other construction materials to improve the performance of the binder system. For future applications, the knowledge of interactions between hydrating tricalcium silicate (C3S) as a main component of cement and CNTs is very important. Therefore, a better understanding of the hydration mechanism and the nucleation in presence of CNTs is needed. The influence of multiwalled carbon nanotubes (MWCNTs) on the crystallinity of portlandite (calcium hydroxide) during C3S hydration has been observed in previous experiments as well as an influence on the mechanical properties. Flexural strength increases up to 45% with samples including MWCNTs and C3S. This observation has led to the assumption that the carbon nanotubes are incorporated into the micro- and nanostructure of the hydrated clinker phase. The growth of hydration products on MWCNTs is shown by AFM-images. Previous research activities focused on the influence of CNTs on portlandite which is one of the main reaction products in cement hydration. The calcium silicate hydrates (CSH) phases are important for strength development in cementitious binder systems.In this work, we investigated the influence of the CNTs on the CSH by combined spectroscopic methods. Recent 29Si MAS NMR experiments indicate an incorporation of MWCNTs into the CSH-phases by a change of the silica tetrahedra Q n distribution and line broadening as a result of a paramagnetic interaction of CNTs. This shows that the CNTs are homogeneous embedded in the CSH phases. IR measurements support these results. Measurements of the hydration kinetics by calorimetric methods have proved acceleration. The ability of the CNTs to function as nucleation sites is as well a reason for this change in the hydration rate.