The effect of multi-walled carbon nanotubes on the rheological properties and hydration process of cement pastes

The properties of cementitious materials can be noticeably improved by the addition of carbon nanotubes (CNTs). It is important to detect the effects of the pellets containing multi-walled CNTs, dispersed in carboxyl-methyl cellulose, on the hydration process of fresh cement pastes, and physical or mechanical properties of hardened cement paste specimens, obtained without the use of commercially available surfactants and plasticisers, in order to use CNTs in cementitious materials more effectively. The influence of dispersed CNTs in different amounts (from 0 to 0.5%) on the pH and electrical conductivity values of water solutions and fresh cement pastes, as well as on the rheological properties, setting time and exothermic reaction of fresh cement pastes or on the physical and mechanical properties of hardened cement paste specimens was analysed in this work. It can be stated, that 0.005% is the marginal amount of CNTs, as amounts above 0.005% deteriorate the hydration parameters of cement pastes. This was confirmed by decreased values in electrical conductivity (approximately 17%), pH (approximately 6.8%) and dynamic viscosity (approximately 3 times) tests. Lower pH values lead to a significant retardation of the dissolution of cement minerals and this process is reflected in the setting time (prolonged by 110–130%) and maximum time (extended by 148%) or temperature (decreased by 18.4%) of exothermic reaction. Consequently, the mechanical properties of hardened cement paste specimens (after 28 days) decreased about 10%, as well as water absorption increased up to 10.6%