Effect of carbon nanotubes on properties of alkali activated slag - A mechanistic study

In this paper, the effects of carbon nanotubes (CNTs) on properties of alkali activated slag (AAS), including rheology, strength and shrinkage were studied and the resulting mechanisms were investigated by microstructure tests and heat of hydration test. In the presence of CNTs, the viscosity of AAS-CNTs composites paste was dramatically increased. At an optimal dosage of CNTs (0.1 wt%), the compressive and flexural strength of the composite increased by 7.18% and 37.69%, respectively, while the shrinkage decreased by 12.31%, compared with that of the control sample. The reinforcement mechanism was then investigated by Isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Though the hydration of AAS was retarded in the first few hours as indicated by the heat of hydration, negligible difference on the hydration products at 28-day was found for samples with and without CNTs. The XPS results further revealed that there was no chemical shift occurred between the samples with and without CNTs. Only minor modification on the calcium aluminosilicate gels by the CNTs was evidenced by the Si-29 NMR measurements. The SEM and MIP results demonstrated that CNTs acted as nano-filler in the composite and reduced the porosity of the matrix up to 14.16%. Based on these findings, it can be suggested that CNTs reinforced the AAS composite primarily by means of their physical effects. (C) 2019 Elsevier Ltd. All rights reserved.