Effect of the combined use of carbon nanotubes (CNT) and metakaolin on the properties of cementitious matrices

When incorporated into a cementitious matrix and properly dispersed, carbon nanotubes (CNTs) reduce shrinkage and increase the mechanical strength of the matrix; thus, they attract considerable research attention. However, research on the interaction between CNTs and supplementary cementitious materials (SCMs) is scarce, raising doubts regarding the impact of SCMs (such as metakaolin) on the dispersion of CNTs. The objective of the present study was to evaluate the effects of the combined use of CNTs and metakaolin on hydration (using isothermal calorimetry and X-ray diffraction), rheology (using rotational rheometry and a flow table), and physicomechanical properties of cementitious matrices. The addition of CNTs did not influence the hydration of the cement pastes, whereas the metakaolin accelerated the hydration, and the resumption of the reaction of the aluminates was anticipated. Additionally, the incorporation of CNTs and metakaolin increased the yield stress and plastic viscosity of the cement pastes and reduced the fluidity of the mortars. Finally, the addition of CNTs to mortars without metakaolin improved the physicomechanical properties, and the optimal content of CNTs was verified to be 0.075 wt%, yielding increases of 13.59% and 10.53% in the tensile strength (in flexion) and compressive strength, respectively, at 28 d. However, the addition of CNTs in pastes with metakaolin degraded the mechanical performance, indicating that the metakaolin made it difficult to disperse the CNTs and that the dispersion method used was not effective for these mixtures. (C) 2020 Elsevier Ltd. All rights reserved.