AbstractThe mechanical performance of cementitious materials can be improved by incorporating carbon nanotubes (CNTs). Nevertheless, some challenges must be overcome, such as reducing the agglomeration trend of CNTs and minimizing their negative effect on the fresh state of the cementitious matrices. In this study, early-age portland cement (PC) hydration and the rheological behavior of cementitious composites with nonsilanized, 3-aminopropyltriethoxysilane (APTES)-functionalized CNTs were investigated. The APTES-functionalized CNTs were incorporated into PC pastes in contents of 0.05% and 0.1% by cement weight. The rotational rheometry of the CNT cementitious composites was carried out during the first hour of hydration. The effect of CNT silanization on cement hydration was assessed using isothermal calorimetry and in situ X-ray diffraction (XRD) for 48 h. The functionalization of CNTs with APTES offset the negative effect of nonsilanized CNTs on the fresh properties of the cementitious matrix, reducing the dynamic yield stress up to 39%. Calorimetry and in situ XRD results revealed that nonsilanized CNTs hindered early cement hydration, while the APTES-treated CNTs allowed proper hydration. At 28 days, the XRD and thermogravimetric analysis (TGA) results indicated that nonsilanized and silane-treated CNTs showed an equivalent hydration degree compared to the plain cement paste. Overall, CNT silanization improved the early age performance of cement pastes, enhanced the fresh properties of cement pastes, and did not affect the early hydration of PC.