Effect of Carbon Nanofibers on the Hydration of Ultrahigh-Performance Concrete: Experimental Study and Model Development

In this study, the hydration kinetics of ultrahigh-performance concrete (UHPC) with carbon nanofibers (CNFs) are investigated. According to classic nucleation theory, nanomaterials, such as CNFs applied in cementitious materials, can act as nucleating sites for hydration products, accelerating the hydration process. While the increased surface area governs the hydration reaction rate in classic nucleation theory, test results for CNFs in UHPC have provided the opposite trend. Namely, the addition of CNFs caused retardation of the hydration reaction. This observation could be due to the pretreatment of CNFs: the application of chemical surfactant and the ultrasonic dispersion process (UDP). Due to van der Waal's forces and the hydrophobic property of the fibers, CNFs tend to bundle together during the mixing stage with cementitious material. The chemical surfactants and UDP are applied to CNFs to avoid this agglomeration and enable dispersion. The selected chemical surfactant, a polycarboxylate-ether-based high-range water reducer (HRWR), has been proven to retard the hydration process. In this study, we find that the UDP further enhances the functionality of the effects of the HRWR, such that the retardation action of the HRWR dominates the hydration kinetics. Different amounts of HRWR and CNFs were used in this study, and their effect on hydration kinetics was studied using an isothermal conduction calorimeter (ICC). The behavior of the modified samples was compared with UHPC controls without any modifications. A new hydration model was developed based on the data collected from the ICC results. This model enables the prediction of the heat production rate as a function of the fineness modulus and the HRWR:CNF ratio (R).