Behavior of functionalized graphene in fresh and hydrated cement matrix: Role of carboxyl and amine functional group

Graphene and functionalized graphene (FG) incorporated cement composites show higher mechanical performance and can help reduce the CO2 footprint. However, the use of graphene and its derivatives in cementitious material is hindered due to its poor dispersibility in fresh cement paste, inconsistency in published performance data, and inadequate knowledge of the graphene-cement interaction mechanisms, among others. This paper explores the stability of carboxyl and amine-functionalized graphene in fresh cementitious materials and examines its effect on the early-age and later-age cement hydration kinetics. The stability was assessed by zeta potential analysis, Fourier-transform infrared spectroscopy, and an image analysis-based technique performed on FG samples suspended in cement pore solution. Carboxyl and amine-functionalized graphene samples were both found to be unstable in cement pore solution. However, carboxyl-functionalized graphene aggregated more than amine-functionalized graphene due to its stronger interactions with the cations in cement pore solution. The effect of polycarboxylate ether was also examined. A graphene-to-polycarboxylate ether mass ratio of 1:3 ensured optimum dispersibility in cement pore solution, irrespective of the functional group type on the graphene surface. The early age hydration behavior of the FG-cement composite was studied using ultrasonic wave transmission, and the later age hydration behavior was assessed from the chemically bound water content. Both functionalization types increased the early-age hydration kinetics, while only carboxyl graphene enhanced the later-age (28-day) hydration kinetics. The stronger interaction of the carboxyl graphene with the hydrated cement matrix also resulted in a higher 28-day compressive strength compared to the amine group. This study sheds light on the role of functional groups on graphene and can help design functionalized graphene suitable for applications in cementitious systems.