Chemical composition of calcium-silicate-hydrate gels: Competition between kinetics and thermodynamics

Although concrete is the world's most widely used manufactured material, basic questions persist regarding the nature of its binding phase, namely, the calcium-silicate-hydrate (C-S-H) gel. Here, based on reactive molecular dynamics simulations, we address the following fundamental question: Is the chemical composition of C-S-H controlled by the kinetics or thermodynamics of its precipitation? As a major outcome of this study, we demonstrate that the average stoichiometry of the C-S-H gel forming upon the hydration of ordinary portland cement (i.e., Ca/Si = 1.75) is determined by the composition of the C-S-H phase presenting the fastest precipitation kinetics, rather than the highest thermodynamic stability.