Kinetic Model of Calcium-Silicate Hydrate Nucleation and Growth in the Presence of PCE Superplasticizers

Calcium-silicate hydrate (commonly referred to as C–S–H in cement chemistry) is the main phase and the "glue" of hydrated cement, the binding matrix of building materials such as concrete, which is an essential commodity for modern infrastructures and housing. The basic mechanisms that control cement hydration and the modes of C–S–H formation are not fully understood and the picture becomes even more complicated when organic polycarboxylate-based additives (PCE) are added with the aim of controlling the rheology of the fresh paste and the strength of the final product. Here, a kinetic model of nucleation and growth, with unconstrained nucleation mechanism (homogeneous/heterogeneous) and time-dependent growth rate, is described and used to fit the rate of C–S–H precipitation obtained by in situ X-ray powder diffraction. The kinetic model predicts a switch from heterogeneous to homogeneous nucleation and an overall inhibition of C–S–H precipitation in the presence of PCE. This mechanism exerts a fundamental role in controlling the experimentally observed decrease in the early age rate of cement hydration in the presence of PCE.