Two-Step Nucleation Process of Calcium Silicate Hydrate, the Nanobrick of Cement

Despite a millennial history and the ubiquitous presence of cement in everyday life, the molecular processes underlying its hydration behavior, like the formation of calcium-silicate-hydrate (C-S-H), the binding phase of concrete, are mostly unexplored. Using time-resolved potentiometry and turbidimetry combined with dynamic light scattering, small-angle X-ray scattering, and cryo-TEM, we demonstrate C-S-H formation to proceed via a complex two-step pathway. In the first step, amorphous and dispersed spheroids are formed, whose composition is depleted in calcium compared to C-S-H and charge compensated with sodium. In the second step, these amorphous spheroids crystallize to tobermorite-type C-S-H. The crystallization is accompanied by a sodium/calcium cation exchange and aggregation. Understanding the formation of C-S-H via amorphous liquid precursors may allow for a better understanding of the topography of the nucleation in cement paste and thus the percolation of hydration products leading to the mechanical setting as well as the retarding effect of known chemical species like aluminum ions and polycarboxylate ethers.