Comparative study of the early stages of crystallization of calcium silicate hydrate (C-S-H) and calcium aluminate silicate hydrate (C-A-S-H)

The use of SCMs as partial substitutes for PC has made C-A-S-H a key binding phase in modern cement, yet its crystallization mechanism remains elusive. This study investigates the early stages of synthetic C-A-S-H formation and compares them with C-S-H using double addition of stoichiometric calcium and silicon amounts at a Ca/Al ratio of 5. Through real-time monitoring of solution parameters—transmittance, free Ca 2+ conductivity, and pH—complemented by structural and morphological characterization (FTIR, XRD, SEM, TEM, and NMR), we demonstrate that C-A-S-H formation is at least a two-step process involving amorphous globules, which then evolve into foil-like particles with higher crystallinity. Additionally,…