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 Ca²⁺ 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, we reveal that Al promotes Ca binding during the prenucleation stage and slightly accelerates nucleation. These results highlight important differences in the formation pathways of both hydrates, particularly the extended stability of the C-A-S-H globules, which might affect the workability and setting time in aluminium-containing blended cements.