Kinetics of Al uptake in synthetic calcium silicate hydrate (C-S-H)

The effect of equilibration time on the composition, structure and solubility of calcium aluminate silicate hydrate (C-A-S-H) was investigated in 0.5 M NaOH solutions at Al/Si = 0.1. Longer equilibration time led to longer silica chains, to less Na and, to more Ca in C-A-S-H, as well as to an increased fraction of tetrahedral Al(IV) chargebalanced by Ca2+. Aluminium, calcium and silicon concentrations in solution changed rapidly during the first day, while after 14 days only minor changes were observed. The long-term concentrations could be reproduced well by the thermodynamic CASH+ model. Over a longer time span of up to 10 years, a slow redistribution of Si and Al in C-A-S-H as well as the partial replacement of Na+ by Ca2+ in the interlayer space was observed. This study highlights the importance of considering the effect of time on C-A-S-H structure.