Calcium silicate hydrates (C-S-H) is the most important hydrate in Portland cements and cements containing silica-rich supplementary cementitious materials (SCM). The solubility and composition of C-S-H incorporating aluminum "C-(A)-S-H" was investigated as a function of different parameters such as Ca/Si ratio, sodium hydroxide (NaOH) concentrations and equilibration times. The aluminum sorption isotherms at different NaOH concentrations revealed a lower uptake of Al at high pH values. Moreover, the experimental data showed that high Ca/Si ratios and longer equilibration time lead to an increase in the Al uptake in C-S-H.