To reduce the CO2 emissions from cement production, Portland cement (PC) is partially replaced by supplementary cementitious materials (SCM). Reactions of SCM with PC during hydration leads to the formation of C-S-H with more silicon and aluminum than in PC, which affects the stability and durability of such concrete. Therefore, it is crucial to determine the role of aluminum on C-S-H properties to predict the formed hydrate phase assemblages and their effects on durability. Aluminum sorption isotherms including very low Al concentrations have been determined for C-S-H with Ca/Si ratios from 0.6 to 1.4. Elemental measurements were performed with ICP-MS and ICP-OES. The presence of secondary phases was investigated by using thermogravimetric analysis and XRD. Higher dissolved concentrations of Al were observed at increased alkali hydroxide concentrations and thus higher pH values. High alkali hydroxide led to an increased Al(OH)(4) formation, which reduced the Al uptake in C-S-H. This comparable behavior of Al and Si towards changes in pH values, points toward the uptake of aluminum within the silica chain both at low and high Ca/Si ratios. A higher Al uptake in C-S-H was observed at higher Ca/Si ratios, which indicates a stabilizing effect of calcium in the interlayer on Al uptake. (C) 2020 The Author(s). Published by Elsevier Inc.