Cement production accounts for approximately 8% of man-made CO2 emissions. Lowering these CO2 emissions is currently one of the most important and urgent research topics within the cement community. To reduce these emissions, the Portland cement (PC) is partially replaced by supplementary cementitious materials (SCM) such as blast furnace slag, fly ash or silica fumes. Reaction of these SCM with PC during hydration leads to the formation of additional calcium silicate hydrates (C-S-H), which is the single most important phase in cements based on silica-rich SCM. The high Al2O3 and SiO2 content of the SCM results in C-S-H compositions with more Si and Al than in PC, which affects the stability and durability of such cements. Therefore, it is crucial to determine the role of Al on C-S-H properties to predict the formed hydrate phase assemblages and their effect on durability. Al sorption isotherms including very low Al concentrations have been determined for C-S-H with Ca/Si ratios from 0.6 to 1.4. The solubility, structure and composition of calcium silicate hydrates incorporating aluminum “C-A-S-H” as a function of different parameters such as Ca/Si ratio, equilibration time, Al and alkali contents were investigated. Elemental measurements were performed with ICP MS and ICP-OES. The presence of secondary phases was investigated by using TGA and XRD and the structure of C A S H was investigated by FTIR. High alkali hydroxide concentrations led to an increased Al(OH)4- formation in solution, which reduced the Al uptake in C-S-H. Increasing the pH values and decreasing the Ca/Si in C-S-H increased not only the Al concentrations but also in parallel the Si concentrations in solution. This comparable behavior of Al and Si towards changes in pH, pointed towards the uptake of Al 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 indicated a stabilizing effect of Ca in the interlayer on Al uptake. The effect of equilibration time on Al uptake in C-S-H was investigated using equilibration times from 7 days up to 3 years. Lower Al concentrations were measured in the solution after longer equilibration times. In addition, a decrease in the content of secondary phases was observed by TGA indicating a higher uptake of Al in C-S-H. Little further decrease in Al concentrations was observed after 2 years and longer at low Ca/Si ratios. At high Ca/Si ratios no significant change in solution concentrations was observed after more than 3 months, while the destabilization of secondary phases continued up to 1 year, indicating that a (meta)stable equilibrium was reached faster at higher Ca/Si ratios. In addition to the C-A-S-H phase, the formation of secondary phases such as strÀtlingite, katoite and Al(OH)3 was observed at Al/Si ≥ 0.03, which limited the Al uptake in C-S-H. More secondary phases were observed at higher Al concentrations and/or lower pH values. At low Al/Si ratios, a more significant decrease of Al concentrations with time was observed indicating a slower equilibration than at higher Al concentrations. The Al sorption isotherms showed a linear trend between the Al in solution and Al in C-A-S-H from Al/Si of 0.001 up to 0.2. The linear trend pointed towards an Al uptake on one or several types of sorption sites, with a high sorption capacity, which would be consistent with an Al uptake in the bridging position of the silica chains suggested based on NMR studies