Aluminium uptake in calcium silicate hydrate (C-S-H) was studied at different Ca/Si ratios from 0.6 to 1.6 at 20 degrees C. Aluminium incorporation in C-S-H depends on the total amount of aluminium present in the sample. At low Al/Si ratio (<= 0.05), the major part of the aluminium is taken up in C-S-H while at higher Al/Si ratio, the precipitation of stratlingite and/or katoite limits the Al/Si ratio in C-S-H to approximate to 0.15, regardless of the Ca/Si ratio. A strong correlation between the aqueous aluminium concentration and the aluminium uptake in the solid phase is observed. At high Ca/Si ratios, aluminium in C-S-H is observed mainly as octahedrally coordinated Al(VI) in TAH and the aluminium uptake increases with the aqueous aluminium concentration. At low Ca/Si ratio (<= 0.8), aluminium is observed mainly as tetrahedrally coordinated Al(IV) and low aqueous aluminium concentrations (below detection limit) indicate a high affinity of aluminium towards C-S-H. (C) 2016 Elsevier Ltd. All rights reserved.