Calcium silicate hydrate (C-S-H) was synthetized at 20 degrees C to investigate the effect of aluminium uptake (Al/Si = 0-0.33) in the presence and absence of alkalis on the composition and the solubility of a C-S-H with a Ca/Si equal to 1.0. C-S-H incorporates aluminium readily resulting in the formation of C-A-S-H at Al/Si At higher Al/Si ratios, in addition to C-A-S-H, katoite and/or stratlingite are present. Aluminium is mainly taken up in the bridging position of the silica dreierketten structure, which increases the chain length. The aluminium uptake in C-S-H increases with the aqueous aluminium concentrations. The presence of potassium hydroxide leads to higher pH values, to the destabilisation of stratlingite and to higher dissolved aluminium concentrations, which favours the aluminium uptake in C-S-H. Potassium replaces partially the calcium ions on the surface and interlayer, thus leading to more negative surface charge and to shortening of chain length. (C) 2015 Elsevier Ltd. All rights reserved.