The effect of Al and of pH on the structure of calcium silicate hydrate (C-S-H), the most important hydration product in Portland and blended cement, was studied at Ca/Si similar to 1.0. The presence of Al led to higher amounts of secondary phases, larger interlayer distances, longer silicate chains, higher concentrations of dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases progressively decreased with increasing pH; that of stratlingite and Al(OH)(3) dominated at low pH, and katoite at high pH values. High pH was also associated with increased Al and Si concentrations, lowered Ca concentrations, shortened silicate chain length and higher maximum Al/SiC-S-H ratios. The Al uptake in C-A-S-H increased with aqueous aluminium concentration. XANES analysis showed the presence of both Al(IV) and Al(VI) in different coordination environments in C-S-H.