Summary Lignosulphonates modify the hydration of tricalcium aluminate: acicular crystals form instead of the usual hexagonal plates, and by X-ray diffraction, DTA* and optical data it has been shown that these can represent more than one hydration product. The differences in reaction products are attributed to small changes in hydration conditions. In pastes of tricalcium aluminate, C2AH8† and C4AH13 crystallize with a new crystal habit, rather than forming a new hydrate. Current theories explaining the effect of lignosulphonates on the mechanical strengths of concrete are discussed.