AbstractCalcium silicate hydrated (C–S–H) gel is the principal binding phase of the cement paste and responsible for its strength. It is basically composed of silicate chains held together with CaO layers, where different guest ions can enter its structure. In particular, the substitution of Al in the silicate chains is expected to play a decisive role for the resulting properties of the cementitious materials. This work explores by means of molecular dynamics simulation the incorporation of aluminium into C–S–H gel. For that purpose we simulate the polymerisation of C–S–H gel in presence of Al atoms and analyse both the connectivity of the (alumino)silicate chains and the position of the aluminium atoms within the structure of C–S–H gel. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)