The reactive force field method was first utilized to characterize the structure, dynamics and mechanical properties of calcium–aluminate–silicate–hydrate, which is essential in the chemistry of high alumina layered gel in Portland cement. In order to study the role of Al atoms, the properties of Al atoms located in the calcium silicate sheet and the interlayer region have been investigated. The Si–Al substitution in the calcium silicate sheet has not changed the layered structure of the C–A–S–H gel. On the other hand, the presence of Al atoms in the interlayer region improves the structure and mechanical performance significantly. The connectivity factor, Q, species evolution indicates that the aluminate species in the interlayer region play an essential role in bridging the defective silicate chains and transforming the layered C–A–S–H gel at low Al/Ca levels to the branch network structure at high Al/Ca levels. The structural transition is partly attributed to the aluminate–silicate connection by the NBO sites and is partly caused by the polymerization reaction between the aluminate species, both of which can be described by the reactive force field. Additionally, the polymerization reaction by the aluminate species also leads to a hydrolytic reaction. In this way, a lot of water molecules are transformed to hydroxyls, even bridging oxygen atoms. Dynamically, due to the high strength of the Al–O bond, the aluminate–silicate network in the C–A–S–H gel has a better stability at higher Al/Ca ratios. Furthermore, uniaxial tension tests on the C–A–S–H gels demonstrate the mechanical behavior and large structural deformation of the gel. Both the Young’s modulus and tensile strength are improved significantly with increasing aluminum content, indicating a good loading resistance ability in the aluminate–silicate network. The tensile deformation, simulated by the reactive force field, is also coupled with de-polymerization of the aluminate species and the water dissociation reaction, which shows good plasticity due to the Al atom addition.