Structure, Dynamics, and Mechanical Properties of Cross-Linked Calcium Aluminosilicate Hydrate: A Molecular Dynamics Study

C-A-S-H is the main hydration product of environmental friendly concrete with cement material partially substituted by the industrial waste. The molecular structure of C-A-S-H gel determines the durability of the material. In this study, the cross-linking C-A-S-H models with Al/Si ratios of 0, 0.05, 0.10, 0.15, and 0.2 are constructed, and the structure, reactivity, and mechanical properties of the C-A-S-H gel are investigated by the reactive force field molecular dynamics. The incorporation of aluminate species in the C-A-S-H gel modifies the silicate-aluminate skeleton and interlayer water molecules. On the one hand, the bridging silicate tetrahedron is substituted by the aluminate species that polymerize with defective silicate chains, improve the crystalline order, enhance the Q species connectivity, and transform the layered C-S-H structure to cross-linked branch of C-A-S-H gel. On the other hand, the Al–Si substitution enhances the reactivity of the bridging oxygen sites in Si–O–Al, which contributes to more interlayer water molecules dissociation into the hydroxyl groups at high Al/Si ratio. The incorporated Al atoms, associating with silicate oxygen atoms, interlayer waters, and hydroxyl groups form the tetra/penta/octahedron local structures. Al–O–Si cross-links can not only exert geometry restriction on the interlayer water molecules but also increases the substrate affinity toward water molecules, which stabilizes the interlayer H-bond connection and significantly reduce the mobility of water molecules. Furthermore, uniaxial tensile test is utilized to study the mechanical behavior and deformation mechanism of the cross-linking C-A-S-H gel. Both the interlayer cohesive strength and stiffness of the C-A-S-H gel is significantly enhanced with the increasing of the aluminate branch structures that resist the tensile loading and strengthen the soft interlayer zone. Besides, during the tensile failure process, the mechanical response of the C-A-S-H gel is coupled with depolymerization of silicate-aluminate chains and the hydrolytic reaction of interlayer water molecules.