Microstructural and mechanical characterization of C-A-S-H: Experimental and molecular dynamics simulation study

This study investigates the influence of aluminum incorporation on the chemical composition, morphology, silicate chain structure, and functional groups of C(-A)-S-H gel through micro- and nanoscale characterization. Based on experimental data, an atomic-scale structural model was constructed to closely reflect realistic Ca/Si and Al/Si ratios as well as Q n distributions, and to explore the effects of aluminum content on the mechanical properties and failure mechanisms of C-A-S-H at the atomic level. Results show that at low Al/Si ratios, Al can be effectively incorporated into the C-S-H gel; however, when Al/Si exceeds 0.10, Al-containing secondary phases start to precipitate. Al atoms bridge defects in silicate chains, extend the length of aluminosilicate chains, and form crosslinking structures along the z direction, with the degree of crosslinking increasing with Al content. Simulation results further indicate that Al incorporation restricts the diffusion pathways of interlayer water molecules, thereby enhancing gel structural integrity. Meanwhile, the formation of a three-dimensional crosslinking network substantially improves mechanical performance, significantly increasing Young's modulus and tensile strength in the y and z directions—especially enhancing interlayer cohesion and ductility along the z direction . Under tensile loading, stress-induced hydrolysis and bond depolymerization accelerate bond cleavage and hydroxyl formation within the C-A-S-H.