Mechanochemical properties of C-(A)-S-H with varying disorder: A reactive molecular dynamics study

Atomic-scale disorder plays an important role in C-(A)-S-H that influences its fracture mode, mechanical response, and hydrolytic chemistry. However, the specific contributions of bridging-tetrahedra (BT) and pairing-tetrahedra (PT) vacancies, the principal sources of network disorder, remain largely unresolved. Here, we first introduce an orientation-weighted bond-order (SBO) metric to link atomic bonding to tensile response. Using reactive MD on C-(A)-S-H (Ca/Si = 1.3) with controlled BT-vacancy disorder ( α ) and Al/Si ( λ ) along x , y , and z , we find that for loading along x increasing α (shorter chains) monotonically lowers the peak stress, whereas Al substitution elevates the pre-peak strength via additional Si–O–Al cross-links but concomitantly accelerates post-peak softening; throughout, the covalent Si–O–Si backbone remains largely intact, and failure initiates predominantly by Ca–O–Si hydrolysis (and by Ca/Al–O when λ > 0). In y and z , peak stress is only weakly sensitive to α or λ ; notably, z -direction ductility increases with λ because Al – O bridges reorient to carry load after the peak. These results show that BT-vacancy–driven disorder controls strength through chain connectivity in the load-parallel direction and that Al substitution tunes ductility through reconfigurable Ca/Al–O bridges. The proposed metrics provide a transferable framework to quantify structure–mechanics coupling in C-(A)-S-H and to guide the design of stronger and tougher binder phases.