Depending on their composition, hydrated gels can be homogeneous or phase-separated, which, in turn, can affect their dynamical and mechanical properties. However, the structural features that govern the propensity for a given gel to phase-separate remain unknown. Here, based on reactive molecular dynamics simulations (ReaxFF), we investigate the atomic structure of a series of calcium–alumino–silicate–hydrate (C–A–S–H) gels with varying Al/Ca molar ratios. We show that Si-rich gels are homogeneous, whereas Al-rich gels tend to phase-separate. By adopting topological constraint theory, we demonstrate that the homogeneous-to-phase-separated transition is correlated to a topological flexible-to-rigid transition within the atomic network. This suggests that the propensity for gels to phase-separate is topological in nature.

Authors: Cheng Zhao,¹ Qi Zhou,¹ Zhe Wang,¹ Yao Zhang¹, Mathieu Bauchy¹

¹ Physics of AmoRphous and Inorganic Solids Laboratory (PARISlab), Department of Civil and Environmental Engineering, University of California, Los Angeles, California, 90095, USA