Insight into the Decalcification Mechanism of Calcium Silicate Hydrate under a Water–Heat–Salt Environment

Understanding the decalcification mechanism of calcium silicate hydrate (C–S–H) under marine environments is crucial for concrete durability. In this study, the decalcification behavior of C–S–H under a water–heat–salt environment was systematically investigated through immersion experiments and reactive molecular dynamics simulations. Experimental results showed that elevated temperature and NaCl solution significantly accelerated Ca leaching and reduced the Ca/Si ratio of C–S–H. Simulation results further revealed that both high temperature and NaCl reduce the dissolution free energy of Ca, making its release more thermodynamically and kinetically favorable. Local structure analysis indicated that Cl– disrupts Ca–Os (O atoms in silicate tetrahedra) connection through electrostatic attraction, while Na+ competes with Ca for silicate coordination sites. Elevated temperature further weakens the Ca–Os interactions and enhances the ion mobility. In contrast, the dissolution of Si atoms shows a clear energy barrier and requires higher energy due to strong Si–Os bonding. Based on these findings, an atomic-scale decalcification mechanism is proposed involving thermal activation, Cl– attraction, and Na+ competition, providing new insights into the degradation processes of C–S–H under the water–heat–salt environment.