Nanoscale Insights into the Influence of Seawater (NaCl) on the Behavior of Calcium Silicate Hydrate

Using seawater for concrete production is potentially advantageous from a sustainability perspective. However, the fundamental mechanisms underlying seawater-mixed concrete are far from being understood, especially from the nanoscale. Herein, molecular models are developed to study, for the first time, the influence of seawater (NaCl solutions) on the behavior of calcium silicate hydrate (C-S-H, the main binding phase of concrete). Thermodynamically, Na⁺ showed a strong adsorption capacity on the C-S-H surface, resulting in the partial release of Ca²⁺ from the surface (termed as “Na-Ca cation exchange”). Mechanically, the presence of NaCl in the interlayer solutions enhanced the bond performance between the C-S-H layers because Na⁺ ions provided a greater stress transfer capacity than H₂O molecules. Under shear loading, the C-S-H layers slid over each other, with the interlayer solutions acting as a lubricant. An increase in NaCl concentration enhanced the sliding resistance, which was attributed to the increased viscosity of the interlayer solutions and strengthening of the solid-solution interfaces. The above atomic-level findings facilitate a better understanding of the influence of seawater on concrete properties.