Freezing behavior of ionic solutions within calcium silicate hydrate gel pores

Salt-frost damage is a crucial issue affecting the durability of concrete structures, however, the freezing behavior and micro-mechanisms of ionic solutions within cementitious material gel pores remain unclear, which is not conducive to optimizing cold-resistant concrete design from the bottom up. In this study, the models of saturated calcium silicate hydrate (C-S-H) gel pores containing NaCl, Na₂SO₄, and aqueous solution, respectively, were constructed by molecular dynamics (MD) simulation. We investigated the freezing behavior of ionic solutions within the gel pores at 230 K. The freezing process of models exhibited a distinct periodic pattern. Na⁺ and Cl⁻ delayed the freezing of pore water, while ${\mathrm{SO}}_4^{2-}$ accumulated near the freezing front, significantly hindering freezing progression. The freezing resulted in two types of nano brine pockets in the NaCl model. This work provides new molecular insights into salt freezing in cementitious materials and informs the design of cold-resistant concrete at the molecular scale.