Effects of sodium/calcium cation exchange on the mechanical properties of calcium silicate hydrate (C-S-H)

Calcium silicate hydrate layer (C-S-H_layer) is considered to be the fundamental building block of hydrated cement. The effect of sodium ions on the atomic scale mechanical properties of C-S-H_layer remains, however, unclear. Yet, this information is critical for understanding and predicting the macroscopic performance of concrete structures during their service life. Herein, the intrinsic mechanical properties of C-S-H_layer with sodium-exchange ions replacing some calcium cations were studied by molecular dynamics simulations. The interatomic interactions provided insights into the role of Na⁺ within the atomistic scale of C-S-H_layer. It was found that Na⁺ did not significantly alter the mechanical properties (i.e., strength and stiffness) of C-S-H_layer. The larger cationic attraction on the interlayer water molecules seen in the presence of Na⁺ occurred due to the exchange of two Na⁺ for one calcium cation and resulted in a volume expansion of C-S-H_layer while a stiffening of its interlayer.