Meso-mechanics of packed C-S-H colloids by nanoindentation: A coarse-grained molecular dynamics study

Colloidal calcium silicate hydrate (C-S-H) gel significantly contributes to cement paste's strength and durability. In this study, the coarse-grained (CG) models for packed C-S-H colloidal particles with different packing densities were established and meso-mechanically assessed via nanoindentation. Load-depth curves showed indentation hardness values (0.55, 1.16, and 2.63 GPa) for the systems with packing densities (η) of 0.50, 0.55, and 0.60, respectively. Structurally, the nano-indenter had a broader impact on low-density C-S-H (η=0.5, impact radius = 118 and 150 nm respectively at indentation depths = 50 and 100 nm) than high-density C-S-H (η=0.6, impact radius = 106 and 140 nm at the same depths). Low-density colloids were easily compressed without deforming low-depth nearby regions, while high-density colloids were squeezed laterally, causing deformation in these regions. Packed C-S-H colloids displayed two-stage stress relaxation behavior: rapid initial relaxation due to nanoindentation-induced instability, followed by slower relaxation due to C-S-H's viscous nature. Furthermore, higher loading rates caused initial unstable deformation, but better stability after stress relaxation compared to lower loading cases. However, the effect of loading rate on the impact region was negligible. These meso-level insights enhance our comprehension of the C-S-H gel properties in cement paste as well as the nanoindentation mechanics.