Nanogranular packing of C–S–H at substochiometric conditions

Herein, we present a comprehensive nanoindentation investigation of cement pastes prepared at substoichiometric water-to-cement (w/c) mass ratios between 0.15 and 0.4 with and without heat treatment. Based on a statistical indentation technique, we provide strong evidence of the existence of a statistically significant third hydrated mechanical phase in addition to the already known Low-Density (LD) and High-Density (HD) C–S–H phases. The nanomechanical properties of this third phase are found to follow similar packing density scaling relations as LD C–S–H and HD C–S–H, while being significantly greater. This third phase, whose nano-packing density is measured at 0.83 ± 0.01, is therefore termed Ultra-High-Density (UHD) phase. All three phases are present in concrete materials in different volume proportions: LD dominates cement-based materials prepared at high w/c mass ratios; HD and UHD control the microstructure of low w/c ratio materials. In addition, heat treatment favors the formation of HD and UHD. The insight thus gained into the link between composition, processing and microstructure makes it possible to monitor packing density distributions of the hydration products at the nanoscale.