The nanogranular nature of C–S–H

Despite its ubiquitous presence as binding phase in all cementitious materials, the mechanical behavior of calcium–silicate–hydrates (C–S–H) is still an enigma that has deceived many decoding attempts from experimental and theoretical sides. In this paper, we propose and validate a new technique and experimental protocol to rationally assess the nanomechanical behavior of C–S–H based on a statistical analysis of hundreds of nanoindentation tests. By means of this grid indentation technique we identify in situ two structurally distinct but compositionally similar C–S–H phases heretofore hypothesized to exist as low density (LD) C–S–H and high density (HD) C–S–H, or outer and inner products. The main finding of this paper is that both phases exhibit a unique nanogranular behavior which is driven by particle-to-particle contact forces rather than by mineral properties. We argue that this nanomechanical blueprint of material invariant behavior of C–S–H is a consequence of the hydration reactions during which precipitating C–S–H nanoparticles percolate generating contact surfaces. As hydration proceeds, these nanoparticles pack closer to center on-average around two characteristic limit   packing densities, the random packing limit (η=64%η=64%) and the ordered face-centered cubic (fcc) or hexagonal close-packed (hcp) packing limit (η=74%η=74%), forming a characteristic LD C–S–H and HD C–S–H phase.