Heterogeneous Surface Growth and Gelation of Cement Hydrates

Abstract

During cement hydration, C−S−H nanoparticles precipitate and form a porous and heterogeneous gel that glues together the hardened product. C−S−H nucleation and growth are driven by dissolution of the cement grains, posing the question of how cement grain surfaces induce spatial heterogeneities in the formation of C−S−H and affect the overall microstructure of the final gel. We develop a model to examine the link between these spatial gradients in C−S−H density and the time-evolving effective interactions between the nanoparticles. Using a combination of molecular dynamics and Monte Carlo simulations, we generate the 3D microstructure of the C−S−H gel. The gel network is analyzed in terms of percolation, internal stresses, and anisotropy, and we find that all of these are affected by the heterogeneous C−S−H growth. Further analysis of the pore structure encompassed by the C−S−H networks shows that the pore size distributions and the tortuosity of the pore space show spatial gradients and anisotropy induced by the cement grain surfaces. Specific features in the effective interactions that emerge during hydration are, however, observed to limit the anisotropies in the structure. Finally, the scattering intensity and specific surface area are computed from the simulations in order to connect to the experimental methods of probing the cement microstructure.

The physics of cement cohesion

“…1A) indeed features a nonmonotonic dependence on D, and the magnitude of the ≃1 GPa jump in pressure between D = 9 and 8 Å is well consistent with our estimated range. This shows that the high stability of the hydration shells can give rise to a competing intermediate-range repulsion (41,42), confirming early atomic force microscopy measurements on cement hydrates (7) and consistent with gel morphologies obtained in C-S-H coarse-grained simulations and seen in microscopy imaging (43)(44)(45). The finite stability of the bulk-like hydrated structures in increasing confinement provides a fundamental mechanism for this nonmonotonic dependence of the nanoscale forces on surface separation, fairly robust to the presence of salt and other ions.…”

“…on August 8, 2021 http://advances.sciencemag.org/ Downloaded from (19,21,23,24,30,(45)(46)(47)(48)(49). By the end of hydration, C-S-H becomes denser and denser, and its nanoscale features (including the interlayer distances and chemical compositions usually described in terms of Ca/Si ratio) play a predominant role in most observations and studies (16,21,29).…”

“…However, the evolving shape of the net interactions also has implications for larger length scales because it determines the anisotropic growth of cement hydrates aggregated into fibrils, lamellae, and layered mesophases that can self-assemble as C-S-H precipitation proceeds ( 5 , 43 ). It therefore provides the missing link from the nanoscale to the mesoscale aggregation kinetics and morphological variability of cement hydrates ( 19 , 21 , 23 , 24 , 30 , 45 – 49 ). By the end of hydration, C-S-H becomes denser and denser, and its nanoscale features (including the interlayer distances and chemical compositions usually described in terms of Ca/Si ratio) play a predominant role in most observations and studies ( 16 , 21 , 29 ).…”

The physics of cement cohesion

“…1a) indeed features a nonmonotonic dependence on D and the magnitude of the 1 GPa jump in pressure between D = 9 A and D = 8 A is well consistent with our estimated range. This shows that the high stability of the hydration shells can give rise to a competing intermediate-range repulsion [41,42], confirming early AFM measurements on cement hydrates [7] and consistent with gel morphologies obtained in C-S-H coarse-grained simulations and seen in microscopy imaging [43][44][45]. The finite stability of the bulk-like hydrated structures in increasing confinement provides a fundamental mechanism for this non-monotonic dependence of the nanoscale forces on surface separation, fairly robust to presence of salt and other ions.…”

“…However, the evolving shape of the net interactions also has implications for larger length-scales, because it determines the anisotropic growth of cement hydrates aggregates into fibrils, lamellae, and layered mesophases that can self-assemble as C-S-H precipitation proceeds [5,43]. It therefore provides the missing link from the nanoscale to the mesoscale aggregation kinetics and morphological variability of cement hydrates [19,21,23,24,30,45,[47][48][49]. By the end of hydration, C-S-H becomes denser and denser, and its nanoscale features (including the interlayer distances and chemical compositions usually described in terms of Ca/Si ratio) play a predominant role in most observations and studies [16,21,29].…”