Confinement and Texture in Cement Hydrates: From Nanoscale Cohesion to Rheology

Abstract

Concrete is the most abundant artificial material on Earth, yet the physical mechanisms that control its properties are not fully understood. Cement, the main binding agent, reacts with water to produce Calcium-Silicate-Hydrate (C-S-H) nanoparticles that form a heterogeneous and porous gel which serves as the glue for the hardened material. The C-S-H building blocks are highly charged and adhere due to electrostatics mediated by the water and counter-ions produced by cement dissolution. This multiscale complexity, coupled to the non-equilibrium setting process, is the central challenge to designing better cementitious materials---for more durable infrastructure, reduced emissions and improved sustainability, and 3D printing based construction. To address this, I have developed nano- and meso-scale computational models, connecting cement chemistry to its nanoscale cohesion and the consequential effects on microstructure and macroscopic material properties.

The electrostatics governing cohesion of C-S-H nanoparticles is in the regime (strongly coupled i.e multivalent ions and high surface charge density) where the usual mean-field theories break down and a new approach is needed. By modeling explicit ions, immersed in SPC/E water, confined by charged surfaces, I demonstrate that strong spatial and dynamic correlations arise between ions and water which are responsible for the strong net attraction between the confining surfaces. This depends on surface charge and ion type, controlled by the cement chemistry, which changes over time. Coarse-graining to the mesoscale, I translate the nanoscale forces to an effective interaction between C-S-H particles. By coupling molecular dynamics with a Grand Canonical Monte Carlo process to mimic C-S-H precipitation, I investigate the heterogeneous C-S-H growth near cement grain surfaces. These simulations demonstrate how the time-evolution of the interactions can drive the formation of a percolating gel, which limits spatial gradients and anisotropy at early hydration times but reaches high densities with large local variations---as in hardened cement pastes---at later times. Together, these models provide a framework for predicting the nanoscale electrostatics, mesoscale morphology, and macroscopic properties of cement from first principles, and I discuss how this could be applied to make predictions of rheological properties.