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Colloidal simulations of C-S-H precipitation: a coarse-grained Transition State Theory approach

The setting and early-age mechanics of cement paste are largely controlled by the formation of calcium-silicate-hydrate (C-S-H). The formation of C-S-H can be described as an aggregation of nanoparticles and development of a network of mesopores, with size between 1 and 100 nm. This process involves two different length scales: the sub-nanometer scale of chemical reactions, and the ~100nm scale of nanoparticle aggregation.
Furthermore, the hydration and setting of cement (and therefore the precipitation of C-S-H) take hours: a timescale that cannot be addressed via molecular and nanoparticle dynamics. This multi-scale nature in length and time challenges all the current nanoscale models of precipitation. As a consequence, the mechanisms by which the chemistry of the aqueous solution affects the development of chemical composition, structural features, and mechanical properties of the C-S-H are still largely unknown.
In our work we developed a new, spatially coarse-grained, Transition State Theory (TST) approach to estimate the nucleation rate of C-S-H nanoparticles as a function of their size and chemical composition. The coarse-graining consists in treating the nucleation of a C-S-H particle as a series of chemical reactions, each one characterized via TST. This approach overcomes several limitations of classical nucleation theory, e.g. it accounts for chemical reactions and for the effect of interactions between nanoparticles on the activation energies.
We simulated the nucleation of C-S-H from cement solutions with different concentration of Ca2+ and H2SiO42- ions. Our results capture and quantify the linkage between the solution chemistry and the size and chemical composition of the C-S-H nuclei. This indicates that our proposed coarse-grained TST technique can enable direct simulations of the chemical kinetics of precipitating nanoparticle systems, toward a simulation-informed nano-design of cement paste that can be extended to other technologically important nanoparticle materials, such as metal oxides.

Author(s):

Igor Shvab    
Newcastle University
United Kingdom

Enrico Masoero    
Newcastle University
United Kingdom