Calcium-Silicate-Hydrates (C-S-H) are one of the major phases responsible for uptake of alkali and alkali earth ions, transition metals, lanthanides and actinides in hardened cement paste. Aggregates of C-S-H emerge between clinker grains within a few minutes after mixing with water and progressively gain strength as the hydration proceeds. Understanding the interplay between the mechanism of ion uptake by C-S-H at an atomistic scale, and bulk thermodynamic properties of cement systems is essential for the long term safety and performance of waste disposal sites. Because of the amorphous character and nano- to micro-meter particle size of C-S-H, computer simulations are virtually the only tool able to reveal the link between experimentally measured macroscopic properties of concrete and the structure of C-S-H at an atomistic scale.

Titrating Monte Carlo simulations in the Grand Canonical ensemble (GCMC) at the level of the Primitive Model have been particularly successful in predicting cations and anions uptake by C-S-H [1] despite its simplicity. In particular, neglecting structural and thermodynamic details, C-S-H was described in its simplest form as a surface decorated with titrating sites regularly distributed on a square lattice and having the same dissociation constant (pK_a) set to the first deprotonation constant of silicic acid.  In the present work, we extend the existing model by including a more realistic distribution of the titrating sites and calculating their pKa values based on quantum mechanical calculations.

In crystalline C-S-H phases protolysis reactions take place on the OH groups associated with the bridging sites of the silica tetrahedra chains [2] and OH groups associated with the “pair” tetrahedra situated next to Si-defects in the bridging tetrahedral sites [3]. We calculate pKa values of all these sites using the thermodynamics integration technique based on molecular dynamics simulations on the density functional level of theory [4].  To complete the model, we also calculate protolysis constants of H₂O molecules adsorbed on C-S-H surface. Incorporating ab initio pKa values and a more realistic site distribution in the GCMC simulations we estimate their effects on ion uptake. The new data and extended model allow us to better account for the variation of the acid-base properties of C-S-H with the polymerization degree of the silicate chains.

[1] Labbez, C., Jonsson, B., Pochard, I., Nonat, A. & Cabane, B. (2006) J. Phys. Chem. B, 110, 9219-9230

[2] Churakov, S. V. (2009) Amer. Miner., 94, 156-166

[3] Churakov, S. V. & Mandaliev, P. (2008) Cem. Concr. Res., 38, 300-311

[4] Sulpizi, M. & Sprik, M., (2008)  Phys. Chem. Chem. Phys., 10, 5238–5249