Thaumasite, along with ettringite, is responsible for sulfate attack in concrete based on Portland cement [1]. Chemically represented as Ca3[Si(OH)6](CO3)(SO4)·12H2O, thaumasite exhibits a columnar crystal structure comprised of stacked silicon hydroxide and calcium hydroxide polyhedra. The hydroxide columns are interconnected through a network of hydrogen bonds (H-bonds), facilitated by H2O molecules, sulfate ions, and carbonate ions occupying the intercolumnar spaces within the crystal.
Here, we use the classical molecular dynamics (MD) simulation technique together with the ClayFF-MOH force field [2, 3] to quantitatively investigate the mineral–water interfaces. Water structuring at the (100) and (001) surfaces for thaumasite and ettringite shows that the affinity of thaumasite for water is greater. However, the total amount of H2O molecules on the surface is the same for both minerals. Investigation of the (hkl) solid–water interfacial energies confirms that thaumasite is more soluble than ettringite. The (001) surface of thaumasite is more susceptible to dissolution than its (100) surface. Due to the smaller number of H2O molecules in the structure of thaumasite, the density of the H-bonding network is lower at the thaumasite (001) surface. Dissolution of structural H2O from the thaumasite (001) surface is most likely, as intracrystalline H-bonds between structural hydroxyls and H2O molecules are the weakest. The MD simulations also demonstrate that the solid (100) interface between the thaumasite and ettringite crystals is stable, confirming the assumption that epitaxial growth of thaumasite on the ettringite surface is possible. Thus, our MD results establish the mechanism governing AFt phase dissolution/formation, which is critical for enabling accurate thermodynamic models of cement systems.
References
[1] Köhler, S., Heinz, D. and Urbonas, L., Cem. Conr. Res. 36 (2006) 697-706.
[2] Cygan, R., Greathouse, J. and Kalinichev, A., J. Phys. Chem. C 125 (2021) 17573-17589.
[3] Tararushkin, E., Pisarev, V. and Kalinichev, A., Cem. Conr. Res. 156 (2022) 106759.
