Atomistic simulations of ettringite and its aqueous interfaces: Structure and properties revisited with the modified ClayFF force field

Abstract

Ettringite, (Ca6[Al(OH)6]2[SO4]3·nH2O, n = 24–27), is one of the common phases of cement and plays an important role in cement chemistry as the primary cause of sulphate corrosion in Portland cement. Molecular dynamic computer simulations have already been applied earlier to model the crystal structure of ettringite and its interfaces with aqueous salt solutions. A recently developed version of the widely used ClayFF force field allows now to explicitly take into account the bending of M-O-H angles of (M = Al, Ca), leading to a much better agreement of the simulation results with available experimental data. The structure and dynamics of bulk ettringite crystal and its interfaces with NaCl and Na2SO4 aqueous solutions are quantitatively evaluated here for the new modified version of the force field, ClayFF-MOH, and compared with the results obtained with the earlier version, ClayFF-orig. The crystallographic parameters, elastic properties, the structure and dynamics of intracrystalline hydrogen bonding network and the vibrational spectra of ettringite are calculated by classical molecular dynamics simulations and quantitatively compared with available experimental data using both versions of ClayFF. Atomic density profiles for solution species at the ettringite surface, atomic distributions within the crystal-solution interface, and the interfacial diffusional mobility of the species are also calculated and compared. The results clearly demonstrate the importance of the explicit inclusion of M-O-H angular bending terms for accurate modeling of the mineral systems containing structural and interfacial hydroxide groups. The simulation results also show that the application of the new more accurate ClayFF-MOH version of the force field leads to the formation of a stronger hydrogen bonding network structure in the intercolumnar space of the ettringite crystal and at its surface, resulting in a stronger immobilization of the water molecules involved, as well as the ions. The ionic adsorption at the ettringite surface is also generally stronger than it was predicted by the earlier model.