Reversible Order-Disorder Transition in Ettringite-Metaettringite Conversion

. The stability of ettringite under sorption and the conversion into metaettringite are studied using hybrid Grand Canonical Monte Carlo (GCMC) and (classical) Molecular Dynamics (MD) simulations sampling the osmotic ensemble. The desorption branch obtained from simulation and the associated volume changes are in agreement with the experimental evidence. We pay special attention to the structural changes at low RH, which is associated with metaettringite conversion, which is recognized as a disordered polymorph of ettringite with an unknown structure to date. We show that the conversion of ettringite into metaettringite is associated with an increase in entropy. The adsorption branch obtained from simulations is reversible in the ettringite domain in agreement with experiments. The reversibility in the conversion of metaettringite into ettringite, which is observed experimentally, is not captured by the simulation approach adopted. The large deformations associated with ettringite desorption make it difficult to capture reversibility with a direct sampling of the osmotic ensemble. Further, we discuss the role of hydrogen bonds on the hysteresis observed in sorption cycles in ettringite.