Molecular computer simulations of two hydrous Ca-aluminates – hydrocalumite ([Ca2Al(OH)6]Cl.2H2O) and ettringite (Ca6[Al(OH)6]2[SO4]3.26H2O) – mark the beginning of our long-standing successful collaboration with Jim Kirkpatrick some 20 years ago [1-2]. These phases are common components in calcium aluminate cements and can also serve as models of other important poorly crystalline cement phases. At the same time, our initial dissatisfaction with the outcomes of these early molecular modeling efforts had greatly stimulated our attempts to construct a better force field parameterization for classical molecular dynamics (MD) simulations of such materials that eventually led to the development of the ClayFF force field [3]. The layered structure of hydrocalumite is formed by distorted Ca-hydroxide and Al-hydroxide octahedra. Its interlayers are formed by a highly ordered arrangement of Cl- ions and H2O molecules. It is structurally one of the best understood of the layered double hydroxides (LDHs) and can serve as a good model for other less ordered natural and synthetic LDH phases important for a wide variety of technological applications in catalysis, environmental remediation, drug delivery, etc. Ettringite, on the other hand, has a columnar structure formed by stacked Al-hydroxide and Ca-hydroxide octahedra. The columns are held together by a network of hydrogen bonds with the H2O molecules and sulphate ions in the intercolumnar space of the crystal. Here we revisit these earlier simulations using the new ClayFF modification, which can now more accurately account for the bending of Al-O-H and Ca-O-H angles in the mineral structure [4]. The structural and dynamic properties of these crystal phases and their aqueous interfaces are probed by classical MD simulations with the new ClayFF-MOH model and compared with a diverse set of available experimental data, including X-ray diffractometry, neutron scattering, IR and Raman spectroscopy, as well as earlier atomistic simulations [1-2].

Structure and properties of two calcium aluminates and their aqueous interfaces revisited with a new series of ClayFF-based atomistic simulations

ACS Fall 2021

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