Understanding hydration reactions, mechanical properties, thermal expansion, and organic interfacial interactions of calcium sulfate hydrates from the atomic scale

Abstract

Calcium sulfates such as anhydrite, hemihydrate, and gypsum are used extensively in building materials, wall board, and biomaterials. The correlation between nanoscale structure and macroscopic properties, however, remains incompletely understood. We employed molecular dynamics simulations with the Interface Force Field (IFF) to examine sensitive hydration reactions, anisotropic thermal and mechanical properties, as well as (hkl) specific adsorption of organic modifiers. Computed thermal transitions between calcium sulfate phases, directional coefficients of thermal expansion, and directional mechanical properties agree exceptionally well with partially known experimental measurements, provide missing data and mechanistic understanding at the atomic scale. Polymeric naphthalene sulfonate-formaldehyde condensates exhibit strong, selective adsorption to the hemihydrate (001) surface. The polymer conformations and facet-specific binding affinities explain the delayed hydration of calcium sulfate hemihydrate to gypsum. The simulation methods can be applied to predict crystal growth and properties of sulfate-containing multiphase materials from atoms to the micrometer scale.