  The formation of crystalline calcium sulfate (CaSO4*xH2O) polymorphs from
aqueous solutions is assumed to occur via a single-step process following the
classical nucleation paradigm. However, although recent research contradicts
this classical picture and indicates that CaSO4*2H2O forms at room temperature
through multiple steps at different length and time-scales, these steps have so
far not been quantified. By using in situ and fast time-resolved small angle
X-ray scattering (SAXS), we demonstrate that the nucleation and growth of
CaSO4*2H2O involves at the very initial stages the formation of well-defined,
primary species of < 3 nm in length (stage I). Stage II of the reaction is
characterized by the arrangement of these primary species into domains, while
in stage III these domains condense into larger aggregates. Based on volume
fractions and electron density considerations we propose that the fast forming
primary species from supersaturated aqueous CaSO4 solutions are composed of
anhydrous Ca-SO4-cores. The first three stages of nucleation and aggregation of
the primary species are followed by a final stage (stage IV), where the primary
species grow within the aggregates, and eventually transform into gypsum
(CaSO4*2H2O). This final stage was also confirmed through simultaneously
collected wide-angle scattering (diffraction, WAXS) data, which clearly show
the growth of gypsum during stage IV only. Our results demonstrate that CaSO4
formation is driven by the nucleation and aggregation of well-defined anhydrous
Ca-SO4-cores that transform through hydration into gypsum through a complex
nucleation and growth pathway.
