Formation of calcium sulfate through the aggregation of sub-3 nm anhydrous primary species

TL;DR

This study reveals that calcium sulfate crystallization involves multiple steps starting from sub-3 nm anhydrous cores, challenging the classical single-step nucleation model, and details the complex pathway leading to gypsum formation.

Contribution

It provides the first quantification of the multi-step nucleation process of calcium sulfate, highlighting the role of primary anhydrous species and their aggregation in gypsum formation.

Findings

Primary species are < 3 nm anhydrous Ca-SO4 cores.

Nucleation involves formation, aggregation, and transformation stages.

Gypsum forms in the final stage through hydration of primary species.

Abstract

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.