Physicochemical and Additive Controls on the Multistep Precipitation Pathway of Gypsum

Abstract

Synchrotron-based small-and wide-angle X-ray scattering (SAXS/WAXS) was used to examine in situ the precipitation of gypsum (CaSO 4 ·2H 2 O) from solution. We determined the role of (I) supersaturation, (II) temperature and (III) additives (Mg 2+ and citric acid) on the precipitation mechanism and rate of gypsum. Detailed analysis of the SAXS data showed that for all tested supersaturations and temperatures the same nucleation pathway was maintained, i.e., formation of primary particles that aggregate and transform/re-organize into gypsum. In the presence of Mg 2+ more primary particle are formed compared to the pure experiment, but the onset of their transformation/reorganization was slowed down. Citrate reduces the formation of primary particles resulting in a longer induction time of gypsum formation. Based on the WAXS data we determined that the precipitation rate of gypsum increased 5-fold from 4 to 40 • C, which results in an effective activation energy of~30 kJ·mol −1 . Mg 2+ reduces the precipitation rate of gypsum by more than half, most likely by blocking the attachment sites of the growth units, while citric acid only weakly hampers the growth of gypsum by lowering the effective supersaturation. In short, our results show that the nucleation mechanism is independent of the solution conditions and that Mg 2+ and citric acid influence differently the nucleation pathway and growth kinetics of gypsum. These insights are key for further improving our ability to control the crystallization process of calcium sulphate.

The Structure of CaSO4 Nanorods: The Precursor of Gypsum

“…Although our current diffraction and earlier scattering data 12,13 , as well as the current MD simulations, clearly point to the presence of small CaSO4 clusters as a precursor to gypsum and possibly other calcium sulfate crystalline phases 29 , it is not clear what their role exactly is.…”

“…We performed such analysis using diffraction data of gypsum precipitating at 10 ºC from solution containing initially 50 and 100 mM CaSO4. For both conditions, diffraction patterns of an identical nanophase profile were collected, but as expected, the crystallization process progressed faster for the higher initial CaSO4 concentration 13 . As shown in Fig.…”

“…Not only do diffraction data allow us to resolve the possible structures of the observed 13 clusters. They are also useful to evaluate the population of such species with respect to the initial ion concentrations and the final expected gypsum content by calculating the scattering invariants of the crystalline and disordered phases.…”

Particle-Mediated Nucleation Pathways Are Imprinted in the Internal Structure of Calcium Sulfate Single Crystals

“…Following the current consensus discussed above, we point out that this classification is solely based on their final structure, which essentially does not provide on its own a sufficient evidence for a non-classical, particle-mediated, growth mechanism 41,42 . However, in our previous work based on scattering experiments and theory [1][2][3]43 , we showed that the formation of calcium sulfate phases occurs through the coalescence and growth of primary particles within surface fractal aggregates ("brick-in-awall") 3 . We postulated that this initial step involved a framework structure as a plausible common precursor to gypsum, bassanite and anhydrite 1,2 .…”

Editorial for Special Issue “Nucleation of Minerals: Precursors, Intermediates and Their Use in Materials Chemistry”

“…Gruber et al [13] present a transparent, inexpensive, and reusable test system for the investigation of infiltration and crystallization processes by using a micro-comb test system. Ossorio et al [14] perform synchrotron-based small-and wide-angle X-ray scattering to examine the precipitation of gypsum from solution, which occur via primary particles that aggregate and transform/re-organize towards the final precipitate, with and without the addition of Mg 2+ and citrate. Self-assembly processes of alkaline earth carbonates in the presence of silica, on the other hand, produce a unique class of composite materials with complex morphologies, as studied by Opel et al [15] (also see the cover page of this Minerals special issue).…”