Scale formation is a serious issue that affects both daily-life and industrial applications to a considerable extent. Due to changes in solubility and/or crystallization kinetics at elevated temperature, certain minerals precipitate from hard water and processing solutions under these conditions, leading to incrustation of pipes, kettles, and heating surfaces. While this may cause damage of household devices like laundry machines or dishwashers, scaling in industrial settings can be associated with a tremendous loss in performance, which eventually will necessitate shutdown of facilities and expensive cleaning. Thus, it is highly desirable to find ways to avoid the precipitation of solids in such environments, or at least prevent any nucleated particles from firmly depositing onto crucial components [1]. This, however, requires a fundamental understanding of the processes underlying mineral nucleation and subsequent crystallization from solution.
In this context, numerous studies have been devoted to calcium carbonate, one of the major scale-forming minerals in most waters. It was found that CaCO₃ precipitation frequently proceeds via a complex multi-stage scenario involving amorphous nanoparticles and metastable crystalline polymorphs as temporary intermediates [2]. Recently, it was furthermore demonstrated that calcium and carbonate ions assemble in solution to give stable associates, so-called pre-nucleation clusters [3], and that aggregation of these species is a possible pathway to nucleation [4]. This novel perspective opens up new promising strategies for scale inhibition that may rely on additives specialized to influence the crystallization at hitherto unknown stages [5].
Calcium sulfate is another mineral that often contributes substantially to water hardness and related problems. While it has long been believed that gypsum (CaSO₄•2H₂O), the stable polymorph at ambient conditions, forms through a classical one-step mechanism directly from the dissolved ions, there is now increasing evidence that also this system may pass through non-classical reaction channels under practically relevant circumstances. Indeed, both amorphous calcium sulfate (ACS) and bassanite (CaSO₄•0.5H₂O), a metastable crystalline modification, have been identified as possible precursor phases [6, 7], and the influence of selected additives on the stability of these intermediates has been explored on a qualitative level [8, 9].
In the present work, we have used a titration-based crystallization assay, originally developed for studying CaCO₃ nucleation [3], in order to monitor calcium sulfate precipitation under controlled conditions. By combining information gained from immersed ion-selective electrodes, conductivity sensors and turbidity probes, the different stages occurring on the way to final gypsum crystals are detected and characterized in situ. Corresponding results show that ion association in solution plays an important role during CaSO₄ mineralization, although the observed binding patterns are distinct from those in the CaCO₃ case. High-resolution techniques like cryo-transmission electron microscopy and analytical ultracentrifugation furthermore allow us to trace and directly observe the actual nucleation event, as well as to investigate early growth and phase transformation processes. Overall, the collected data paint a consistent molecular-scale picture that rationalizes the macroscopic precipitation behavior of calcium sulfate. In light of these new findings, we finally examine the effects of specific crystallization additives on the newly identified processes, and evaluate the potential of certain well-known antiscalants so as to ultimately delineate, as for calcium carbonate, alternative and perhaps more powerful concepts for scale inhibition.


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1. The role of prenucleation clusters during (bio)mineral nucleation
2. Analysis of the early stages of crystallization processes