In-situ hydration of calcium sulfate and the phase transformation pathways of bassanite to gypsum

Session

Stream 3: Operando Microscopy

Authors

Dr Martha Ilett (1), Dr Helen Freeman (1), Dr Johanna Galloway (1), Dr Zabeada Aslam (1), Dr Ian McPherson (2), Dr Oscar Céspedes (1), Dr Yi-Yeoun Kim (1), Professor Fiona Meldrum (1), Professor Rik Drummond-Brydson (1)

Affiliations

1. University of Leeds
2. University of Warwick

Keywords

Calcium sulfate

In situ

TEM

Abstract text

The nucleation, growth, and phase transformation of calcium sulfate must be understood to improve the performance of construction materials, reduce scaling in industrial processes and better understand the natural environment. Recent studies suggest that classical nucleation theory does not apply to calcium sulfate systems where a multi-stage process occurs instead, involving several intermediate phases. Through a variety of in-situ experimental work, there is a growing body of evidence which shows an oriented-attachment-like crystallisation process. In this work we have used advanced in-situ electron microscopy to study the phase transformation in real time to understand the mechanistic processes involved.

Both liquid cell (LC) and cryogenic (cryo) transmission electron microscopy (TEM) were used to monitor the phase transformation of bassanite (calcium sulfate hemihydrate CaSO₄·0.5H₂O) to gypsum (calcium sulfate dihydrate CaSO₄·2H₂O) during hydration in an aqueous, undersaturated calcium sulfate solution. An FEI Titan³ Themis G2 operating at 300 kV and equipped with a Gatan OneView CCD was used for in-situ electron microscopy studies, alongside Raman spectroscopy of both bulk and confined calcium sulfate systems.

By collecting real-time images and videos using LCTEM we have been able to follow the aggregation and alignment of bassanite nanoparticles at the nanoscale. When coupled with Raman spectroscopy our results show there is a period where the two phases (bassanite and gypsum) co-exist. In addition, comparisons between LCTEM and cryo-TEM will allow us to evaluate any beam induced changes in LCTEM where it would be predicted these would be ‘frozen out’ in cryo-TEM. Early comparisons between the two techniques using a model nanoparticle system suggest beam induced dissolution observed in LCTEM and caused by changes in pH can be prevented using cryo-TEM. 

Ultimately, this work has allowed real time observation of the phase transformation of bassanite to gypsum alongside comparisons between LC and cryo TEM, which can advance the application of both techniques within materials science research.