Phase transformation of CaSO4 during hydration using liquid-cell transmission electron microscopy

Session

PSA.7 - Soft and Organic Materials in liquid phase

Authors

Dr Helen.M Freeman (2), Dr Zabeada Aslam (2), Dr Johanna Galloway (3), Dr Ian McPhearson (1), Dr Oscar Cespedes (4), Dr Yi-Yeoun Kim (3), Prof Fiona.C Meldrum (3), Prof Rik.M.D Brydson (2)

Affiliations

1. Department of Chemistry, University of Warwick
2. School of Chemical and Process Engineering, University of Leeds
3. School of Chemistry, University of Leeds
4. School of Physics, University of Leeds

Keywords

LCTEM

Calcium Sulphate

Phase Transformation

Crystallization 

Abstract text

Calcium sulfate minerals are highly relevant for both natural and industrial systems (e.g. pipe scaling and building materials). As a result, they have been extensively investigated using a number of techniques. Liquid cell transmission electron microscopy (LC-TEM) enables the study of dynamic material processes in liquids at nano-micro length scales and in real time. We have used LC-TEM to follow the crystallization, polymorph transformation, and dissolution of calcium sulfate. For this work, an FEI Titan3 Themis 300: X-FEG 300 kV S/TEM was used with a Hummingbird Scientific LC-TEM holder at the University of Leeds.

 

Calcium sulfate occurs in three forms: dehydrated anhydrite (CaSO₄); hemi-hydrated bassanite (CaSO₄·0.5H₂O), and hydrated gypsum (CaSO₄·2H₂O). Despite their widespread significance, a considerable knowledge gap remains regarding the nucleation and growth of such crystals from solution, and the transformation processes between the three forms. There is a growing body of literature, suggesting that crystallization of calcium sulfate occurs through a nonclassical nanoparticle-mediated crystallization process [1–7]. 

 

This work uses LC-TEM and Raman spectroscopy to follow the hydration of synthesized bassanite nano-rods[8] into gypsum needles. Analysis of STEM images, electron diffraction patterns, and Raman spectra suggest the process involves a combination of the dissolution of bassanite and re-precipitation of gypsum alongside the oriented attachment of bassanite nano-rods which transform to gypsum. We hypothesize that the size of the synthesized bassanite nano-rods may have a role to play in the preferred phase transformation pathway, however additional studies are required to confirm this. The dissolution of end products in various aqueous solutions was also investigated and post-mortem materials characterization techniques were used to classify the final crystal structure. 

References

 

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