| Summary: | Calcium oxalates are naturally occurring biominerals and can be found as a byproduct of some industrial processes. Recently, a new and green method for carbon capture and sequestration in stable calcium oxalate from oxalic acid produced by carbon dioxide reduction was proposed. The reaction resulted in high-quality weddellite crystals. Assessing the stability of these weddellite crystals is crucial to forecast their reuse as solid-state reservoir of pure CO<sub>2</sub> and CaO in a circular economy perspective or, eventually, their disposal. The thermal decomposition of weddellite obtained from the new method of carbon capture and storage was studied by coupling <i>in-situ</i> high-temperature X-ray powder diffraction and thermogravimetric analysis, in order to evaluate the dehydration, decarbonation, and the possible production of unwanted volatile species during heating. At low temperature (119–255 °C), structural water release was superimposed to an early CO<sub>2</sub> feeble evolution, resulting in a water-carbon dioxide mixture that should be separated for reuse. Furthermore, the storage temperature limit must be considered bearing in mind this CO<sub>2</sub> release low-temperature event. In the range 390–550 °C, a two-component mixture of carbon monoxide and dioxide is evolved, requiring oxidation of the former or gas separation to reuse pure gases. Finally, the last decarbonation reaction produced pure CO<sub>2</sub> starting from 550 °C.
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