I have a slag sample with following elemental oxide composition:
CaO, SiO2, Al2O3, MgO, SO3 and also contains carbonate bearing phase(s). I have identify the following phases in XRD:
Gypsum (CaSO4.2H2O) , Aragonite (CaCO3); Calcite (CaCO3)
I have conducted thermogravimetric analysis on this slag with following settings:
40deg to 980 deg temperature @ 30deg per min. The maximum upper temperature limit of our instrument is 1000 deg. I find three endothermic peaks, the first one at ~175 deg is that of Gypsum, the second one at ~750 deg is that of CaCO3, the third one is at ~960 deg, just wondering if anybody has an idea about the third possible phase. Is it another CaCO3 endothermic peak? The mass loss was still continuing but because of the upper limit of our TGA instrument I couldn't go further.
I ealier raised this question couple of days ago with the XRD data but to get further clarity I have conducted TGA as well but i am not fully confident about the last endothermic peak @ ~960. I'll try TGA with mass spectrometry at later stage to check the evolved gases but I don't have access straightaway so thought to put up this question in researchgate.
How reproducible is the 960 C peak? It could be some contamination in your crucible or furnace (especially if you do not routinely run to close to the max temperature). I would try running a couple of cleaning cycles, i.e., no samples, just ramp to 1000 C under flowing air, then try re-running your sample. If the high temperature peak persists, then it is a real, sample-related, peak. It could be the sulfate decomposition that Dr Hippmann referred to, or it could be melting of an ash-like/other component. If it's the latter, you might see it recrystallize (exotherm) upon cooling, and it will be repeatable if you run multiple cycles on the same specimen. If it is a decomposition, then it will occur only once, upon first heating.
Thanks Sebastian / Eric / Sergio for your response. I did an XRD of the TGA samples after the heating cycle and the gypsum phase was missing, so I agree it is most likely the decomposition of Anhydrite (Gypsum).
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