In my sample, I got 4 DTG peaks for the (OPC) cement paste hydrated for 28 days. The peaks for portlandite and calcite are very clear but the peaks for ettringite and C-S-H are bit confusing. What other peaks can be identify in these region (50-200 C) since the DTG peaks overlapped a lot here?
Is there any specific meaning if the ettringite peak shows shoulder?
(samples were prepared by solvent exchange and vacuum drying).
Correction: the peak marked as C-S-H is AFm.
Ir could be water. Dry your sample previous to the TGA and try again.
Mona S. Mohammed
Dear Dr. Mona
Please read this fig. and answer
thank you in advance
Randa
Ion Such Basáñez i think the probability of water is very less because the samples were prepared through solvent exchange (iso propyl alcohol) for 5 days followed by vacuum drying for 36 hours.
you cant identify the full peaks for other phases even ettringite peaks are also sometimetimes overlap with monosulfate and gypsum
90–130 °C, 130–160 °C, 420–530 °C and 530–770 °C were mainly due to the decomposition of ettringite, gypsum, Ca(OH)2 and CaCO3, respectively. 30 °C to 170 °C owing to the dehydration of C-S-H
Dry your sample before analysis the gypsum phase should be visible
I would have answered the same as Dr. Such. But when reading the sample preparation method, it could be that unreacted gypsum is left. I suggest you do X-ray diffraction and look at the 7.5 amnstrongs lattice spacing
Dear Saurabh Khandelwal ,
Indeed other phases decompose in this range (50 - 300°C):
- C-S-H normally loses water over a wide range (between 50 to 600°C);
- Ettringite normally loses the water around 100°C (as identified by you in your Figure);
- AFm phases (such as monosulfate and hemicarbonate) lose water between 60°C to 300°C.
- Calcium aluminate hydrates also lose water in this range (e.g., aluminum hydroxide around 270°C, CAH10 at 120°C, katoite at 320°C...);
- Gypsum (CaSO4∙2H2O) dehydrate into hemihydrate (CaSO4∙0.5H2O) between 100 to 140°C and into anhydrite (CaSO4) between 140-150°C. However, it is not common to have enough gypsum in a cement paste with 28 days to be detectable in the DTG (all or at least most of gypsum already dissolved in the first days to form ettringite and AFm phases).
Furthermore, as far as I know, this relatively sharp peak that you have identified as C-S-H normally corresponds to the AFm decomposition. The C-S-H DTG peak is very broad and its maximum normally is between 60 to 150°C - overlapping with ettringite.
For more information I suggest you the following book chapter:
B. Lothenbach, P.T. Durdziński, K. de Weerdt, Thermogravimetric analysis, in: K. Scrivener, R. Snellings, B. Lothenbach (Eds.), A Practical Guide to Microstructural Analysis of Cementitious Materials, 1st ed., CRC Press, 2016: pp. 177–212.
Regards,
José Andrade
José da Silva Andrade Neto thanks for the suggestion about AFm peak.
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