I have conducted TGA and XRD of 7 day hydrated sample containing cement, fly ash and nano silica. The TGA analysis show high amount of mass loss with an endothermic TGA peak located at about 385 °C. The chemically bound water was estimated to be 3.6% of the initial mass of dry sample (physically bound water was removed before conducting TGA using solvent replacement method). XRD data doesn't show any strong peak for any C-A-H phase (silica rich or without silica). Just wondering if C-A-H phases can show amorphous characteristics. I have attached XRD and derivative curve of TG data
From the xrd I see weak traces of ettringite, C3S and C2S, calcite from your white (?) cement I suppose;
Quartz and Mullite from the silica and flyash;
C3AH6 or the precursor Hydroxy-AFms normally don´t form in a portland/white(?) cement if sulfate is present. Provide more information on the type of cement used...
Thanks for your reply Dominic,
The cement used was ordinary portland cement (not white cement). I did TGA on unhydrated (raw) cement as well, which shows that in addition to the clinker composition and approx 5% gypsum, it contained about 7.5% of calcite and about 1.5% of hydrated lime.
So if your portland cement contains enough sulfate in respect to the aluminate and ferrite phase there is very little probability for the creation of C-A-Hx or C3AH6 phases as hydration product. Ettringite is much more stable (as you can see in the XRD), even under low sulfate condition if ettringite gets unstable the conversion to mono sulfate is more likely than C-A-Hx phases.
Normally it is in the best interest of the producer of your OPC to have enough sulfate to hinder the formation of C-A-Hx phases, since this will heavily impact on the performance and workability.
Unless your flyash has an excessive amount reactive aluminate phases your are very unlikely to find any C-A-Hx or C3AH6
Thanks Dominique,
XRD and XRF analysis of my fly ash show that all the alumina present in it was converted into Mullite and there is no alumina left to react in the pozzolanic reaction. There was a small endothermic peak at 330 °C for 28day hydrated OPC showing about 1.4% bound water mass loss. When 80% of OPC was replaced with fly ash this mass lost decreased to about 0.6% and the endothermic shifted to 350 °C. Based on the past literature, i was of the understanding that mass loss aroung 330 °C is from the breakdown of C3AH6 phase. I couldn't find any other phase in this temperature zone in the past literature. Moreover there is no sign in the XRD data as well
Dear Rajeev
Indeed the dehydration peak at roughly 350 °C is not entirely clear...
The first dehydration is probably C-S-H + Ettringite following e.g. Lothenbach (https://www.researchgate.net/publication/222065456_Influence_of_Limestone_on_the_Hydration_of_Portland_Cement). Roughly at 450 °C normally there is portlandite, however I can´t find any portlandite in the xrd. So probably your portlandite is carbonated to calcite which is normally in the range of 700 °C (as in your data). It could be that the calcite form your portlandite decarbonates earlier (or perhaps your C-S-H is partially carbonated as well). The carbonation of your portlandite may produce some defective nano crystalline calcite and thus decarbonate earlier.
Another thing to consider may be the influence of your solvent exchange procedure.
It may be informative to do an xrd after different temperature treatment to see what happens in your sample or you could try to analyze your sample without solvent exchange to see if your procedure affects your xrd and tga data.
Regards
Article Influence of Limestone on the Hydration of Portland Cement
Thanks for your reply Dominique,
Yes i agree, the first mass loss is from C-S-H and Ettringite only as there was no sign of any other AFt or gypsum phase. nS present in the mix is highly reactive and it consumes the portlandite that is available in the system, be it externally added or released from the reaction of Calcium silicate phases. I noticed this trend in my previous experiments as well.
I agree with your suggestion regarding effect of solvent exchange method, i'll probably do an XRD for before and after solvent exchange. I am using 40 deg Celsius drying after solvent exchange, just to minimise the effect of temperature if any.
Thanks for referring this nice article. I was following the papers of their group for most of my research. Their group have done quite impressive work on cementitious materials using various characterisation techniques.
Thanks a lot for all your help
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