I have a mix design of cement flyash blend. After doing rietveld refinement I have found that the ettringite percentage that was 1.57% at 7 days reduced to 0.85% at 28days. The gypsum content that was 0.67% at 7 days increased to 2.52% at 28days. Not sure, if I have identified the peak of gypsum correctly. Based on the past literature, ettringite normally converts into AFm phase but not back to gypsum.
I am quantifying the samples using external standard method and value of factor "G" which i'm using is 1890.29. The formula, which i'm using for the calculation of phases is (S * ρ * V2 * µ)/G. Where S=scale factor; ρ = density of a particular phase; V2= square of the volume of unit cell of the same phase; µ = absorption coefficient of the mix.
The scale factors were:
Ettringite @ 7day = 0.00000632 Gypsum @ 7day = 0.00004583
Ettringite @ 28day = 0.00000341 Gypsum @ 28day = 0.00014414
Ettringite density = 1.79 g/cm3 and unit cell volume = 2306.04 pm3
Gypsum density = 2.32 g/cm3 and unit cell volume = 493.34 pm3
µ = absorption coefficient of the mix = 49.29 cm2/g
The data was collected at Brukers D4 Endevor with the following settings:Cu ka Anode, Stepsize 0.013 deg, time per step 1 sec, 40 Kv and 40 mA
I have attached the jpeg images of the refined samples for both 7 and 28days and have also attached .raw files. I would highly appreciate if someone could have a look and suggest if the identification is correct or there is something wrong in that.
Hello Rajeev,
I have had a quick look at your samples - are you sure you have analysed the right sample? The image in the pdf is different to the data in the raw file for the 28 day sample. In the data, as far as I can see there is gypsum at 7 days but no gypsum at all in the 28 day sample. The pdf shows a different picture - puzzled...I have attached a quick qualitative assessment - I have not tried to do any quants until we can clear this one up!
And also no portlandite in the 28 day raw file...
Ian
Thanks Ian, you rightly pointed out, i think i have used a wrong file and have mistakenly overwritten it with wrong number. I'm going to do the analysis again and will get back to you if i got similar behaviour
Dear Mr. Rajeev, In my opinion, one should not analyze only for Ettringite alone while quantifying XRD of hydrated cement. At the same time, try to analyze for other forms of sulphates such Melanterite (Fe2S)4.H2O), Monosulphate, Mereiter (K2Mg2(SO4)3), Sulfatespurrite, Thaumasite, Thenardite etc minerals simultaneously. To the best of my knowledge, hydrated cement will not revert back to Gypsum at all, once hydrated. Moreover, the gypsum used in cement is already a hydrated one (Ca(SO4). 2H2O). As given above, there are various forms sulphates in addition to ettringite are formed.
Thanks Ian/Maheshwari, I couldn't find any other sulphate in search/match. I have attached the right XRD files now, not sure what is missing.
I think that the issue is mainly a phase misID. Since you have calcite in your system, its quite likely that the peak that you're assigning to Gypsum at latter ages corresponds to hemicarboaluminte or monocarboaluminate (both are AFm phases), both have their main peak around this position. In fact, the main peak of Mc matches perfectly your data, as you can see in the attached image (you've not only one Mc peak, but all of them, always remember that a good phase ID require the presence of all the main peaks of the candidate phase, unless you suspect strong texture). Hope this would be helpful for you :)
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