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I am having some trouble with fully quantitative phase identification of coal fly ashes using x-ray diffraction (XRD). Here follows some basic information about the experiment:

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Instrument: Bruker D8A

Software: TOPAS5

Database: COD (But can also use PAN ICSD if need be)

Sample: Coal fly ashes from a Western Australian coal power station combusted at around 900$^\circ$C.

The goal: How to accurately fit an XRD profile to the pattern and accurately quantify the phase composition (i.e. crystalline and amorphous wt%s) of the fly ashes. For example, I'd like a good fit of the XRD pattern so that I can get an accurate output phase composition of the fly ash along the lines of (as an example):

  • 2% Hematite
  • 4% Magnetite
  • 8% Mullite
  • 16% Quartz
  • 70% Amorphous

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I have used TOPAS before for easier and idealised samples (i.e. crystals and simple amorphous structures using the internal standard method) and have fitted the curves great. However, I'm having trouble conceiving of an analytical strategy to analyse and quantify the crystalline and amorphous material in coal fly ashes; a "real world" sample. Refer to the picture below where I've prepared the sample in a way to minimise preferred orientation and with a 20 wt% corundum internal standard and have done phase ID to find quartz, hematite, magnetite, mullite and corundum (from the standard) along with allowing for 1/X background and zero error, absorption and LP corrections:

enter image description here

enter image description here

The goal here is to input the phase composition of the fly ashes (i.e. the quantified crystalline and amorphous wt%s) into a mathematical model (the "mix designer") to synthesise geopolymers, without these wt%s the geopolymers I make don't have good consistency in their measured durability properties (i.e. compressive strength, rheology, viscosity etc.) nor quality assurance and control. Note that procedures for this have been developed in literature (i.e. https://www.sciencedirect.com/science/article/pii/S0016236110003789), but unfortunately, the detailed XRD method employed is not usually provided.

Does anyone have any experience or learnings here and can provide some insight or knowledge? I have done some literature review on this but a lot of the fly ashes mentioned are very different due to differing parameters like location, coal power station combustion conditions, source coal geology and ash content (etc.). XRD of coal fly ashes is a niche and an uncommon area of materials science, accurate quantification is even more uncommon.

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  • $\begingroup$ I might be ignorant here, but after subtraction of the identifiable patterns, what do you expect to be able to analyse and quantify? $\endgroup$
    – Karl
    Aug 30, 2022 at 20:30
  • $\begingroup$ ".. mix design and synthesis geopolymers .." Can you elaborate on that a bit? I haven't got the foggiest idea what that sentence means. Typo? Missing word? $\endgroup$
    – Karl
    Aug 30, 2022 at 20:35
  • $\begingroup$ I've made some edits to the question for clarity. In short, I input the crystalline and amorphous wt%s (i.e. 16% quartz, 8% mullite, 70% amorphous etc.) into a mathematical model (the "mix designer") to make geopolymers, without these wt%s the geopolymers I make don't have good consistency in their measured durability properties (i.e. compressive strength, rheology, viscosity etc.) nor quality assurance and control. So a good quantitative XRD strategy is crucial to this process. $\endgroup$
    – Hendrix13
    Aug 31, 2022 at 2:41
  • $\begingroup$ Hm, and why do you think that model, which you cannot test, because you cannot get the numbers, accurately describes the material properties of a geopolymer made from fly ash? ;) Sorry for playing the devil's advocate here. $\endgroup$
    – Karl
    Aug 31, 2022 at 20:37
  • $\begingroup$ I think XRD is probably the best standard analytical method to use here, but it's also very probably totally insufficient. Fly ash is very likely far too complex and inhomogeneous to be sufficiently described by its XRD pattern. Just my gut feeling. $\endgroup$
    – Karl
    Aug 31, 2022 at 20:48

1 Answer 1

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I am the author of that paper.

Reach out on linked in if you would like to discuss in detail. The bot deleted my previous message when I sent my linked in address. You can also get more information my PhD thesis available on the Curtin university library website. It probably has the what I refined and in what order if I did a sequential process.

Like in the paper you need XRF and Quant XRD.

The fluorite internal standard was used to not overlap and minimize micro-adsorption mismatch.

A very important thing to do is fit the mullite with a le-bail fit to get the lattice parameters, then find the most appropriate mullite as it is a solid solution. Then you can use the correct mullite in the Rietveld refinement. There is a set of Mullite's which cover the range I covered in 9 Australian fly ashes, I think it was Ban and Okada (1992). Fitting the minor phases is also important to accurately quantify the amorphous fraction.

Then as per the paper you calculate the composition of the amorphous fraction using Oxide fractions from XRF minus oxide fractions calculated from XRD.

This works ok for many of the Australian fly ashes. In subsequent work we saw that fly ashes with amorphous Si/Al >> 2 did not work out well as Geopolymer target Si/Al ~= 2, in retrospec is was probably an issue with the commercial sodium aluminate product that we were using. Industrial byproduct sodium aluminate worked fine.

If I was given a new fly ash I would follow this procedure and make about 9 paste mixes with a variation of Si/Al and Na/Al ratio to confirm most appropriate mix. If I was in process control I would use the above process to flag when there is likely going to be an issue to investigate.

Ban T, Okada K. Structure refinement of mullite by the Rietveld method and a new method for estimation of chemical composition. J Am Ceram Soc 1992;75:227–30

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  • $\begingroup$ I've since gotten better at XRD and GP mxi designing and have been doing a lot of this it's been great $\endgroup$
    – Hendrix13
    Apr 20, 2023 at 7:47
  • $\begingroup$ Hi @Ross Williams, amazing response thank you! I tried adding you a few months ago and messaging but it was locked. I can't seem to PM you here on SE but my name is Ramon S on Linkedin, please reach out as I can't message you and couldn't get your contact through Arie :) Would LOVE to have a coffee and chat ! $\endgroup$
    – Hendrix13
    Apr 21, 2023 at 2:41
  • $\begingroup$ So when using TOPAS the fits look much better and I've learned a lot. My refinements now are nowhere near as heinous as the pics in my original question 7 months ago! $\endgroup$
    – Hendrix13
    Apr 21, 2023 at 2:45

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