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Modeling Fly Ash, Ettringite and CSH


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Hi,

 

I'm trying to study the formation of Ettringite and Calcium Silicate Hydrate (CSH) in a Fly Ash system. Fly Ash consists of Mullite, Hematite, Lime , SiO2(amorphous) and Anhydrite-CaSO4. The main objective is to vary the constituents of the Fly Ash mix to see its effect of Ettringite and CSH phase formation.

 

When I modeled the above mentioned system in Geochemist's workbench, the system doesn't seem to converge. Is this because of High ionic strength or is there a solution to this problem. It would be great if you could advice me to model this system.

 

I have attached the React file and also sent the database via email.

 

Thanks and Regards,

Pavan Akula

 

 

1_Fly Ash model.rea

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Guest Melika Sharifi

Hi Pavan,

 

Thanks for attaching your script. We received your thermo dataset, too. We will look into your script, and will get back to you here.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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Guest Melika Sharifi

Dear Pavan,

 

I am not really familiar with Fly Ash system, but I think the way you have set your model is not representative of your system.

You may need to rethink about it and see if you would need to add some minerals to your system. You can consider adding minerals as kinetic minerals. Could you, please, tell me how or based on what type of analysis you set up your system (i.e O2(aq) concentration, pH, etc.)?

 

Thanks,

Melika Sharifi

Aqueous Solutions LLC

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I was planning to setup a system to see what phases form at equilibrium when Fly Ash reacts with water with the database that I had sent you. The system consists of Fly Ash minerals (SiO2, Lime, Mullite, Hematite, Anhydrite). The aim of the system was to model the the interaction of fly ash minerals with H2O at a pH 10-14. I was particularly interested in the formation of phases such as Ettringite and C-S-H at equilibrium.

 

As I wasn't able to move forward without the input of O2 conc, I had to use O2(aq) in the system (I do not actually need O2). If I use the mineral as Kinetic minerals, REACT prompts for the individual elemental concentration of the mineral to be given as an input in the basis pane (for. e.g. If I use Anhydride as a kinetic mineral, I get an error asking me to include SO4 2- in the basis pane). If I include the concentration of SO4 2- ions in the basis pane, I do not understand the need to Anhydrite as a kinetic mineral in the first place.

 

I went through the Reaction modeling guide and I understand that equilibrium is first achieved by reacting all the phases in the pane and then, this is reacted with the mineral in the reactants pane (kinetic minerals) to produce the final output. Please correct me if I'm wrong. I'm trying to draft a model that works to study my system. Please let me know if you need more information on this.

 

Thanks,

Pavan Akula

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Guest Melika Sharifi

Dear Pavan,

 

Thanks for the additional info on your work.

 

The basis pane is where you set constraints on your initial system. If you have a mineral/species in your reactants pane, the components forming them should be in your initial system (Basis pane) too. Where the concentration of a particular component is negligible, you can set a small positive value (e.g. 1*10^-7).

 

The program is asking for O2 concentration as the available basis species in the default thermo dataset (thermo.tdat) is Fe++, and you are adding Hematite to the system in which the oxidation of Iron is 3+. You can either set the Eh (or pe, f O2(g), O2(aq)), or decouple Fe2+/Fe3+. If you set Eh (or pe, f O2(g), O2(aq)) and leave Fe2+/Fe3+ coupled then the equilibrium between Fe2+/Fe3+ is determined by that Eh value. However, if you decouple Fe2+/Fe3+, then the redox equilibrium between them is set by constraining each component separately (Fe2+ and Fe3+). In this case, the system Eh has no effect on Fe speciation. If you decouple the pair and only specify Fe3+, then no Fe3+ species will form (similarly decoupling the pair and only constraining Fe2+ will prevent Fe3+ from forming), so be careful with this.

 

To get what you are looking for, you need a titration model. I would suggest that you start with a dilute water that is in equilibrium with the atmosphere, and add Fly Ash minerals ( Lime, Mullite, Hematite, Anhydrite) as simple reactants to the system. Or, you can add them as kinetic minerals.

 

Hope this helps.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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