Conceptual Models > Equilibrium assumptions

Mineral Equilibrium calculation

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Sol-Chan Han:
Hello, Phreeqc users and experts!
I am a new user who has been started using Phreeqc recently.
Many comments, which are posted on this website, were very useful to me.

However, there are some remained problems annoying me. So, I ask you a help to solve some troubles.

I want to model equilibrium phases among the minerals, which are in contact with the water.
I am trying to use EQUILIBRIUM_PHASES BLOCK and REACTION BLOCK. However, they are me give different answers.

Following text is part of my model.

1. When I using Equilibrium phases block

SOLUTION 1
EQUILIBRIUM_PHASES 1
C3S   0   6
C2S   0   3
C3A   0   0.9
C4AF   0   0.2
Brucite 0   0
Hematite 0  0
Portlandite 0  0
END

2. When I using Raction block

SOLUTION 1
EQUILIBRIUM_PHASES 1
Brucite 0   0
Hematite 0  0
Portlandite 0  0
REACTION 1
C3S      6
C2S      3
C3A      0.9
C4AF      0.2
10.1 moles
END

I found that some users used reaction block when they calculated equilibrium between solid and solution, while some users used equilibrium phases block.

Which could be better modeling? Could you please give me any comments?

Thank you.
Best regards,

dlparkhurst:
Your file has two different conceptual models. In the first input set, you are assuming the moles of each phase and that the phases will react to equilibrium (dissolving or precipitating as needed) to achieve equilibrium (or dissolve completely).

In the second input set, you are adding the specified moles of the four reactants to solution. These reactants dissolve totally increasing the dissolved concentrations of the solution. Then only the minerals in EQUILIBRIUM_PHASES will react to equilibrium. C2S, C3S, and the other two will not be in equilibrium, whereas they probably will be in the first calculation.

Sol-Chan Han:
Hello sir, I appreciate your comments.

I am sorry that there were some mistakes in my previous post.
I actually also listed C3S, C3S, C3A, and C4AF in the equilibrium_phases block in the second input set as can be seen below.

2. When I using Reaction block

SOLUTION 1
EQUILIBRIUM_PHASES 1
Brucite 0   0
Hematite 0  0
Portlandite 0  0
C3S     0   0
C2S     0   0
C3A     0   0
C4AF   0   0
REACTION 1
C3S      6
C2S      3
C3A      0.9
C4AF      0.2
10.1 moles
END

I am wondering that what kinds of difference can occur between the first and the second input?
In my opinion, I think that the first input is more realistic because the reaction block force to dissolve all phases regardless of the reaction constant.
(I mean that for example even if C3S has very low Ksp, all amount of C3S will be dissolved into the solution by REACTION block)
Could you please give me any comment whether my opinion is right?

Thank you.

John Mahoney:
The REACTION keyword is best used for irreversible reactions - all of it goes into solution.  EQUILIBRIUM_PHASES Keyword means that an equilibrium assemblage will be made up of the phases in that block.  It will draw components out of the solution (precipitate stable phases) to make up that assemblage.  The other issue with EQUILIBRIUM_PHASES is the activities of many of the components will be defined by the solubility reactions and the log Ksp values.  This is a Phase Rule requirement. 

I suggest you run your model with the  EQP and REACTION blocks and compare it to the model below. You have to have those phases defined for this to work.  I do not have those in my database so I cannot run it.

EQUILIBRIUM_PHASES 1
Brucite 0   0
Hematite 0  0
Portlandite 0  0
C3S   0   60.6
C2S   0   30.3
C3A    0  9.09
C4AF  0    2.02

 


I am also a little concerned about your naming conventions (this is cement terminology I assume)  I assume you have phase names such as C3S,  for a Tri Calcium Silicate solids,  defined  in the PHASES block.  But, and David Parkhurst might want to chime in here,  there might be some confusion using a single capital letter C, which is carbon and followed by a capital letter S for silica, when S represents Sulfur in most databases.  F I assume is iron, but  to the database it is Fluoride.   So you may want to spell out the names to avoid confusion.  If they are defined as phases I think you are OK but be careful.  Check what the model is actually reading.  Capital letters tend to be read as Elements.   

Sol-Chan Han:
Thank you for Dr. David L Parkhurst and John Mahoney.

However, still, I have a remained question.

I wonder that How the Reaction data block works.
The attached file below is the database what I used. There is no SiO2, Al2O, SO3 etc in the data block as phases.
However following code works well (part of my code). Can you give me any comment that how the Phreeqc recognize the SO3 etc in the Reaction data block? How the Phreeqc dissolves the SiO2 and SO3 to the solution without any thermodynamic data?

USE SOLUTION 1
REACTION 1
SiO2      0.4
Al2O3   0.01
Fe2O3   0.02
CaO      0.7
MgO      0.05
SO3      0.04
Na2O   0.02
36.28292124 moles

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