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Re: Alkalinity input

Dear David Parkhurst,

> I am sure you get asked a lot of questions regarding your PHREEQC
but having read the manual cover to cover

I have to admire your resolve and feel obligated to help you out.

> Above the Ca/Si ratio of 1 then the solution becomes equilibrated with
Portlandite (corresponding pH of ?12.55 ±0.1). Whenever I try to input this

using EQUILIBRIUM_PHASES the percent error on charge balancing is a
staggering 89-90%. I know there are limitations in PHREEQC regarding highly

alkaline solutions where Alkalinity isn't specified as (CO3)2-, though in
this case I have done everything possible to try and keep CO2 out of the

The problems with high ionic strength should be in poor estimates of
activity coefficients and activities of aqueous species, but mole balance
and charge balance should still obtain. In the absence of SURFACEs, the
charge balance for solutions is set by the original definition in SOLUTION.
The charge imbalance calculated in the initial solution calculation is
maintained through all "reaction" calculations. Equilibrating a solution
with a set of EQUILIBRIUM_PHASES is one type of reaction calculation. So I
think the charge imbalance is in your SOLUTION definition, not in the
subsequent equilibration with portlandite. Also the Alkalinity definition
only affects the initial solution calculation and after that mole balance
on carbon is used from which alkalinity is calculated.

If you send me your input file (and database file if it is not one of the
databases distributed with the program), I'll take a look at it and try to
explain what is going on.

> Is there a way of specifying that the Alkalinity is a function of
the dissolution of Tobermorite-14A for Ca/Si < 1.0,  1.0 < Portlandite +
Tobermorite < 1.4 and Afwillite + Portlandite > 1.4. Or maybe using (CO3)2-

as a buffer, but not including it in any reactions? Thanks in advance for
any comments or suggestions you may have.

Again, I'll take a look at your input file, but I'm not sure about the
conceptual model for your simulations. Several ways I can think of are: (1)
add NaOH to adjust the pH and look at what the stable phases should be, (2)
artificially fix the Al/Si ratio and look at the stable phases, (3) treat
the CSH as a solid solution of some kind, (4) add the CSH components
incrementally and look at the stable phases, or something completely
different. If you can outline where you are going with this calculation,
I'll give you some ideas about how to set it up.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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