Dear David Parkhurst, > I am sure you get asked a lot of questions regarding your PHREEQC program, 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 system. 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 David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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