Abnormal pH value in the dissolution of magnesia(MgO)

[Hennessy_1]

I had set a database(in attachment), which only including magnesia(MgO) and water(H2O). 1 mol MgO seperated into 1000 steps and put into 1 kg water(298.15K, pH=7) gradually, then I find a terrible consequence that the pH value of the water was increased from 7.0 to 13.78 (Since the pH value of saturated magnesium hydroxide is 11， approximately）! The data block in PHREEQC is listed as below:

SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg

REACTION 1
    MgO        1
    1 moles in 1000 steps

SELECTED_OUTPUT 1
    -file                 selected_output_pH.sel
    -pH                   true

END

I haven't found where is wrong, waiting for your help, thanks!

[dlparkhurst]

You assumed that a mineral would precipitate, but that won't happen unless you specify EQUILIBRIUM_PHASES. Here is your file modified also to include Brucite from the wateq4f.dat database. It is more stable than your phase, but you will have to evaluate log Ks for yourself. If you want to consider brucite, uncomment it in the EQUILIBRIUM_PHASES data block.

PHASES
Brucite             19
        Mg(OH)2 + 2H+ = Mg+2 + 2H2O
        log_k           16.84
        delta_h -27.1 kcal

SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    1 # kg

REACTION 1
    MgO        1
    .1 moles in 1000 steps

EQUILIBRIUM_PHASES 1
#    Brucite   0 0
    MgO       0 0

USER_GRAPH 1
    -headings               Rxn Mg pH
    -axis_titles            "MgO reacted, moles" "Mg, mol/kgw" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y TOT("Mg")
30 GRAPH_SY -LA("H+")
  -end
    -active                 true

END

[Hennessy_1]

Thanks for your reminding! The pH value of MgO dissolution is back to normal, but the pH value of my database is still abnormal and one kind of the main hydration phase(K-Struvite) is not generated. Since many solid inorganic salts was included in my database,  I will check my thermodynamic data (log_k, Gibbs free energy and enthalpy ). Thanks a lot, sincerely!