# PHREEQC Input Files — Mg/Ca-Acetate and -Citrate Solubility Model

## Context

These files were used to model the solubility of Mg- and Ca-acetate and -citrate phases during the dissolution of MgO and CaO in acetic acid and citric acid solutions. They support the geochemical model shown in **Fig. 1**, **Fig. 2**, **Fig. A1** and the phase data in **Table A1** of the research paper for:

> Pettauer, M., Baldermann, A., Eder, S., Dietzel, M. — *Acetic and citric acid control on oxide hydration in MgO-CaO-H2O systems*. Submitted to *Cement and Concrete Research*.

The simulated concentration series correspond to the validation experiments reported in the same supplement (Tables A2–A3): acetate experiments MgA1–MgA3 / CaA1–CaA3, and citrate experiments MgC1–MgC3 / CaC1–CaC3.

## Contents

| File | Description |
|---|---|
| `MeOrg26.dat` | PHREEQC database used by all four input files |
| `Ace_Ca.pqi` | CaO titrated into acetate solution (3 concentrations) |
| `Ace_Mg.pqi` | MgO titrated into acetate solution (3 concentrations) |
| `Cit_Ca.pqi` | CaO titrated into citrate solution (3 concentrations) |
| `Cit_Mg.pqi` | MgO titrated into citrate solution (3 concentrations) |

## Database: `MeOrg26.dat`

Based on the standard PHREEQC database `phreeqc.dat` (pressure-dependence variant, including molal volumes of aqueous species and minerals and critical temperatures/pressures for gases). Custom additions relative to the original database:

- **Master/solution species** added: `Citrate` (Citrate-3), `Acetate` (Acetate-), `Malate` (Malate-2), with aqueous diffusion coefficients from aqion.de and complexation constants (with H+, Ca2+, Mg2+, Na+) from MINTEQ.v4 / NIST46.
- **Custom PHASES** for the Me-organic-acid salts, with log K (25 °C) and PHREEQC analytical-expression coefficients fitted to literature solubility data (reproduced in Table A1 of the supplement):

  | Phase | Reaction | log K (25 °C) | Source |
  |---|---|---|---|
  | `Mg-Cit-9Hyd` | Mg3(Cit)2·9H2O ↔ 3Mg2+ + 2Cit3- + 9H2O | −13.96 | Apelblat (2014) |
  | `Mg-Cit-14Hyd` | Mg3(Cit)2·14H2O ↔ 3Mg2+ + 2Cit3- + 14H2O | −14.05 | Apelblat (2014) |
  | `Ca-Cit` | Ca3(Cit)2·4H2O ↔ 3Ca2+ + 2Cit3- + 4H2O | −17.59 | Apelblat (2014) |
  | `Mg-Ace` | Mg(Ace)2·4H2O ↔ Mg2+ + 2Ace- + 4H2O | −1.01 | Apelblat (1993) |
  | `Ca-Ace` | Ca(Ace)2·0.9H2O ↔ Ca2+ + 2Ace- + 0.9H2O | −1.27 | Saury et al. (1993) |

  `Brucite` and `Portlandite` (from the parent database) are used as the competing hydroxide phases.

  Full references:
  - Saury, C., Boistelle, R., Dalemat, F., Bruggeman, J. (1993). Solubilities of calcium acetates in the temperature range 0–100 °C. *J. Chem. Eng. Data* 38, 56–59.
  - Apelblat, A. (1993). Solubilities of organic salts of magnesium, calcium, and iron in water. *J. Chem. Thermodyn.* 25, 1443–1445.
  - Apelblat, A., Manzurola, E. (1999). Solubilities of magnesium, calcium, barium, cobalt, nickel, copper, and zinc acetates in water from 278.15 to 348.15 K. *J. Chem. Thermodyn.* 31, 1347–1357.
  - Apelblat, A. (2014). *Citric Acid*. Springer International Publishing, Cham.

**Note:** each `.pqi` file's `DATABASE` line points to the original local path used during development (`...\20251005_phreeqc_database.dat`). This is the same database, provided here as `MeOrg26.dat`; the path must be edited to the local location of `MeOrg26.dat` before running.

## Input files: `Ace_Ca.pqi`, `Ace_Mg.pqi`, `Cit_Ca.pqi`, `Cit_Mg.pqi`

Each file contains three stacked simulation blocks (separated by `END`), one per initial ligand concentration:

- `Ace_Ca.pqi` / `Ace_Mg.pqi`: 10, 14, 18 mmol/kgw acetate (↔ CaA1–CaA3 / MgA1–MgA3)
- `Cit_Ca.pqi` / `Cit_Mg.pqi`: 0.5, 1.0, 1.5 mmol/kgw citrate (↔ CaC1–CaC3 / MgC1–MgC3)

Each block has the same structure:

1. `SOLUTION 1` — acetate or citrate solution, pH 7, 25 °C.
2. `REACTION 1` — stepwise addition of CaO or MgO (0–14 mol over 500 steps), simulating progressive oxide dissolution/titration.
3. `EQUILIBRIUM_PHASES 1` — allows precipitation (precipitate-only) of the relevant Me-organic-acid salt and the competing hydroxide (Portlandite or Brucite) once saturation is reached.
4. `USER_GRAPH 1–6` — plot, as a function of pH or reaction progress: dissolved Me (graph 1), saturation indices of the salt and hydroxide phase (graph 2), precipitated phase amounts (graph 3), pH vs. reaction progress (graph 4), solution density vs. reaction progress (graph 5), and dissolved Me and ligand together (graph 6). These reproduce the modelled curves compared against measured data in Fig. A1.

## Running the files

Open in PHREEQC Interactive (or PHREEQC for Windows) with `MeOrg26.dat` as the database (update the `DATABASE` path in each `.pqi` file first). The `USER_GRAPH` blocks require the graphical PHREEQC interface to display plots; running via the command-line `phreeqc` executable will compute the simulations but not render the graphs.