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[MIS13-02] Geochemical modeling for rock-cement system with secondary phase nucleation
Keywords:rock-cement system, secondary phase, nucleation, geochemical model
1. Context
On the necessity to maintain the waste repository to be safe, we often use engineered or natural rock barriers surrounding the facility containing waste. If these barriers may suffer from interaction with cementitious materials, porewater can be changed to be high pH and in chemically undersaturated with respect to the original minerals. Then, the system is sooner or later saturated with secondary phases. However, the present geochemical models work with the mechanisms of dissolution kinetics [1][2] and immediate precipitation. Previous model for the rock-cement system reproduced this situation on the basis of thermodynamic equilibrium and kinetic-laws. For improving model to more realistic, crystal growth kinetics along with nucleation [3] is newly incorporated.
2. Experimental and theoretical
In order to incorporate classical nucleation theory into the model, interfacial free energy of minerals needs to be measured. Wetting angle measurements of high-pH solution were carried out on the candidate minerals: montmorillonite, phillipsite, clinoptilolite, analcime, laumontite, anorthite, albite, labradorite, K-feldspar and quartz. Surface free energy of the solutions, gL was calculated by hanging droplet method (Fig. 1a). Using these data, interfacial free energies, gSL were calculated. It was confirmed that the gSL of each mineral varies as a function of solubility as LogQ = Logk - npH (Q: ionic activity product) [4] (Fig. 1b).
3. Chemical simulation with nucleation
In the previous investigation [4], nucleation experiment of phillipsite from montmorillonite gave a specific size of critical nuclei as comprised of one to several unit cells based on gSL, nucleation rate. If the each nuclei sizes are appropriately assumed, critical supersaturation as Log(Q/k’) = LogQ - Logk’ which is always less than that of normal equilibrium model due to large Logk’ (i.e., higher solubility before nucleation), suggesting delayed precipitation. For incorporation of classical nucleation theory into the geochemical code, PHREEQC supersaturation needs to be switched to effective supersaturation as function of gSL, Q and normal Logk.
3. Discussion
Obtained results of geochemical simulation was applied for alteration of bed-rock in contact with cementitious material as a realistic and reasonably non-conservative modeling. The calculation should have exposed that delayed alteration as accompanied with zeolite nucleation cannot be ignored to consider precise modeling. Thus, thermodynamics and kinetics improvement may enable us to verify and validate the weathering and alteration which are interests in the early Earth’s environment and concerns for safety of the waste disposal facility.
References:
[1] Lasaga, A.C. (1998) Kinetic Theory in the Earth Sciences (Princeton Series in Geochemistry). [2] Savage, D. et al. (2011) Physics and Chemistry of the Earth 36, 1817–1829. [3] Steefel, C.I. and Cappellen, P.V. (1990) Geochimica et Cosmochimica Acta Vol. 54, 2651-2611. [4] Owada, H. et al. (2018) 2nd AW CEBAMA 319.
Fig. 1 Wetting angle measurement by photomicrography (a) and resultant plot of solubility vs. interfacial free energy for the potential secondary phases at pH12.6 and 13.7 (b).
On the necessity to maintain the waste repository to be safe, we often use engineered or natural rock barriers surrounding the facility containing waste. If these barriers may suffer from interaction with cementitious materials, porewater can be changed to be high pH and in chemically undersaturated with respect to the original minerals. Then, the system is sooner or later saturated with secondary phases. However, the present geochemical models work with the mechanisms of dissolution kinetics [1][2] and immediate precipitation. Previous model for the rock-cement system reproduced this situation on the basis of thermodynamic equilibrium and kinetic-laws. For improving model to more realistic, crystal growth kinetics along with nucleation [3] is newly incorporated.
2. Experimental and theoretical
In order to incorporate classical nucleation theory into the model, interfacial free energy of minerals needs to be measured. Wetting angle measurements of high-pH solution were carried out on the candidate minerals: montmorillonite, phillipsite, clinoptilolite, analcime, laumontite, anorthite, albite, labradorite, K-feldspar and quartz. Surface free energy of the solutions, gL was calculated by hanging droplet method (Fig. 1a). Using these data, interfacial free energies, gSL were calculated. It was confirmed that the gSL of each mineral varies as a function of solubility as LogQ = Logk - npH (Q: ionic activity product) [4] (Fig. 1b).
3. Chemical simulation with nucleation
In the previous investigation [4], nucleation experiment of phillipsite from montmorillonite gave a specific size of critical nuclei as comprised of one to several unit cells based on gSL, nucleation rate. If the each nuclei sizes are appropriately assumed, critical supersaturation as Log(Q/k’) = LogQ - Logk’ which is always less than that of normal equilibrium model due to large Logk’ (i.e., higher solubility before nucleation), suggesting delayed precipitation. For incorporation of classical nucleation theory into the geochemical code, PHREEQC supersaturation needs to be switched to effective supersaturation as function of gSL, Q and normal Logk.
3. Discussion
Obtained results of geochemical simulation was applied for alteration of bed-rock in contact with cementitious material as a realistic and reasonably non-conservative modeling. The calculation should have exposed that delayed alteration as accompanied with zeolite nucleation cannot be ignored to consider precise modeling. Thus, thermodynamics and kinetics improvement may enable us to verify and validate the weathering and alteration which are interests in the early Earth’s environment and concerns for safety of the waste disposal facility.
References:
[1] Lasaga, A.C. (1998) Kinetic Theory in the Earth Sciences (Princeton Series in Geochemistry). [2] Savage, D. et al. (2011) Physics and Chemistry of the Earth 36, 1817–1829. [3] Steefel, C.I. and Cappellen, P.V. (1990) Geochimica et Cosmochimica Acta Vol. 54, 2651-2611. [4] Owada, H. et al. (2018) 2nd AW CEBAMA 319.
Fig. 1 Wetting angle measurement by photomicrography (a) and resultant plot of solubility vs. interfacial free energy for the potential secondary phases at pH12.6 and 13.7 (b).