Modelling of low-pH cement degradation in a KBS-3 HLNW repository - - PowerPoint PPT Presentation

▶
modelling of low ph cement degradation in a kbs 3 hlnw
SMART_READER_LITE
LIVE PREVIEW

Modelling of low-pH cement degradation in a KBS-3 HLNW repository - - PowerPoint PPT Presentation

Dec 18, 2022 27 likes •270 views

Modelling of low-pH cement degradation in a KBS-3 HLNW repository F. Grandia, J. Salas, J. Molinero, D. Arcos AMPHOS XXI Consulting Motivation Low-pH cements. Why to use low-pH cements in radioactive waste repositories: Aqueous

slide-1
SLIDE 1

Modelling of low-pH cement degradation in a KBS-3 HLNW repository

  • F. Grandia, J. Salas, J. Molinero, D. Arcos

AMPHOS XXI Consulting

slide-2
SLIDE 2

Low-pH cements.

  • Why to use low-pH cements in radioactive waste repositories:
  • Aqueous speciation of silicon at pH>10 enhances solubility of clay barrier.
  • Low-pH cements may supply 50% less hydroxyls than conventional OPC.

Motivation

SFR LLW, Sweden

Bentonite blocks Pre-fabricated concrete beams Concrete plug Drainage Grouting pipes Filter material / Crushed rock Backfill

Tunnel plugs in HLNW repository, Sweden

slide-3
SLIDE 3

Modelling cement degradation.

  • Predictive modelling of the cement (CSH gels) dissolution is required to

evaluate the pH evolution of porewater.

Treatment of CSH → Pure phases vs. Solid solutions Kinetics of CSH → Rates of precipitation/dissolution of intermediate phases Diffusion coefficients in cement porewater Secondary precipitates → Ettringite, calcite, silica, …

Conceptual and data uncertainties

Open issues

slide-4
SLIDE 4

CSH dissolution/precipitation approaches.

  • A number of approaches have been developed to implement the

incongruent dissolution of cements in reactive transport codes.

  • Local equilibrium approach 1. Thermodynamic equilibrium with pure solid

phases.

  • Dissolution (sometimes using kinetic laws) of CSH-like crystalline phases (tobermorite,

jennite, …) and precipitation of secondary phases.

  • Flaws: inability to model incongruent dissolution.

CSH behaviour approaches

slide-5
SLIDE 5

CSH dissolution approaches

  • Local equilibrium approach 2. Thermodynamic equilibrium with solid

solutions.

  • Dissolution of CSH phases with initial specified Ca/Si ratio. Arbitrary end

members, not necessarily present in the system. Formation of new CSH with different Ca/Si ratio. Ability to reproduce incongruent dissolution using non- ideal SS.

  • Flaws: Instantaneous re-equilibration of the SS with the fluid (Nernst-Berthelot

approach).

CSH behaviour approaches

slide-6
SLIDE 6

CSH dissolution approaches

  • Kinetic precipitation/dissolution of CSH solid solutions (Lichtner and

Carey, 2007).

  • Implementation of the solid solution theory but using a discrete number of

intermediate solids. Dissolution/precipitation is governed by (irreversible) kinetics (Doerner and Hoskins approach). Incongruent dissolution using non- ideality terms.

  • Flaws: Lack of kinetic data for many CSH phases.

CSH behaviour approaches

slide-7
SLIDE 7

Examples: CSH dissolution using non-ideaI solid solutions.

  • A classic example of this kind of approach is found in Berner (1988, 1990

and 1992). End members Dependence of K

  • n solid

composition

CSH behaviour approaches

slide-8
SLIDE 8

Examples: CSH dissolution using non-ideaI solid solutions.

  • Variable end members depending on Ca/Si ratios in CSH.
  • For Ca/Si>1.5, portlandite diss. controls the chemistry.
  • For Ca/Si>1, portlandite and CaH2SiO4 have commonly been selected as

end members of solid solution (Berner, 1990; Börjesson et al., 1997).

  • For Ca/Si<1, different SS models have been proposed with different end

member: CaH2SiO4 – SiO2 (Berner, 1992).

  • For Ca/Si>1.5 to <1, Ca(OH)2-SiO2 (Sugiyama & Fujita, 2006 and Carey

& Litchner, 2007).

CSH behaviour approaches

Low-pH cements

slide-9
SLIDE 9

Implementation of CSH dissolution using non-ideaI solid solutions in reactive transport codes.

  • Models covering the whole Ca/Si

→ Test the low-pH cement alteration

→ Solid solution end-members: Ca(OH)2 and SiO2

  • Model of Sugiyama & Fujita (2006)
  • Model of Carey & Lichtner (2007)

[ ] [ ] [ ]

2 1 2 2 2

,...., ) ( , ) ( SiO SiO OH Ca OH Ca

x x −

⋅

CSH approach comparison

slide-10
SLIDE 10

CSH approach comparison

Data from Greenberg & Chang (1965) and Chen et al. (2004) in a Lippmann diagram.

  • 7
  • 6
  • 5
  • 4
  • 3
  • 2

0.2 0.4 0.6 0.8 1

XSiO2, Si(aq) Log([aCa2+*aOH-

2]+ aSiO2(aq))

Chen et al. (2004)_solidus Chen et al. (2004)_solutus Greenberg and Chang (1965)_ solidus Greenberg and Chang (1965)_ solutus

Alyotropic point? Low-pH cements

2.8 1.5 1.0 0.66 0.43 0.25

Ca/Si

slide-11
SLIDE 11

CSH approach comparison

Non-ideal SS. Carey & Lichtner (2007). Non-ideality parametres: a0= -29.67, a1= 0.28, a2= -0.0032

  • 7
  • 6
  • 5
  • 4
  • 3
  • 2

0.2 0.4 0.6 0.8 1

XSiO2, Si(aq) Log([aCa2+*aOH-

2]+ aSiO2(aq))

Solidus_Lichtner & Carey (2007) Solutus__Lichtner & Carey (2007) Chen et al. (2004)_solidus Chen et al. (2004)_solutus Greenberg and Chang (1965)_ solidus Greenberg and Chang (1965)_ solutus

Low-pH cements

2.8 1.5 1.0 0.66 0.43 0.25

Ca/Si

slide-12
SLIDE 12

CSH approach comparison

Non-ideal SS. Sugiyama & Fujita (2006). Conditional solubility constants

  • 7
  • 6
  • 5
  • 4
  • 3
  • 2

0.2 0.4 0.6 0.8 1

XSiO2, Si(aq) Log([aCa2+*aOH-

2]+ aSiO2(aq))

Solidus_Lichtner & Carey (2007) Solutus__Lichtner & Carey (2007) Solidus_ Sugiyama & Fujita (2006) Chen et al. (2004)_solidus Chen et al. (2004)_solutus Greenberg and Chang (1965)_ solidus Greenberg and Chang (1965)_ solutus

Low-pH cements

2.8 1.5 1.0 0.66 0.43 0.25

Ca/Si

slide-13
SLIDE 13

CSH approach comparison

Calculated logK:

  • 8
  • 7
  • 6
  • 5
  • 4
  • 3
  • 2
  • 1

0.4 0.8 1.2 1.6 2 2.4 2.8

Ca/Si Log Ksp CSH

Sugiyama & Fujita (2006) Lichtner & Carey (2007)

slide-14
SLIDE 14

Cement degradation model: The system CSH degradation: Reactive transport modelling

  • 1D
  • Granitic, diluted water (pH=7.9; I=2.6×10-2M)
  • Non-reactive backfill
  • Initial CSH composition: 50% volume, Ca/Si=2.85
  • Molar volume: 160 cm3/mol
  • Porosity: 12.5%

Backfill Backfill Cement

30 cm

slide-15
SLIDE 15

Cement degradation model: The code Numerical tool

  • RCB (Saaltink et al., 2005) → RETRASO + CodeBright

Main capabilities:

  • Multiphase flow modelling (liquid and/or gas).
  • Heat flow modelling.
  • Simulation of solute transport by advection, dispersion and diffusion in gas

and liquid phase.

  • Simulation of chemical reactions, including solid solutions.
  • Simulation of the effects of precipitation and dissolution of mineral phases on

porosity and permeability.

Reactive transport of solutes Multiphase flow and thermomechanics

slide-16
SLIDE 16

Cement degradation model: The code Numerical tool

  • RETACO (Saaltink et al., 2005) → RETRASO + CodeBright

Mineral dissolution/precipitation is treated following kinetic laws.

Uncertainties: dissolution/precipitation rates for CSH, molar volumes for intermediate solid solutions, diffusion coefficients, ...

( )

mk mk s mki k

m N i P i N k mk m a m m m

a k RT E r

η θ

ζ σ 1 exp

1 1 ,

− Ω ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ =

∏ ∑

= =

reactive area activation energy IAP/K

slide-17
SLIDE 17

Cement degradation model: Comparison CSH degradation: Carey & Lichtner (2007). Results

Backfill Backfill Cement

8 9 10 11 12 13 14 0.5 1 1.5 2 2.5

C/S ratio pH

Greenberg and Chang (1965) Chen et al. 2004 Carey & Lichtner (2007)

1E-07 1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 1E+00 9 10 11 12 13

pH [Si] (mol dm-3)

Greenberg and Chang Chen et al. 2004 Carey & Lichtner (2007)

0.5 1 1.5 2 2.5 0.000 0.005 0.010 0.015 0.020 0.025

Ca (M) C/S ratio

Greenberg and Chang (1965) Chen et al. 2004 Carey & Lichtner (2007)

Experimental data from Chen et al. (2004) and Greenberg and Chang (1965)

Low-pH cements

slide-18
SLIDE 18

Cement degradation model: Comparison CSH degradation: Sugita & Fujiyama (2006). Results

Backfill Backfill Cement

1E-06 1E-05 1E-04 1E-03 1E-02 1E-01 9 10 11 12 13

pH [Ca] (mol dm-3)

Greenberg & Chang (1965) Chen et al. (2004) Sugiyama & Fujita (2006) Sugiyama & Fujita (2006)_Cc

0.5 1 1.5 2 2.5 0.000 0.005 0.010 0.015 0.020 0.025

Ca (M) C/S ratio

Harris et al. (2002) Sugiyama & Fujita (2006) Sugiyama & Fujita (2006)_Cc

Experimental data from Chen et al. (2004) and from Greenberg and Chan (1965) Experimental data from Harris et al. (2002)

Cc↓ Cc↓

9 10 11 12 13 0.5 1 1.5 2 2.5 3

C/S ratio pH

Initial Ca/Si=2.7 Initial Ca/Si=1.6 Initial Ca/Si=1.4 Initial Ca/Si=1.1 Initial Ca/Si=0.90 Initial Ca/Si=0.81 Initial Ca/Si=0.76 Initial Ca/Si=0.72 Sugiyama & Fujita (2006)

Experimental data from Harris et al. (2002) Low-pH cements

slide-19
SLIDE 19

Cement degradation model: Comparison CSH degradation: Carey & Lichtner (2007). Results

  • The Carey & Lichtner approach reproduces well the degradation of CSH

in the Ca/Si range from 3 to 1.

  • At lower ratios, the model does not fit much with experimental data, as

already suggested by the Lippmann diagrams.

CSH degradation: Sugiyama & Fujita (2006). Results

  • The Sugiyama and Fujita (2006) approach reproduces well the changes

in CSH composition in the range of Ca/Si<1, which are characteristic of low pH cements.

  • Aqueous calcium seems to be overpredicted in the simulations compared

with experimental data. Including precipitation of calcite, the fit is better. However, it is not clear the precipitation of this mineral during the experiments.

slide-20
SLIDE 20

Cement degradation model: Implementation CSH degradation: Carey & Lichtner (2007). Effect of porosity changes on hydraulic properties.

  • Uncertainties:

→ Which are the molar volumes of the intermediate CSH phases? → And the reactive areas?

The final results from modelling are strongly dependent on these parameters.

slide-21
SLIDE 21

CSH molar volumes

Decreasing Ca/Si ratio lead to increasing molar volumes. Is this meaning that CSH degradation lead to reducing porosity? Ca leaching let to a a decrease of net volume in the reaction!!! But there is still an unknown on the behaviour of CSH gels. We are considering a single value for the molar volume of different CSH phases: 160 cm3/mol

100 120 140 160 180 200 220 240 260 280 300 0.0 0.5 1.0 1.5 2.0 2.5 Ca/Si Molar volume (cm3/mol)

Hillebrandite Afwillite Foshagite Xonotlite Tobermorite

  • 30
  • 25
  • 20
  • 15
  • 10
  • 5

0.0 0.5 1.0 1.5 2.0 2.5 Ca/Si Net volume increase (cm3)

Hillebrandite Afwillite Foshagite Xonotlite Tobermorite

slide-22
SLIDE 22

Cement degradation: porosity changes CSH degradation: Carey & Lichtner (2007). Effect of porosity changes on hydraulic properties.

Backfill Backfill Cement

12 13 14 15 16 17 12 13 14 15 16 17

12.49 12.50 12.51 12.52 12.53 12.54 1 2 3 4 5 6 7 8 9 10

Time (years) Porosity

No updated porosity Updated porosity

slide-23
SLIDE 23

1 18 6 9 12

Backfill Backfill Cement

Cement degradation: CSH behaviour

2 4 6 8 10 12 14 2 4 6 8 10

Distance to interface (cm) % Volume dissolved CSH (Ca/Si=2.85)

Updated porosity No updated porosity

2 4 6 8 10 12 14 2 4 6 8 10

Distance to interface (cm) % Volume precipitated CSH (Ca/Si=1.94)

The net CSH pool results in a larger dissolution when porosity has not been updates in transport processes

slide-24
SLIDE 24

Preliminary conclusions

Final remarks

  • Among the CSH dissolution/precipitation approaches, the ones from

Sugiyama & Fujita (2006) and Carey & Litchner (2007) are the most consistent from thermodynamic and experimental point of view.

  • The approach from Sugiyama & Fujita (2006) seems to better reproduce

experimental data for low-pH cement.

  • Porosity updates in reactive transport models is very relevant and can

result in substantial errors if not considered, despite the large uncertainty in CSH properties (molar volumes and reactive surface).

  • Further work is envisaged to consider the long term evolution of low-pH

cements in the KBS-3 repository by considering the presented approach.