**Modelling of Chemical Alteration of Cement Materials in Radioactive Waste Repository Environment**

Daisuke Sugiyama

*Nuclear Technology Research Laboratory, Central Research Institute of Electric Power Industry Japan* 

### **1. Introduction**

Cement is a potential waste packaging, backfilling and constructing material for the disposal of radioactive waste. The physical properties of cement materials such as low permeability and low diffusivity in their matrices reduce the migration of radionuclides from a cementitious repository. Also, under a high-pH condition provided by the leaching of the components of cement hydrates, the solubility is low and the sorption distribution ratio is high for many radionuclides, so that the release of radionuclides from radioactive waste is restricted. Therefore, cement materials are expected to enable both the physical and chemical containments of long-term radioactive waste in disposal systems (TRU Coordination Office, 2000).

Under geological conditions, cement materials alter due to various reactions such as dissolution into groundwater and secondary mineral formation caused by chemical components in groundwater. The containment properties of cement materials are affected by these reactions. Also, the leached high-pH solution with alkaline components from the cement materials affects the physical and chemical properties of bentonite and the surrounding rocks. Therefore, for the long-term safety assessment of radioactive waste disposal, it is necessary to develop a methodology to estimate the long-term evolution of the cementitious repository system. The chemistry of the CaO-SiO2-H2O (C-S-H) system is a key parameter since it is suggested to be responsible for the high-pH condition in cements and is important in discussing the high-pH chemical condition in the long-term assessment of a repository environment (Atkinson et al., 1985; Atkinson, 1985). The author therefore has been developing a series of predictive calculation models based on a discussion of the incongruent dissolution/precipitation of the C-S-H system (Sugiyama & Fujita, 2006; Sugiyama et al., 2007; Sugiyama, 2008).

In this study, the alteration of cement materials in an underground repository environment is discussed. Cement materials come in contact with groundwater and some secondary minerals are expected to precipitate in the repository environment. The precipitation of calcite and its effects on the alteration of cement materials should be key issues in assessing the long-term performance of cement materials. There have been some experimental studies

Modelling of Chemical

Alteration of Cement Materials in Radioactive Waste Repository Environment 75

s s0 2 s0 s1 s2 1 11 1 1 log log log 1 11 <sup>1</sup> 1 1

c c0 2 c0 c1 c2 1 1 log log log 1 11 <sup>1</sup> 1 1

 *y yy K K AA A y yy y y y*

 *y y yy y y K K AA A*

*y* = Ca/Si ratio in C-S-H, *K*s0 log 2.710 , *K*c0 log 22.81 (Bond et al., 1997).

At Ca/Si 0.461, log log *KK y* s s0 log 1 .

At 1.755 Ca/Si, *K*<sup>s</sup> log 7.853 , *K*<sup>c</sup> log 22.81 .

The fitted empirical parameters (*A*ij) are shown in Table 1. This model can predict the equilibria of the incongruent dissolution and precipitation with a continuous change in the Ca/Si ratio of the solid phase by a series of calculations, in which the quantities of the dissolved/precipitated end members are calculated stepwisely, so that the quantities and compositions of the solid and liquid phases and the conditional solubility constants used in the next step can be estimated (Sugiyama & Fujita, 2006). CCT-P contains the geochemical code PHREEQE (Parkhurst et al., 1980) to calculate the chemical equilibrium, and the C-S-H model is employed to calculate the incongruent dissolution and precipitation by iterative calculations, because the simplicity of its numerical description allows its inclusion in chemical equilibrium calculations based on the common approach of using the law of mass

Fig. 1 (a) shows a schematic representation of the transport model used in CCT-P. To calculate the mass transport in the porous media, a one-dimensional advection

( , ) ( , ) { ( , ) ( , ) ( , )} } ( , ) { ( , ) *<sup>S</sup> <sup>x</sup> <sup>t</sup>*

 

*t*

),

*x <sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>V</sup>*

*Seq* : source term given by chemical equilibrium calculation within matrix,

*<sup>t</sup> <sup>R</sup> <sup>t</sup> <sup>K</sup> d d*

 ( ) <sup>1</sup> ( ) ( ) <sup>1</sup>

*x eq <sup>d</sup> <sup>e</sup> <sup>d</sup>*

*t x t R x t C x t*

action (log *K*) in PHREEQE (Parkhurst et al., 1980).

/dispersion/diffusion equation is employed in CCT-P:

*x*

*<sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>D</sup> <sup>x</sup> <sup>t</sup>*

*De* : effective diffusion coefficient in matrix,

*Rd* : retardation factor (

*C*: concentration of aqueous species,

*Vd* : velocity of flow in matrix,

*Kd* : distribution coefficient.

where

*t* : time, 

: porosity,

: density,

 

*y yy y y y*

 

'' '

'' '

2

2

, (1)

, (2)

, (3)

on these issues, in which the precipitation of calcite from calcium bicarbonate solution passing through cracks in concrete and the leaching behaviour of components from cementitious materials were observed (Harris et al., 1998; Glasser et al., 2001). It was observed that calcite is mostly precipitated on the surface of the cracks, filling the cracks, and it was suggested that the thin layers of low-porosity calcite produced act as a diffusion barrier limiting contact between the cement and the solution (Harris et al., 1998; Glasser et al., 2001). Brodersen (2003) simulated the obtained experimental results using the CRACK2 computation model. Also, Lagneau and van der Lee (2005) simulated the clogging effect due to the precipitation of secondary minerals using the HYTEC code and showed that the precipitation leads to a reduction in porosity, which reduces the diffusive migration rate of chemical species. Burnol et al. (2005) discussed the precipitation behaviour of some minerals at a concrete/clay interface using various reactive transport codes. Marty et al. (2009) investigated the modelling of concrete/clay interactions under a geological disposal condition and discussed an occlusion due to the secondary mineral precipitation including calcite. However, the incongruent dissolution of the calcium-silicate hydrate (C-S-H) phase, which is one of the principle components of cementitious materials, was not sufficiently discussed in the previous modelling studies. Brodersen (2003) disregarded the leaching of silicic acid, and in the other studies, only three C-S-H phases (C-S-H 1.8, C-S-H 1.1 and C-S-H 0.8 (Lagneau & van der Lee, 2005; Burnol et al., 2005), and C-S-H 1.6, C-S-H 1.2 and C-S-H 0.8 (Marty et al., 2009) were included discretely in their modelling.

In this study, a reactive transport computational code, in which a geochemical model including the thermodynamic incongruent dissolution model of C-S-H (Sugiyama & Fujita, 2006) is coupled with the advection-diffusion/dispersion equation including the evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components, has been developed. A series of experiments on the alteration of hydrated ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash (FAC) monoliths in deionised water and sodium bicarbonate (NaHCO3) solution were carried out and the model was optimised on the basis of the observations. Using the developed code, some implications for the long-term performance assessment of radioactive waste disposal systems were discussed.

#### **2. Development of calculation code CCT-P**

#### **2.1 Model description**

A coupling transport and chemical equilibrium calculation code, coupled chemical equilibria-mass transport code for porous media (CCT-P), is developed to predict the alteration behaviour of cement materials. The calculations using CCT-P are performed with a two-step procedure involving a nonreactive transport step (using Eqs. (3)-(8)) followed by a chemical equilibrium calculation. All thermodynamic modelling calculations were carried out using the thermodynamic database HATCHES (Bond et al., 1997) Ver. NEA15.

The thermodynamic incongruent dissolution model of C-S-H, proposed by Sugiyama & Fujita (2006), is employed in the modelling approach. In this model, C-S-H is assumed to be a binary nonideal solid solution of Ca(OH)2 and SiO2, and the log *K* values of the model end members of the solid solution are given as functions of the Ca/Si ratio of C-S-H (Sugiyama & Fujita, 2006):

$$\log K\_s = \frac{1}{1+y} \cdot \log K\_{s0} - \frac{1}{1+y} \cdot \log \frac{1}{1+y} + \frac{y}{\left(1+y\right)^2} \cdot \left[ A\_{s0}^{'} + A\_{s1}^{'} \left(\frac{1-y}{1+y}\right) + A\_{s2}^{'} \left(\frac{1-y}{1+y}\right)^2 \right],\tag{1}$$

$$\log K\_c = \frac{y}{1+y} \cdot \log K\_{c0} - \frac{y}{1+y} \cdot \log \frac{y}{1+y} + \frac{y}{\left(1+y\right)^2} \cdot \left[ A\_{c0}^{'} + A\_{c1}^{'} \left(\frac{1-y}{1+y}\right) + A\_{c2}^{'} \left(\frac{1-y}{1+y}\right)^2 \right], \tag{2}$$

*y* = Ca/Si ratio in C-S-H, *K*s0 log 2.710 , *K*c0 log 22.81 (Bond et al., 1997).

At Ca/Si 0.461, log log *KK y* s s0 log 1 .

$$\text{At } 1.755 \le \text{Ca/Si}, \text{ } \log K\_{\text{s}} = -7.853 \text{ } , \log K\_{\text{c}} = 22.81 \dots$$

The fitted empirical parameters (*A*ij) are shown in Table 1. This model can predict the equilibria of the incongruent dissolution and precipitation with a continuous change in the Ca/Si ratio of the solid phase by a series of calculations, in which the quantities of the dissolved/precipitated end members are calculated stepwisely, so that the quantities and compositions of the solid and liquid phases and the conditional solubility constants used in the next step can be estimated (Sugiyama & Fujita, 2006). CCT-P contains the geochemical code PHREEQE (Parkhurst et al., 1980) to calculate the chemical equilibrium, and the C-S-H model is employed to calculate the incongruent dissolution and precipitation by iterative calculations, because the simplicity of its numerical description allows its inclusion in chemical equilibrium calculations based on the common approach of using the law of mass action (log *K*) in PHREEQE (Parkhurst et al., 1980).

Fig. 1 (a) shows a schematic representation of the transport model used in CCT-P. To calculate the mass transport in the porous media, a one-dimensional advection /dispersion/diffusion equation is employed in CCT-P:

$$\frac{\partial}{\partial \mathbf{x}} \{ D\_{\epsilon}(\mathbf{x}, t) \cdot \frac{\partial \mathbf{C}(\mathbf{x}, t)}{\partial \mathbf{x}} \} - V\_{d} \cdot \frac{\partial \mathbf{C}(\mathbf{x}, t)}{\partial \mathbf{x}} = \frac{\partial \{ \phi(\mathbf{x}, t) \cdot R\_{d}(\mathbf{x}, t) \cdot \mathbf{C}(\mathbf{x}, t) \}}{\partial t} - S\_{eq}(\mathbf{x}, t) \,, \tag{3}$$

where

74 Municipal and Industrial Waste Disposal

on these issues, in which the precipitation of calcite from calcium bicarbonate solution passing through cracks in concrete and the leaching behaviour of components from cementitious materials were observed (Harris et al., 1998; Glasser et al., 2001). It was observed that calcite is mostly precipitated on the surface of the cracks, filling the cracks, and it was suggested that the thin layers of low-porosity calcite produced act as a diffusion barrier limiting contact between the cement and the solution (Harris et al., 1998; Glasser et al., 2001). Brodersen (2003) simulated the obtained experimental results using the CRACK2 computation model. Also, Lagneau and van der Lee (2005) simulated the clogging effect due to the precipitation of secondary minerals using the HYTEC code and showed that the precipitation leads to a reduction in porosity, which reduces the diffusive migration rate of chemical species. Burnol et al. (2005) discussed the precipitation behaviour of some minerals at a concrete/clay interface using various reactive transport codes. Marty et al. (2009) investigated the modelling of concrete/clay interactions under a geological disposal condition and discussed an occlusion due to the secondary mineral precipitation including calcite. However, the incongruent dissolution of the calcium-silicate hydrate (C-S-H) phase, which is one of the principle components of cementitious materials, was not sufficiently discussed in the previous modelling studies. Brodersen (2003) disregarded the leaching of silicic acid, and in the other studies, only three C-S-H phases (C-S-H 1.8, C-S-H 1.1 and C-S-H 0.8 (Lagneau & van der Lee, 2005; Burnol et al., 2005), and C-S-H 1.6, C-S-H 1.2 and C-S-H

In this study, a reactive transport computational code, in which a geochemical model including the thermodynamic incongruent dissolution model of C-S-H (Sugiyama & Fujita, 2006) is coupled with the advection-diffusion/dispersion equation including the evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components, has been developed. A series of experiments on the alteration of hydrated ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash (FAC) monoliths in deionised water and sodium bicarbonate (NaHCO3) solution were carried out and the model was optimised on the basis of the observations. Using the developed code, some implications for the long-term performance assessment of radioactive

A coupling transport and chemical equilibrium calculation code, coupled chemical equilibria-mass transport code for porous media (CCT-P), is developed to predict the alteration behaviour of cement materials. The calculations using CCT-P are performed with a two-step procedure involving a nonreactive transport step (using Eqs. (3)-(8)) followed by a chemical equilibrium calculation. All thermodynamic modelling calculations were carried

The thermodynamic incongruent dissolution model of C-S-H, proposed by Sugiyama & Fujita (2006), is employed in the modelling approach. In this model, C-S-H is assumed to be a binary nonideal solid solution of Ca(OH)2 and SiO2, and the log *K* values of the model end members of the solid solution are given as functions of the Ca/Si ratio of C-S-H (Sugiyama

out using the thermodynamic database HATCHES (Bond et al., 1997) Ver. NEA15.

0.8 (Marty et al., 2009) were included discretely in their modelling.

waste disposal systems were discussed.

**2.1 Model description** 

& Fujita, 2006):

**2. Development of calculation code CCT-P** 

*C*: concentration of aqueous species,

*t* : time,

: porosity,

*Vd* : velocity of flow in matrix,

*De* : effective diffusion coefficient in matrix,

*Seq* : source term given by chemical equilibrium calculation within matrix,

$$R\_d \text{: retardation factor} \left( R\_d(t) = 1 + \rho \cdot K\_d \cdot \left( \frac{1 - \phi(t)}{\phi(t)} \right) \right),$$

: density,

*Kd* : distribution coefficient.

Modelling of Chemical

*Vsolid*: volume of solid phase,

diffusion coefficient.

*J*: flux of aqueous species,

where

respectively.

*CS*: molarity of component mineral,

where

Alteration of Cement Materials in Radioactive Waste Repository Environment 77

( ) ( ) <sup>1</sup> (1 (0)) *<sup>V</sup>*

In CCT-P, the diffusion coefficient in the altered region of the solid matrix can be described

 (0)

In this study, n = 2 (Haga et al., 2005) is assumed when calculating the evolution of the

At the boundaries of the regions (Fig. 1 (a)), the advection/dispersion/diffusion equations

as a function of porosity using formulas based on the study by Haga et al. (2005):

*moli Vsolid <sup>t</sup> CSi <sup>t</sup> <sup>v</sup> <sup>V</sup>* , :

*t*

*i solid*

*vmol*: molar volume of component mineral (*vmol* Ca(OH)2 = 0.0331 dm3 mol-1,

*vmol* SiO2 = 0.0273 dm3 mol-1, *vmol* CaCO3 = 0.0369 dm3 mol-1), *Vsolid,static*: volume of insoluble residual solid phase.

(Eq. (3)) in adjacent regions are connected as follows:

(0)

*solid static*

( ) ( ) , (5)

*D(t) D(0)* . (6)

*inner outer J J* (7)

, (4)

*V t*

*solid solid*

*<sup>n</sup> <sup>t</sup>* 

( ) 

( , ) , ( , ) ( , ) *dinner boundary inner*

( , ) . ( , ) ( , ) *douter boundary outer*

*inner <sup>e</sup> inner <sup>V</sup> <sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>x</sup> <sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>J</sup> <sup>D</sup> <sup>x</sup> <sup>t</sup>*

*outer <sup>e</sup> outer <sup>V</sup> <sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>x</sup> <sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>J</sup> <sup>D</sup> <sup>x</sup> <sup>t</sup>*

The subscripts *inner* and *outer* denote the values in the adjacent inner and outer cells,

In this study, the effects of the precipitation of calcite on the alteration of cement material are discussed. In the experiment, it was observed that calcite was mostly precipitated on the surface of the cement monolith, and the thin layers of low-porosity calcite produced acted as a diffusion barrier limiting contact between the solid cement and the solution (see subsection 4.1). Calcite precipitated in the pore solution of the cement solid matrix and also in the bulk solution near the interface between the cement solid and the solution. To describe this, a hypothetical reaction layer model is proposed and included in CCT-P, as shown in Fig. 1 (b). In this model, a less-soluble or insoluble secondary phase is formed in a


Table 1. Fitted values of empirical parameters (Sugiyama & Fujita, 2006)

(a) Schematic representation of transport model

(b) Schematic representation of hypothetical reaction layer model

The evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components is also considered in the calculation code. The component minerals in the solid are leached from its surface into the solution, and are precipitated on the surface and in the matrix of the solid. In this study, it is considered that the porosity of the solid matrix increases or decreases as the component minerals are dissolved and leached or precipitated, respectively. This is described by the following equations:

Fig. 1. Transport model in CCT-P

$$\phi(t) = 1 - (1 - \phi(0)) \cdot \frac{V\_{solid}(t)}{V\_{solid}(0)} \, ^\prime \tag{4}$$

$$V\_{solid}\left(\mathbf{f}\right) = \sum\_{i:solid} \mathbf{C} S\_i\left(\mathbf{f}\right) \cdot \boldsymbol{\upsilon}\_{mol\_i} + V\_{solid,static\\_\nu} \tag{5}$$

where

76 Municipal and Industrial Waste Disposal

*A*ij *A*s0 *A*s1 *A*s2 *A*c0 *A*c1 *A*c2 Ca/Si 0.833 -18.623 57.754 -58.241 37.019 -36.724 164.17 Ca/Si > 0.833 -18.656 49.712 25.033 36.937 -7.8302 -50.792

(a) Schematic representation of transport model

layer Cement solid

HCO3 -

Ca2+

(b) Schematic representation of hypothetical reaction layer model

The evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components is also considered in the calculation code. The component minerals in the solid are leached from its surface into the solution, and are precipitated on the surface and in the matrix of the solid. In this study, it is considered that the porosity of the solid matrix increases or decreases as the component minerals are dissolved and leached

or precipitated, respectively. This is described by the following equations:

(Ca(OH)2, C-S-H gel, CaCO3)

The porosity of the solid matrix in the near surface region decreases.

End member SiO2 Ca(OH)2

Table 1. Fitted values of empirical parameters (Sugiyama & Fujita, 2006)

Hypothetical reaction

CaCO3 precipitates

(NaHCO3)

Solution

HCO3 -

Ca2+

Fig. 1. Transport model in CCT-P

*Vsolid*: volume of solid phase,

*CS*: molarity of component mineral,

*vmol*: molar volume of component mineral (*vmol* Ca(OH)2 = 0.0331 dm3 mol-1,

*vmol* SiO2 = 0.0273 dm3 mol-1, *vmol* CaCO3 = 0.0369 dm3 mol-1),

*Vsolid,static*: volume of insoluble residual solid phase.

In CCT-P, the diffusion coefficient in the altered region of the solid matrix can be described as a function of porosity using formulas based on the study by Haga et al. (2005):

$$D(t) = D(0) \cdot \left(\frac{\phi(t)}{\phi(0)}\right)^n. \tag{6}$$

In this study, n = 2 (Haga et al., 2005) is assumed when calculating the evolution of the diffusion coefficient.

At the boundaries of the regions (Fig. 1 (a)), the advection/dispersion/diffusion equations (Eq. (3)) in adjacent regions are connected as follows:

$$J\_{inner} = J\_{outer} \tag{7}$$

where

*J*: flux of aqueous species,

$$\begin{aligned} \left. f \right|\_{\text{inner}} &= -D\_{\varepsilon} \left\langle \mathbf{x}, t \right\rangle\_{\text{inner}} \cdot \frac{\partial \mathbf{C} \left\langle \mathbf{x}, t \right\rangle}{\partial \mathbf{x}} \bigg|\_{\text{inner}} + V\_{d\_{\text{inner}}} \cdot \mathbf{C} \left\langle \mathbf{x}, t \right\rangle\_{\text{boundary}}, \\\\ \left. f \right|\_{\text{outer}} &= -D\_{\varepsilon} \left\langle \mathbf{x}, t \right\rangle\_{\text{outer}} \cdot \frac{\partial \mathbf{C} \left\langle \mathbf{x}, t \right\rangle}{\partial \mathbf{x}} \bigg|\_{\text{outer}} + V\_{d\_{\text{outer}}} \cdot \mathbf{C} \left\langle \mathbf{x}, t \right\rangle\_{\text{boundary}}. \end{aligned}$$

The subscripts *inner* and *outer* denote the values in the adjacent inner and outer cells, respectively.

In this study, the effects of the precipitation of calcite on the alteration of cement material are discussed. In the experiment, it was observed that calcite was mostly precipitated on the surface of the cement monolith, and the thin layers of low-porosity calcite produced acted as a diffusion barrier limiting contact between the solid cement and the solution (see subsection 4.1). Calcite precipitated in the pore solution of the cement solid matrix and also in the bulk solution near the interface between the cement solid and the solution. To describe this, a hypothetical reaction layer model is proposed and included in CCT-P, as shown in Fig. 1 (b). In this model, a less-soluble or insoluble secondary phase is formed in a

Modelling of Chemical

*a*j, *b*j, *c*j and *b*\*

*j j <sup>j</sup> <sup>B</sup> <sup>x</sup> <sup>A</sup> <sup>a</sup>* <sup>2</sup> <sup>1</sup>

*j j <sup>j</sup> <sup>B</sup> <sup>x</sup> <sup>A</sup> <sup>c</sup>* <sup>2</sup>

1 

*j d j e j <sup>j</sup> R D <sup>A</sup>* ,

*j d j*

*j d j <sup>j</sup>* 

*j d j*

*<sup>R</sup> CC*

*eq <sup>j</sup> <sup>D</sup> <sup>j</sup> <sup>R</sup> S* .

j are given as

<sup>1</sup> ) 1

*j*

*j j*

1

,

*j j*

,

*j*

1

{ } <sup>1</sup> <sup>1</sup>

,

*D D*

*<sup>j</sup> <sup>V</sup> <sup>x</sup>*

*n j*

superscript are the newest values.)

*<sup>R</sup> <sup>B</sup>*

*j e j e j*

> *<sup>o</sup> <sup>d</sup> <sup>j</sup> n j*

<sup>1</sup> ,

Upper boundary (Dirichlet boundary condition)

Upper boundary (Flux boundary condition)

*t R R*

Alteration of Cement Materials in Radioactive Waste Repository Environment 79

*j* = 2, . . . *N*-1. (N: total number of meshes)

Spatial discretization is performed by centered-in-space differencing and temporal discretization is performed by Crank-Nicholson (centered-in-time) differencing. *A, B, CC*

(The superscripts 'n' and 'o' denote the newest value and an old value obtained at *t*- *t* , respectively. Porosity, effective diffusion coefficient and retardation factor without

*b C c C d <sup>n</sup> <sup>n</sup>* <sup>1</sup> <sup>1</sup> <sup>1</sup> <sup>2</sup> <sup>1</sup> , (12)

( ) ( ) <sup>1</sup> <sup>1</sup> <sup>2</sup> <sup>1</sup> <sup>1</sup> <sup>2</sup> \* *d*<sup>1</sup> *a*<sup>1</sup> *C C b*1*C c C D x x <sup>o</sup> <sup>o</sup> <sup>o</sup> <sup>b</sup> <sup>n</sup> <sup>b</sup>* . (13)

For the boundary condition, the finite-difference equations are given as follows.

( ) <sup>1</sup> ( ) <sup>2</sup>

( ) <sup>1</sup> ( ) <sup>2</sup>

<sup>1</sup> <sup>1</sup> ( <sup>1</sup> *<sup>x</sup> <sup>x</sup>* <sup>1</sup> *<sup>t</sup> CC <sup>x</sup> <sup>x</sup> <sup>x</sup> <sup>x</sup> <sup>b</sup> <sup>A</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup>*

<sup>1</sup> ) <sup>1</sup> <sup>1</sup> ( <sup>1</sup> <sup>1</sup>

and *D* in Eq. (9) are also described as the differenced forms

*d*

*d o j*

*<sup>j</sup> <sup>j</sup>* 

\* *<sup>x</sup> <sup>x</sup> <sup>t</sup> CC <sup>x</sup> <sup>x</sup> <sup>x</sup> <sup>x</sup> <sup>b</sup> <sup>A</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup>*

*j j*

( ) <sup>1</sup> <sup>1</sup> \* *d a C* <sup>1</sup> *b C c C Dj x <sup>j</sup> x <sup>j</sup> <sup>o</sup> <sup>j</sup> <sup>j</sup> <sup>o</sup> <sup>j</sup> <sup>j</sup> <sup>o</sup> <sup>j</sup> <sup>j</sup> <sup>j</sup>* , (11)

,

.

hypothetical reaction layer and precipitates in the vicinity of the surface of a solid phase. A hypothetical reaction layer is added between the solid and solution phases (outside of the solid phase interested), and calcium ions leached from the solid cement react with bicarbonate ions in the hypothetical reaction layer to form calcite. The formed calcite precipitates on the surface of the solid cement and the porosity of the solid matrix in the near-surface region decreases, then the precipitated layer of low-porosity restricts the diffusion mass transfer. This is described by the following equation:

$$\left. -D\_{\varepsilon}(\mathbf{x}, t)\_{\varepsilon \text{current}} \cdot \frac{\partial \mathbf{C}(\mathbf{x}, t)}{\partial \mathbf{x}} \right|\_{\varepsilon \text{current}} = \left( \frac{\phi(t)}{\phi(0)} \right)\_{\text{surface}} \cdot \left\{ -D\_{\varepsilon}(\mathbf{x}, t)\_{\text{HRL}} \cdot \frac{\partial \mathbf{C}(\mathbf{x}, t)}{\partial \mathbf{x}} \right\}\_{\text{HRL}} \tag{8}$$

where the subscripts *cement*, *surface* and *HRL* denote the bulk cement region, the nearsurface layer of the cement region in which the porosity decreases and the hypothetical reaction layer, respectively. (*t*)surface is calculated using Eq. (4), assuming that all secondary precipitates formed in the HRL precipitate on the near-surface layer with a thickness of *Lshell* and contribute to adjust porosity. The thickness of the near-surface layer (*Lshell*) is provided as a parameter in CCT-P. Note that the code refers to (*t*)surface only in calculations using Eq. (8) connecting regions at the boundary. The term ((*t*)/(0))surface is used to describe that contact between the solid cement and the solution is limited by the calcite precipitate, which acts as a barrier.

#### **2.2 Numerical method**

The one-dimensional advection/dispersion/diffusion equation (Eq. (3)) employed in CCT-P is numerically solved by an implicit finite-difference method. Eq. (3) can be described in the generic form

$$\frac{\partial \mathbf{C}\_{\dot{I}}}{\partial t} = A \frac{\partial^2 \mathbf{C}\_{\dot{I}}}{\partial \mathbf{x}^2} - B \frac{\partial \mathbf{C}\_{\dot{I}}}{\partial \mathbf{x}} - \mathbf{C} \mathbf{C} \cdot \mathbf{C}\_{\dot{I}} + D \tag{9}$$

where

$$\begin{split} A &= \frac{D\_{\varepsilon}\left(\mathbf{x}\_{\prime},t\right)}{\phi\left(\mathbf{x}\_{\prime},t\right)\cdot\mathbf{R}\_{d}\left(\mathbf{x}\_{\prime},t\right)}, \\ B &= \frac{1}{\phi\left(\mathbf{x}\_{\prime},t\right)\cdot\mathbf{R}\_{d}\left(\mathbf{x}\_{\prime},t\right)} \{-\frac{\hat{\boldsymbol{\mathcal{O}}}\,\boldsymbol{D}\_{\varepsilon}\left(\mathbf{x}\_{\prime},t\right)}{\hat{\boldsymbol{\mathcal{O}}}\}} + V\_{d}\}, \\ CC &= \frac{1}{\phi\left(\mathbf{x}\_{\prime},t\right)\cdot\mathbf{R}\_{d}\left(\mathbf{x}\_{\prime},t\right)} \cdot \frac{\hat{\boldsymbol{\mathcal{O}}}\{\phi\left(\mathbf{x}\_{\prime},t\right)\cdot\mathbf{R}\_{d}\left(\mathbf{x}\_{\prime},t\right)\}}{\hat{\boldsymbol{\mathcal{O}}}}, \\ D &= \frac{S\_{\varepsilon\eta}\left(\mathbf{x}\_{\prime},t\right)}{\phi\left(\mathbf{x}\_{\prime},t\right)\cdot\mathbf{R}\_{d}\left(\mathbf{x}\_{\prime},t\right)}. \end{split}$$

The finite-difference equation for Eq. (9) is given as

$$-a\_{\rangle} \mathbf{C}\_{j-1}^{\prime \prime} + b\_{\rangle} \mathbf{C}\_{j}^{\prime \prime} - c\_{\rangle} \mathbf{C}\_{j+1}^{\prime \prime} = d\_{\rangle \prime} \tag{10}$$

$$d\_j = a\_j \mathbf{C}\_{j-1}^o + b\_j^\* \mathbf{C}\_j^o + c\_j \mathbf{C}\_{j+1}^o + D\_j \cdot \left(\boldsymbol{\Delta}\_{\mathcal{X}j} + \boldsymbol{\Delta}\_{\mathcal{X}j+1}\right) \tag{11}$$

*j* = 2, . . . *N*-1. (N: total number of meshes)

*a*j, *b*j, *c*j and *b*\* j are given as

78 Municipal and Industrial Waste Disposal

hypothetical reaction layer and precipitates in the vicinity of the surface of a solid phase. A hypothetical reaction layer is added between the solid and solution phases (outside of the solid phase interested), and calcium ions leached from the solid cement react with bicarbonate ions in the hypothetical reaction layer to form calcite. The formed calcite precipitates on the surface of the solid cement and the porosity of the solid matrix in the near-surface region decreases, then the precipitated layer of low-porosity restricts the

> 

where the subscripts *cement*, *surface* and *HRL* denote the bulk cement region, the nearsurface layer of the cement region in which the porosity decreases and the hypothetical

precipitates formed in the HRL precipitate on the near-surface layer with a thickness of *Lshell* and contribute to adjust porosity. The thickness of the near-surface layer (*Lshell*) is provided

contact between the solid cement and the solution is limited by the calcite precipitate, which

The one-dimensional advection/dispersion/diffusion equation (Eq. (3)) employed in CCT-P is numerically solved by an implicit finite-difference method. Eq. (3) can be described in the

> *Ci*

*x Ci <sup>B</sup>*

*x Ci <sup>A</sup>*

2 2

,

*t*

*d*

*V*

*t x t R x t* *t*

*<sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>D</sup> <sup>x</sup> <sup>t</sup>* ( , ) ( , ) (0)

 

*surface cement <sup>e</sup> cement x*

  

*CC Ci D*

*a C b C c C dj <sup>n</sup> <sup>j</sup> <sup>j</sup> <sup>n</sup> <sup>j</sup> <sup>j</sup> <sup>n</sup> <sup>j</sup> <sup>j</sup>*<sup>1</sup> <sup>1</sup> , (10)

(*t*)/

*e HRL*

(*t*)surface is calculated using Eq. (4), assuming that all secondary

*<sup>C</sup> <sup>x</sup> <sup>t</sup> <sup>D</sup> <sup>x</sup> <sup>t</sup>*

 

, (8)

*HRL*

(*t*)surface only in calculations using Eq.

(0))surface is used to describe that

, (9)

diffusion mass transfer. This is described by the following equation:

*x*

as a parameter in CCT-P. Note that the code refers to

(8) connecting regions at the boundary. The term (

( , ) ( ) ( , )

reaction layer, respectively.

acts as a barrier.

generic form

*<sup>D</sup> <sup>x</sup> <sup>t</sup> <sup>A</sup>*

*e* 

where

*D*

**2.2 Numerical method** 

( , ) ( , ) ( , ) *x t R x t*

1

*x t R x t*

,

*d*

*B*

*d*

*x t R x t CC <sup>d</sup>*

> ( , ) ( , ) ( , ) *x t R x t S x t*

.

*eq* 

*d*

( , ) ( , ) 1

} ( , ) { ( , ) ( , )

,

*<sup>d</sup>* { ( , ) ( , )}

*e*

The finite-difference equation for Eq. (9) is given as

*x D x t*

$$\begin{aligned} a\_{J} &= \frac{A\_{J}}{\Delta x\_{J}} - \frac{1}{2}B\_{j} \\ b\_{j} &= A\_{J} \cdot (\frac{1}{\Delta x\_{J}} + \frac{1}{\Delta x\_{J+1}}) + \frac{1}{2}CC\_{j} \cdot (\Delta x\_{J} + \Delta x\_{J+1}) + \frac{1}{\Delta t}(\Delta x\_{J} + \Delta x\_{J+1}) \\ c\_{J} &= \frac{A\_{J}}{\Delta x\_{J+1}} - \frac{1}{2}B\_{j} \\ b\_{j}^{\*} &= -A\_{J} \cdot (\frac{1}{\Delta\_{X\_{J}}} + \frac{1}{\Delta\_{X\_{J+1}}}) - \frac{1}{2}CC\_{j} \cdot (\Delta x\_{J} + \Delta x\_{J+1}) + \frac{1}{\Delta t}(\Delta x\_{J} + \Delta x\_{J+1}) \end{aligned}$$

Spatial discretization is performed by centered-in-space differencing and temporal discretization is performed by Crank-Nicholson (centered-in-time) differencing. *A, B, CC* and *D* in Eq. (9) are also described as the differenced forms

$$\begin{split} A\_{j} &= \frac{D\_{\mathcal{E}\_{j}}}{\phi\_{j} \cdot R\_{d} \underset{j}{}} \\ B\_{j} &= \frac{1}{\phi\_{j} \cdot R\_{d} \underset{j}{}} \{-\frac{D\_{\mathcal{E}\_{j} \cdot j + 1} - D\_{\mathcal{E}\_{j}}}{\Delta x\_{j}} + V\_{d} \} \\ CC\_{j} &= \frac{1}{\phi\_{j} \cdot R\_{d} \underset{j}{}} \cdot \frac{\phi\_{j}^{n} \cdot R\_{d} \underset{j}{}^{n} - \phi\_{j}^{o} \cdot R\_{d} \underset{j}{}^{o}}{\Delta t} \\ D\_{j} &= \frac{S\_{eq \ j}}{\phi\_{j} \cdot R\_{d \ \stackrel{j}{}}} \ . \end{split}$$

(The superscripts 'n' and 'o' denote the newest value and an old value obtained at *t*- *t* , respectively. Porosity, effective diffusion coefficient and retardation factor without superscript are the newest values.)

For the boundary condition, the finite-difference equations are given as follows.

Upper boundary (Dirichlet boundary condition)

$$b\_1 \mathbf{C}\_1^n = \mathbf{c}\_1 \mathbf{C}\_2^n = d\_1 \tag{12}$$

$$d\_1 = a\_1 \left(\mathbf{C}\_b^\bullet + \mathbf{C}\_b^\bullet\right) + b\_1^\bullet \mathbf{C}\_1^\bullet + c\_1 \mathbf{C}\_2^\bullet + D\_1 \cdot \left(\boldsymbol{\Delta} \,\mathbf{x}\_1 + \boldsymbol{\Delta} \,\mathbf{x}\_2\right). \tag{13}$$

Upper boundary (Flux boundary condition)

Modelling of Chemical

Oxide composition / wt%

cement sample

Sample Minerals

**3.2 Tank leaching experiment** 

Alteration of Cement Materials in Radioactive Waste Repository Environment 81

cement hydrate samples. In FAC hydrate, XRD identified katoite (Ca3Al2(SiO4)(OH)8) as a minor mineral and quartz (SiO2) as an unhydrated phase. The amount of Ca(OH)2 in the hydrated solid samples was quantified by DTA and is shown in Table 3. The porosity of the solid matrix of cement samples and the pore size distribution were measured by mercury

Hardened OPC and FAC after curing were cut into 20 20 10 mm blocks using a diamond cutter. The cement monolith samples were set in the acrylic cells shown in Fig. 3 with a circular window of 12 mm diameter and placed in contact with the solution. The exposure of only one of the faces of each monolith to the aqueous solution in the alteration (leaching/precipitation) experiments simplifies the subsequent analysis of the distribution of components in the cement monolith. Each cell containing a cement monolith was placed in a vessel to which 80 cm3 of deionised water or NaHCO3 solution at room temperature was added. The initial concentrations of NaHCO3 were 6 × 10-5, 1 × 10-4, 6 × 10-4, 1 × 10-3, and 6 × 10-3 mol dm-3. The solution and solid samples were separated and the monolith samples were again placed in contact with fresh deionised water or NaHCO3 solution after 1 week and then every 4-5 weeks.

The concentrations of calcium and silica in each separated solution were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) after passing it through a 0.45 µm membrane filter. Each filtered solution was acidified and diluted with known volumes of nitric acid (HNO3) solution and distilled water before ICP-AES analysis to ensure that the concentrations of the target and matrix elements were appropriate for the analysis. The carbonate ion concentration in the solution was measured by ion chromatography. The surface of the monolith sample after 76 weeks was observed by scanning electron microscopy (SEM) and the cross section of the solid was analysed by energy-dispersive X-ray analysis (EDX

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O

Quarts, Katoite 4.0 28.9

Ca(OH)2 / wt% (Measured by DTA)

Porosity / % (Measured by mercury intrusion porosimetry)

OPC 21.4 5.4 2.7 64.8 1.5 2.1 0.3 0.5 FAC 32.9 9.5 3.8 46.9 1.2 1.9 0.8 0.6

OPC Ca(OH)2, C-S-H gel, Ettringite 17.4 12.0

Table 3. Analytical results of XRD, DTA and mercury intrusion porosimetry of hydrated

intrusion porosimetry, and these data are shown in Table 3 and Fig. 2.

All experiments were prepared in nitrogen-filled glovebox in triplicates.

analysis) to observe the distribution of components in a cement matrix.

Table 2. Oxide composition of cement clinker used in experiment

(Identified by XRD)

FAC Ca(OH)2, C-S-H gel, Ettringite,

$$(b\_1 - a\_1 \frac{\overline{D\_{e,i}}}{\Delta \underline{\chi\_1}})C\_1^n - c\_1 C\_2^n = d\_1 \,\tag{14}$$

$$\mathbf{a}\_{d1} = a\_1 \frac{J\_b^u + J\_b^o}{\upsilon u\_b + \frac{D\_{e,i}}{\Delta \mathbf{x} 1}} + \left[ \frac{a\_1 \frac{D\_{e,i}}{\Delta \mathbf{x}\_1}}{\upsilon u\_b + \frac{D\_{e,i}}{\Delta \mathbf{x} 1}} + b\_1^\* \right] \mathbf{C}\_1^o + a\_1 \mathbf{C}\_2^o + D\_1 \cdot \left\{ \Delta\_{X1} + \Delta\_{X2} \right\} \tag{15}$$

Lower boundary (Dirichlet boundary condition)

$$b - a\_{N-1} \mathbb{C}\_{N-2}^{\prime} + b\_{N-1} \mathbb{C}\_{N-1}^{\prime} = d\_{N-1} \, \prime \tag{16}$$

$$d\_{N-1} = c\_{N-1} \left( \mathbf{C}\_b^n + \mathbf{C}\_b^o \right) + a\_{N-1} \mathbf{C}\_{N-2}^n + b\_{N-1}^\* \mathbf{C}\_{N-1}^o + D\_{N-1} \cdot \left( \boldsymbol{\Lambda} \boldsymbol{\chi}\_{N-1} + \boldsymbol{\Lambda} \boldsymbol{\chi}\_N \right). \tag{17}$$

Lower boundary (Neumann boundary condition)

$$-a\_{N-1} \mathbf{C}\_{N-2}^{\rm n} + (b\_{N-1} - c\_{N-1}) \mathbf{C}\_{N-1}^{\rm n} = d\_{N-1} \,\tag{18}$$

$$d\_{N-1} = a\_{N-1} \mathbb{C}\_{N-2}^{o} + \left(b\_{N-1}^{\*} + c\_{N-1}\right) \mathbb{C}\_{N-1}^{o} + D\_{N-1} \cdot \left(\Lambda\_{XN-1} + \Lambda\_{XN}\right) \,. \tag{19}$$

At the boundaries of the regions, the advection/dispersion/diffusion equations in adjacent regions are connected using the following finite-diffusion equation:

$$-\frac{D\_{\mathcal{E},j-1}}{\Delta \mathbf{x}\_{j-1}} \mathbf{C}\_{j-1}^{u} + \left(\frac{D\_{\mathcal{E},j-1}}{\Delta \mathbf{x}\_{j}} + \frac{D\_{\mathcal{E},j}}{\Delta \mathbf{x}\_{j+1}} - \upsilon\_{\mathcal{U},j,\text{inner}} + \upsilon\_{\mathcal{U},\text{outer}}\right) \mathbf{C}\_{j}^{u} - \frac{D\_{\mathcal{E},j}}{\Delta \mathbf{x}\_{j+1}} \mathbf{C}\_{j+1}^{u}$$

$$=\frac{D\_{\mathcal{E},j-1}}{\Delta \mathbf{x}\_{j}} \mathbf{C}\_{j-1}^{o} - \left(\frac{D\_{\mathcal{E},j-1}}{\Delta \mathbf{x}\_{j}} + \frac{D\_{\mathcal{E},j}}{\Delta \mathbf{x}\_{j+1}} - \upsilon\_{\mathcal{U},\text{inner}} + \upsilon\_{\mathcal{U},\text{outer}}\right) \mathbf{C}\_{j}^{o} - \frac{D\_{\mathcal{E},j}}{\Delta \mathbf{x}\_{j+1}} \mathbf{C}\_{j+1}^{o} \,. \tag{20}$$

In CCT-P, these equations are solved by the backward substitution method.

#### **3. Experimental**

#### **3.1 Materials**

A series of experiments on the alteration of hydrated cement monoliths in deionised water and sodium bicarbonate solution were carried out in this study. Solid monolith samples of ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash (FAC) were prepared for use in a so-called tank leaching experiment.

The chemical compositions of OPC and FAC used in this study are shown in Table 2. The cement was hydrated at a water/cement clinker mixing ratio of 0.35. The hydrated materials were then cured in water at 50 °C to enhance hydration (Taylor, 1997) and reduce the effects of unhydrated phases for 91 days.

The hydrated solid samples were analysed by X-ray diffraction (XRD). The results are shown in Table 3, and the predominant phases were portlandite, C-S-H gel and ettringite for all solid cement hydrate samples. In FAC hydrate, XRD identified katoite (Ca3Al2(SiO4)(OH)8) as a minor mineral and quartz (SiO2) as an unhydrated phase. The amount of Ca(OH)2 in the hydrated solid samples was quantified by DTA and is shown in Table 3. The porosity of the solid matrix of cement samples and the pore size distribution were measured by mercury intrusion porosimetry, and these data are shown in Table 3 and Fig. 2.

#### **3.2 Tank leaching experiment**

80 Municipal and Industrial Waste Disposal

*x D*

1 , 1 , 1

1 , 1 , <sup>1</sup> <sup>1</sup> ( ) 

, <sup>1</sup> <sup>1</sup> *<sup>b</sup> <sup>C</sup> <sup>a</sup> <sup>C</sup> <sup>D</sup> <sup>x</sup> <sup>x</sup> x <sup>D</sup> <sup>v</sup> x <sup>D</sup> <sup>a</sup>*

( ) ( ) <sup>1</sup> <sup>1</sup> <sup>1</sup> \* *d* <sup>1</sup> *c* <sup>1</sup> *C C a* <sup>1</sup>*C* <sup>2</sup> *b* <sup>1</sup>*C DN xN xN <sup>o</sup> <sup>N</sup> <sup>N</sup> <sup>n</sup> <sup>N</sup> <sup>N</sup> <sup>o</sup>*

*a C b c C dN <sup>n</sup>*

) ( ) ( <sup>1</sup> <sup>1</sup> <sup>1</sup> <sup>1</sup> \* *d* <sup>1</sup> *a* <sup>1</sup>*C* <sup>2</sup> *b* <sup>1</sup> *c C DN xN xN <sup>o</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>o</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup>* . (19)

At the boundaries of the regions, the advection/dispersion/diffusion equations in adjacent

*D <sup>n</sup>*

*D <sup>o</sup>*

, <sup>1</sup> ( )

, <sup>1</sup> ( )

A series of experiments on the alteration of hydrated cement monoliths in deionised water and sodium bicarbonate solution were carried out in this study. Solid monolith samples of ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash

The chemical compositions of OPC and FAC used in this study are shown in Table 2. The cement was hydrated at a water/cement clinker mixing ratio of 0.35. The hydrated materials were then cured in water at 50 °C to enhance hydration (Taylor, 1997) and reduce the effects

The hydrated solid samples were analysed by X-ray diffraction (XRD). The results are shown in Table 3, and the predominant phases were portlandite, C-S-H gel and ettringite for all solid

1

*<sup>b</sup>* 

*x D*

*e i*

*e i*

*b a <sup>n</sup> <sup>n</sup> e i*

*v*

*<sup>J</sup> <sup>J</sup> <sup>d</sup> <sup>a</sup> <sup>o</sup> <sup>o</sup> <sup>e</sup> <sup>i</sup> db*

 

1

regions are connected using the following finite-diffusion equation:

*e j n j j e j*

*e j o j j e j*

1 1

1

 

 

*x D*

*x D*

(FAC) were prepared for use in a so-called tank leaching experiment.

*j*

*j*

*D*

*D*

, 1 ,

, 1 ,

In CCT-P, these equations are solved by the backward substitution method.

*j e j*

*j e j*

1

1

*x <sup>D</sup> <sup>v</sup>*

*<sup>e</sup> <sup>i</sup> db o b n*

Lower boundary (Dirichlet boundary condition)

Lower boundary (Neumann boundary condition)

*<sup>C</sup> <sup>x</sup>*

*<sup>C</sup> <sup>x</sup>*

**3. Experimental** 

of unhydrated phases for 91 days.

**3.1 Materials** 

*db*

*C c C d*

1 1 2 1

*<sup>b</sup> <sup>n</sup> <sup>N</sup> <sup>N</sup> <sup>b</sup>* . (17)

*<sup>D</sup> <sup>v</sup> <sup>v</sup> <sup>C</sup> <sup>x</sup>*

*<sup>D</sup> <sup>v</sup> <sup>v</sup> <sup>C</sup> <sup>x</sup>*

, ,

*j d j inner d j outer*

, ,

*j d j inner d j outer*

. (20)

( ) <sup>1</sup> <sup>1</sup> <sup>2</sup> <sup>1</sup> <sup>1</sup> <sup>2</sup> \*

*a C b C dN <sup>n</sup> <sup>N</sup> <sup>N</sup> <sup>n</sup> <sup>N</sup>*<sup>1</sup> *<sup>N</sup>*<sup>2</sup> <sup>1</sup> <sup>1</sup> <sup>1</sup> , (16)

*<sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>n</sup> <sup>N</sup>* <sup>1</sup> *<sup>N</sup>* <sup>2</sup> <sup>1</sup> <sup>1</sup> <sup>1</sup> <sup>1</sup> ( ) , (18)

*<sup>C</sup> <sup>x</sup>*

*j e j n*

*<sup>C</sup> <sup>x</sup>*

1 ,

*j e j o* *j*

*j*

1 1 ,

1

. (15)

, (14)

Hardened OPC and FAC after curing were cut into 20 20 10 mm blocks using a diamond cutter. The cement monolith samples were set in the acrylic cells shown in Fig. 3 with a circular window of 12 mm diameter and placed in contact with the solution. The exposure of only one of the faces of each monolith to the aqueous solution in the alteration (leaching/precipitation) experiments simplifies the subsequent analysis of the distribution of components in the cement monolith. Each cell containing a cement monolith was placed in a vessel to which 80 cm3 of deionised water or NaHCO3 solution at room temperature was added. The initial concentrations of NaHCO3 were 6 × 10-5, 1 × 10-4, 6 × 10-4, 1 × 10-3, and 6 × 10-3 mol dm-3. The solution and solid samples were separated and the monolith samples were again placed in contact with fresh deionised water or NaHCO3 solution after 1 week and then every 4-5 weeks. All experiments were prepared in nitrogen-filled glovebox in triplicates.

The concentrations of calcium and silica in each separated solution were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) after passing it through a 0.45 µm membrane filter. Each filtered solution was acidified and diluted with known volumes of nitric acid (HNO3) solution and distilled water before ICP-AES analysis to ensure that the concentrations of the target and matrix elements were appropriate for the analysis. The carbonate ion concentration in the solution was measured by ion chromatography. The surface of the monolith sample after 76 weeks was observed by scanning electron microscopy (SEM) and the cross section of the solid was analysed by energy-dispersive X-ray analysis (EDX analysis) to observe the distribution of components in a cement matrix.



Table 2. Oxide composition of cement clinker used in experiment

Table 3. Analytical results of XRD, DTA and mercury intrusion porosimetry of hydrated cement sample

Modelling of Chemical

3.0x10-3

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

3.0x10-3

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

0 20 40 60

0 20 40 60

concentrations of 1 × 10-3 and 6 × 10-3 mol dm-3.

Time [weeks]

Time [weeks]

Alteration of Cement Materials in Radioactive Waste Repository Environment 83

In the FAC experiments shown in Fig. 4 (b), the trend of calcium leaching was observed to be similar to that in OPC experiments; however, the rate of calcium leaching was lower in the cases of FAC than in the cases of OPC. FAC inhibited the leaching of calcium in 6 × 10-4 mol dm-3 NaHCO3 solution earlier than OPC. The leaching of calcium was inhibited from

(a) OPC

(b) FAC

For all the cases of alteration in NaHCO3 solution, a secondary crystalline precipitate was formed on the surface of the solids, as shown in Fig. 5. The precipitate was identified as calcite by XRD analysis. A denser calcite layer was formed at higher NaHCO3

Fig. 4. Measured and calculated amounts of calcium leached from cement monolith

NaHCO3 [mol dm-3] Measured Modelling

NaHCO3 [mol dm-3] Measured Modelling

 0 6e-5 1e-4 6e-4 1e-3 6e-3

 0 6e-5 1e-4 6e-4 1e-3 6e-3

the early stage of the experiment at the NaHCO3 concentration of > 1 × 10-3 mol dm-3.

(a) OPC (b) FAC

Fig. 2. Pore size distributions of cement monolith samples

Fig. 3. Experimental cell for cement alteration experiment

#### **4. Results**

#### **4.1 Cement monolith alteration experiments**

The amount of calcium leached from the OPC monolith sample was calculated using the measured aqueous composition and is shown in Fig. 4 (a) as a function of time. In deionised water, calcium was leached from the surface of the OPC solid monolith and the leaching rate decreased slightly with time. This result suggests that in the early stage of alteration, the dissolution equilibrium of portlandite (Ca(OH)2) dominated the leaching of calcium, and in the late stage, the incongruent dissolution of C-S-H gel in the altered surface region dominated the calcium leaching. In sodium bicarbonate solution at a NaHCO3 concentration higher than 6 × 10-4 mol dm-3, a reduction in the rate of calcium leaching was observed. As the concentration of NaHCO3 increased, the rate of calcium leaching decreased and was restricted significantly in the later stage of the experiment. The leaching of calcium was always inhibited at the NaHCO3 concentration of 6 × 10-3 mol dm-3 during the experiment.

0.0

0.0038 0.0121 0.0369 0.1210 0.6159 3.5439 19.284 270.05

Pore diameter(μm)

Fig. 2. Pore size distributions of cement monolith samples

Fig. 3. Experimental cell for cement alteration experiment

**4.1 Cement monolith alteration experiments** 

of 6 × 10-3 mol dm-3 during the experiment.

0.5

1.0

Slope(%)

(a) OPC (b) FAC

The amount of calcium leached from the OPC monolith sample was calculated using the measured aqueous composition and is shown in Fig. 4 (a) as a function of time. In deionised water, calcium was leached from the surface of the OPC solid monolith and the leaching rate decreased slightly with time. This result suggests that in the early stage of alteration, the dissolution equilibrium of portlandite (Ca(OH)2) dominated the leaching of calcium, and in the late stage, the incongruent dissolution of C-S-H gel in the altered surface region dominated the calcium leaching. In sodium bicarbonate solution at a NaHCO3 concentration higher than 6 × 10-4 mol dm-3, a reduction in the rate of calcium leaching was observed. As the concentration of NaHCO3 increased, the rate of calcium leaching decreased and was restricted significantly in the later stage of the experiment. The leaching of calcium was always inhibited at the NaHCO3 concentration

0

0

0.0038 0.0121 0.0369 0.1211 0.6137 3.5573 19.292 271.51

Pore diameter(μm)

1

2

Slope(%)

3

Slope (%) Porosity (%) 4

10

20

Porosity(%)

30

40

1.5

Slope (%) Porosity (%) 2.0

0

**4. Results** 

5

10

Porosity(%)

15

20

In the FAC experiments shown in Fig. 4 (b), the trend of calcium leaching was observed to be similar to that in OPC experiments; however, the rate of calcium leaching was lower in the cases of FAC than in the cases of OPC. FAC inhibited the leaching of calcium in 6 × 10-4 mol dm-3 NaHCO3 solution earlier than OPC. The leaching of calcium was inhibited from the early stage of the experiment at the NaHCO3 concentration of > 1 × 10-3 mol dm-3.

Fig. 4. Measured and calculated amounts of calcium leached from cement monolith

For all the cases of alteration in NaHCO3 solution, a secondary crystalline precipitate was formed on the surface of the solids, as shown in Fig. 5. The precipitate was identified as calcite by XRD analysis. A denser calcite layer was formed at higher NaHCO3 concentrations of 1 × 10-3 and 6 × 10-3 mol dm-3.

Modelling of Chemical

Alteration of Cement Materials in Radioactive Waste Repository Environment 85

 (f) FAC (in distilled water) (g) FAC (in 1 × 10-4 mol dm-3 NaHCO3)

 (h) FAC (in 6 × 10-4 mol dm-3 NaHCO3) (i) FAC (in 6 × 10-4 mol dm-3 NaHCO3) (Filmy layer of calcite precipitate)

(j) FAC (in 6 × 10-3 mol dm-3 NaHCO3)

Fig. 5. SEM observation on surface of cement monolith

(a) OPC (in distilled water) (b) OPC (in 1 × 10-4 mol dm-3 NaHCO3)


(e) OPC (in 6 × 10-3 mol dm-3 NaHCO3)

 (a) OPC (in distilled water) (b) OPC (in 1 × 10-4 mol dm-3 NaHCO3)

 (c) OPC (in 6 × 10-4 mol dm-3 NaHCO3) (d) OPC (in 6 × 10-4 mol dm-3 NaHCO3) (Filmy layer of calcite precipitate)

(e) OPC (in 6 × 10-3 mol dm-3 NaHCO3)

(f) FAC (in distilled water) (g) FAC (in 1 × 10-4 mol dm-3 NaHCO3)


(j) FAC (in 6 × 10-3 mol dm-3 NaHCO3)

Fig. 5. SEM observation on surface of cement monolith

Modelling of Chemical

the solution.

decreased.

For OPC:

Alteration of Cement Materials in Radioactive Waste Repository Environment 87

The calcium concentration at the cross section of the divided altered solid samples analysed by EDX analysis is shown in Fig. 6. In deionised water, calcium was leached from the surface and the Ca/Si ratio of the solid decreased in the surface region up to a depth of around 2 mm. In sodium bicarbonate solution at a higher NaHCO3 concentration of 6 × 10-4 mol dm-3, the region with decreasing Ca/Si ratio of the solid was small up to a depth of around 1.0 mm. In contrast, it was observed that a small amount of calcium leached in 6 × 10-3 mol dm-3 sodium bicarbonate solutions. In 6 × 10-3 mol dm-3 NaHCO3 solutions, a layer of accumulated calcium was formed at the vicinity of the surface of the solid, as shown in Figs. 6 (d) and (h). It is suggested that calcite precipitated on the surface of the monoliths in sodium bicarbonate solution and the secondary calcite precipitation concentrated in the near-surface layer. Thus, the secondary calcite precipitation restricted the leaching of calcium from the solid by limiting the contact between the solid cement and

The amount of calcite precipitated on the cement monolith sample was calculated using the measured carbonate ion concentration in the separated solution and is shown in Fig. 7 as a function of time. In < 6 × 10-4 mol dm-3 NaHCO3 solutions, more calcite precipitated at higher NaHCO3 concentrations. In the cases where a restriction of calcium leaching in > 6 × 10-4 mol dm-3 NaHCO3 solutions was observed, the calcite precipitation rate

The cement alteration experiments were simulated using the developed model and the CCT-P calculation code. First, the mineral compositions of the hydrated cements were derived by an approach based on that described by Glasser et al. (1987) with some modifications. Glasser et al. calculated the equilibrium phase distribution by considering the phases of Ca(OH)2, C-S-H, hydrotalcite and monosulphate (Glasser et al., 1987). The hydrated OPC and FAC in the experiments were dominantly composed of portlandite, C-S-H gel, ettringite and a small amount of other minerals, and the amount of portlandite was measured by DTA. Thus, in this study, the mineral compositions of the hydrated OPC and FAC were

**Step 1.** The mineral assembly in the hydrated OPC was described with portlandite

**Step 2.** The amount of portlandite was estimated by DTA of the hydrated OPC sample

**Step 3.** SiO2 was assumed to be taken up by the C-S-H gel with Ca/Si = 1.755 (Sugiyama &

**Step 4.** The remaining CaO was assumed to be taken up by ettringite. The remaining Al2O3

(Ca(OH)2), C-S-H, ettringite, brucite (Mg(OH)2), NaOH and KOH. Na2O, K2O and MgO were assumed to be completely hydrated to NaOH, KOH and brucite,

Fujita, 2006), and C-S-H was described using the model proposed by Sugiyama and

was assumed to be amorphous alumina gel or taken up by some phases (e.g., C-A-S-H gel), although no excess Al was included in the following modelling

**4.2 Modelling calculation of cement alteration experiment** 

calculated by the following approach.

used in the experiment in this study.

respectively.

Fujita (2006).

calculations.

(a) OPC (in distilled water) (b) OPC (in 6 × 10-4 mol dm-3 NaHCO3)

Surface of solid (contacting solution)

(c) OPC (in 6 × 10-3 mol dm-3 NaHCO3) (d) OPC (in 6 × 10-3 mol dm-3 NaHCO3) (Surface region)

Surface of solid (contacting solution)

(e) FAC (in distilled water) (f) FAC (in 6 × 10-4 mol dm-3 NaHCO3)

Surface of solid (contacting solution)

(g) FAC (in 6 × 10-3 mol dm-3 NaHCO3) (h) FAC (in 6 × 10-3 mol dm-3 NaHCO3) (Surface region)

Fig. 6. Calcium concentration profiles at cross section of altered cement sample after 76 weeks obtained by EDX analysis

The calcium concentration at the cross section of the divided altered solid samples analysed by EDX analysis is shown in Fig. 6. In deionised water, calcium was leached from the surface and the Ca/Si ratio of the solid decreased in the surface region up to a depth of around 2 mm. In sodium bicarbonate solution at a higher NaHCO3 concentration of 6 × 10-4 mol dm-3, the region with decreasing Ca/Si ratio of the solid was small up to a depth of around 1.0 mm. In contrast, it was observed that a small amount of calcium leached in 6 × 10-3 mol dm-3 sodium bicarbonate solutions. In 6 × 10-3 mol dm-3 NaHCO3 solutions, a layer of accumulated calcium was formed at the vicinity of the surface of the solid, as shown in Figs. 6 (d) and (h). It is suggested that calcite precipitated on the surface of the monoliths in sodium bicarbonate solution and the secondary calcite precipitation concentrated in the near-surface layer. Thus, the secondary calcite precipitation restricted the leaching of calcium from the solid by limiting the contact between the solid cement and the solution.

The amount of calcite precipitated on the cement monolith sample was calculated using the measured carbonate ion concentration in the separated solution and is shown in Fig. 7 as a function of time. In < 6 × 10-4 mol dm-3 NaHCO3 solutions, more calcite precipitated at higher NaHCO3 concentrations. In the cases where a restriction of calcium leaching in > 6 × 10-4 mol dm-3 NaHCO3 solutions was observed, the calcite precipitation rate decreased.

#### **4.2 Modelling calculation of cement alteration experiment**

The cement alteration experiments were simulated using the developed model and the CCT-P calculation code. First, the mineral compositions of the hydrated cements were derived by an approach based on that described by Glasser et al. (1987) with some modifications. Glasser et al. calculated the equilibrium phase distribution by considering the phases of Ca(OH)2, C-S-H, hydrotalcite and monosulphate (Glasser et al., 1987). The hydrated OPC and FAC in the experiments were dominantly composed of portlandite, C-S-H gel, ettringite and a small amount of other minerals, and the amount of portlandite was measured by DTA. Thus, in this study, the mineral compositions of the hydrated OPC and FAC were calculated by the following approach.

For OPC:

86 Municipal and Industrial Waste Disposal

(a) OPC (in distilled water) (b) OPC (in 6 × 10-4 mol dm-3 NaHCO3)

(c) OPC (in 6 × 10-3 mol dm-3 NaHCO3) (d) OPC (in 6 × 10-3 mol dm-3 NaHCO3) (Surface region)

(e) FAC (in distilled water) (f) FAC (in 6 × 10-4 mol dm-3 NaHCO3)

(g) FAC (in 6 × 10-3 mol dm-3 NaHCO3) (h) FAC (in 6 × 10-3 mol dm-3 NaHCO3) (Surface region)

Fig. 6. Calcium concentration profiles at cross section of altered cement sample after 76

Surface of solid (contacting solution)

Surface of solid (contacting solution)

Surface of solid (contacting solution)

Surface of solid (contacting solution)

Ca precipitation

Ca

Si

Ca

Si

Ca

Si

Ca precipitation

Ca

Si

Surface of solid (contacting solution)

Surface of solid (contacting solution)

Surface of solid (contacting solution)

Surface of solid (contacting solution)

weeks obtained by EDX analysis

Ca

Ca

Si

Ca

Si

Si

Ca

Si


Modelling of Chemical

Alteration of Cement Materials in Radioactive Waste Repository Environment 89

The mineral compositions of the OPC and FAC hydrates calculated by the method above are shown in Table 4. Then, the cement hydrates are simply described as the assembly of

The calculation system was defined using the parameters in Table 5 and the model in Fig. 8. A constant boundary condition was imposed at the surface of the solid, which described the experimental condition of the replacement of the solution, and at the other end, a closed boundary condition was prescribed. Diffusion transport was calculated and advection transport was not considered in this simulation. As described in subsection 2.1, the HRL was added at the boundary between the solid and the solution to simulate the calcite precipitation on the surface of the monolith in the model. The amount of calcium leached from the solid into the solution was calculated by integrating the flux of calcium at the surface of the solid (*x* = 0).

[mol kg-1]

OPC - - 0.28 0.29 2.35 2.76 0.07 0.08 FAC 1.65 0.72 0.25 0.07 0.54 2.27 0.21 0.10

**Solution**

**Solid cement monolith**

Thickness of region [mm] 0.3 0.3 Thickness of each grid layer [mm] 0.1 0.1

Thickness of region [mm] 10 10 Thickness of each grid layer [mm] 0.267 0.200 Initial *De* [m2 s-1] 1.9 × 10-11\*\* 7.4 × 10-12\*\* Initial porosity 0.120 0.289

Initial *De* [m2 s-1] 8.0 × 10-10\* 8.0 × 10-10\*

C-S-H gel

**Boundary condition:** *C* **(0,** *t***) =** *C***<sup>0</sup>**

**Boundary condition: d***C***/d***x* **= 0**

(Ca/Si = 1.755) NaOH KOH

Cement Quartz Katoite Brucite Ettringite Ca(OH)2

**Hypothetical reaction layer**

Hypothetical reaction layer (HRL)

Cement hydrate solid

Table 4. Calculated mineral compositions of cement hydrates

Fig. 8. Analysis model used for OPC alteration experiment

Cement OPC FAC

\* An infinite dilution diffusion coefficient for calcium was assumed in the calculation. \*\* The initial effective diffusion coefficient in the cement solid matrix was provided by fitting calculation using the leaching data of calcium from the cement monolith in distilled water. Table 5. Calculation parameters used for modelling OPC alteration experiment

Ca(OH)2 and C-S-H gel in this modelling study to reduce the load of calculation.

Fig. 7. Measured and calculated amounts of calcite precipitated on cement monolith

For FAC:


(a) OPC

(b) FAC

**Step 1.** The mineral assembly in the hydrated FAC was described with portlandite

**Step 2.** SO3 was assumed to be taken up by ettringite. The remaining Al2O3 was assumed to

**Step 3.** The remaining CaO was assumed to be taken up by portlandite and C-S-H gel with

by DTA of the hydrated FAC sample used in the experiment in this study. **Step 4.** SiO2 was assumed to be taken up by the C-S-H gel with Ca/Si = 1.755 (Sugiyama &

**Step 5.** The remaining SiO2 was assumed to be quartz, which is an unhydrated component

(Ca(OH)2), C-S-H, ettringite, brucite, katoite, NaOH, KOH and quartz. Na2O, K2O and MgO were assumed to be completely hydrated to NaOH, KOH and brucite,

Ca/Si = 1.755 (Sugiyama & Fujita, 2006). The amount of portlandite was estimated

Fig. 7. Measured and calculated amounts of calcite precipitated on cement monolith

NaHCO3 [mol dm-3

NaHCO3 [mol dm-3

 6e-5 1e-4 6e-4 1e-3 6e-3

 6e-5 1e-4 6e-4 1e-3 6e-3 ] Measured Modelling

] Measured Modelling

8x10-4

6

4

Precipitated C [mol cm-2

]

2

0

8x10-4

6

4

Precipitated C [mol cm-2

For FAC:

]

2

0

respectively.

Fujita, 2006).

in fly ash.

be taken up by katoite.

0 20 40 60 Time [weeks]

0 20 40 60 Time [weeks]

The mineral compositions of the OPC and FAC hydrates calculated by the method above are shown in Table 4. Then, the cement hydrates are simply described as the assembly of Ca(OH)2 and C-S-H gel in this modelling study to reduce the load of calculation.

The calculation system was defined using the parameters in Table 5 and the model in Fig. 8. A constant boundary condition was imposed at the surface of the solid, which described the experimental condition of the replacement of the solution, and at the other end, a closed boundary condition was prescribed. Diffusion transport was calculated and advection transport was not considered in this simulation. As described in subsection 2.1, the HRL was added at the boundary between the solid and the solution to simulate the calcite precipitation on the surface of the monolith in the model. The amount of calcium leached from the solid into the solution was calculated by integrating the flux of calcium at the surface of the solid (*x* = 0).



Fig. 8. Analysis model used for OPC alteration experiment


\* An infinite dilution diffusion coefficient for calcium was assumed in the calculation. \*\* The initial effective diffusion coefficient in the cement solid matrix was provided by fitting calculation using the leaching data of calcium from the cement monolith in distilled water.

Table 5. Calculation parameters used for modelling OPC alteration experiment

Modelling of Chemical

time.

Leached Ca [mol cm-2

]

3.0x10-3

 in Water in 6e-5M NaHCO3 in 1e-4M NaHCO3 in 6e-4M NaHCO3 in 1e-3M NaHCO3 in 6e-3M NaHCO3

Concentration of calcium [mol/L-solid]

2.5 2.0 1.5 1.0 0.5 0.0

0 20 40 60 Time [weeks]

Fig. 10. Sensitivity analysis of thickness of HRL

Ca in solid Calcite

5x10-3 0 1 2 3 4 Distance from surface [m]

after 76 weeks

 HRL [mm] 0 0.1 0.2 0.3 0.6 0.9 Measured

Alteration of Cement Materials in Radioactive Waste Repository Environment 91

given in Table 6. As shown in Fig. 4, the modelling calculations quantitatively well predicted the experimental results for the leaching of calcium. Figure 7 shows that calcite precipitation was predicted fairly well at lower NaHCO3 concentrations ( 1 × 10-4 mol dm-3), though it was underestimated at higher NaHCO3 concentrations. In the CCT-P calculation, the amount of precipitated calcite saturated after 10 ~ 20 weeks owing to clogging; however, in the experiments, calcite still precipitated even if the rate of precipitation decreased with

3.0x10-3

2.5 2.0 1.5  HRL [mm] 0 0.1 0.2 0.3 0.6 0.9 Measured

1.0

Leached Ca [mol cm-2

]

0.5 0.0

0 20 40 60 Time [weeks]

> in Water in 6e-5M NaHCO3 in 1e-4M NaHCO3 in 6e-4M NaHCO3 in 1e-3M NaHCO3 in 6e-3M NaHCO3

5x10-3 0 1 2 3 4 Distance from surface [m]

after 76 weeks

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Concentration of calcite [mol/L-solid]

Porosity

(a) OPC

0.5 0.4 0.3 0.2 0.1 0.0

(a) OPC (b) FAC

The spatial resolution and the initial effective diffusion coefficient were optimised by preliminary sensitivity calculations to reproduce the rate of calcium leaching in the experiments in distilled water. The fitted initial effective diffusion coefficients given in Table 5 are comparable with the values measured by Haga et al. (2005) and Yasuda et al. (2002). The time step size was given in consideration of a Neumann criterion (Marty et al., 2009; Sousa, 2003; Hindmarsh & Gresho, 1984):

$$\frac{2D\_p \cdot \Delta t}{\Delta x^2} \le 1,\tag{21}$$

where *Dp* (= *De*/) is the pore diffusion coefficient, and *x* and *t* refer to the mesh size in the solid region and the time step.

For modelling using the two-step procedure, it is suggested that the choice of the space discretization affects the calculation results (Marty et al., 2009). In this study, the sensitivity of calculation to the mesh size was analysed. The sensitivity calculation results are shown in Fig. 9. In the case of OPC (Fig. 9 (a)), coarser mesh sizes gave a lower rate of calcium leaching and finer mesh sizes degraded the stability of numerical solutions. The optimised mesh size in this case is 0.267 mm, which reproduced the measured result using a diffusion coefficient (1.9 × 10-11 m2 s-1) compatible with the experimentally determined value (Haga et al., 2005). In the case of FAC (Fig. 9 (b)), the calculation result was less sensitive to the space discretization, though a behaviour similar to that of OPC was observed.

Fig. 9. Sensitivity analysis of mesh size in solid region

The thickness of the HRL was optimised by sensitivity calculations given in Fig. 10. It is considered that a smaller HRL gives lesser disturbance to the calculation. The optimised HRL size was chosen to be 0.3 mm since too fine HRL could affect the stability of the numerical solution. It is noted that FAC calculation was insensitive to the HRL thickness (Fig. 10 (b)).

To model the alteration in the NaHCO3 solutions, the thickness of the near-surface layer (*Lshell*) must be determined. Parametric analysis was carried in this study and is discussed in subsection 5.1. In this subsection, the best-fit results are described with the optimised *Lshell*

The spatial resolution and the initial effective diffusion coefficient were optimised by preliminary sensitivity calculations to reproduce the rate of calcium leaching in the experiments in distilled water. The fitted initial effective diffusion coefficients given in Table 5 are comparable with the values measured by Haga et al. (2005) and Yasuda et al. (2002). The time step size was given in consideration of a Neumann criterion (Marty et al., 2009;

2

discretization, though a behaviour similar to that of OPC was observed.

 Mesh size [mm] 0.800 0.400 0.267 0.200 0.160 0.133 

Measured

(a) OPC (b) FAC

The thickness of the HRL was optimised by sensitivity calculations given in Fig. 10. It is considered that a smaller HRL gives lesser disturbance to the calculation. The optimised HRL size was chosen to be 0.3 mm since too fine HRL could affect the stability of the numerical solution. It is noted that FAC calculation was insensitive to the HRL thickness

To model the alteration in the NaHCO3 solutions, the thickness of the near-surface layer (*Lshell*) must be determined. Parametric analysis was carried in this study and is discussed in subsection 5.1. In this subsection, the best-fit results are described with the optimised *Lshell*

1

3.0x10-3

Leached Ca [mol cm-2

]

2.5 2.0 1.5 1.0 0.5 0.0  Mesh size [mm] 0.400 0.267 0.200 0.160 0.133 0.114 

Measured

) is the pore diffusion coefficient, and *x* and *t* refer to the mesh size in

*<sup>D</sup> <sup>t</sup> <sup>p</sup>* , (21)

0 20 40 60 Time [weeks]

<sup>2</sup> 

 *x*

For modelling using the two-step procedure, it is suggested that the choice of the space discretization affects the calculation results (Marty et al., 2009). In this study, the sensitivity of calculation to the mesh size was analysed. The sensitivity calculation results are shown in Fig. 9. In the case of OPC (Fig. 9 (a)), coarser mesh sizes gave a lower rate of calcium leaching and finer mesh sizes degraded the stability of numerical solutions. The optimised mesh size in this case is 0.267 mm, which reproduced the measured result using a diffusion coefficient (1.9 × 10-11 m2 s-1) compatible with the experimentally determined value (Haga et al., 2005). In the case of FAC (Fig. 9 (b)), the calculation result was less sensitive to the space

Sousa, 2003; Hindmarsh & Gresho, 1984):

0 20 40 60 Time [weeks]

Fig. 9. Sensitivity analysis of mesh size in solid region

the solid region and the time step.

where *Dp* (= *De*/

3.0x10-3

Leached Ca [mol cm-2

(Fig. 10 (b)).

]

2.5 2.0 1.5 1.0 0.5 0.0 given in Table 6. As shown in Fig. 4, the modelling calculations quantitatively well predicted the experimental results for the leaching of calcium. Figure 7 shows that calcite precipitation was predicted fairly well at lower NaHCO3 concentrations ( 1 × 10-4 mol dm-3), though it was underestimated at higher NaHCO3 concentrations. In the CCT-P calculation, the amount of precipitated calcite saturated after 10 ~ 20 weeks owing to clogging; however, in the experiments, calcite still precipitated even if the rate of precipitation decreased with time.

Fig. 10. Sensitivity analysis of thickness of HRL

(a) OPC

Modelling of Chemical

sensitivity analysis.

3.0x10-3

3.0x10-3

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

0 20 40 60 Time [weeks]

*Lshell* = 0.03 mm

*Lshell* = 0.18 mm

0 20 40 60 Time [weeks]

Alteration of Cement Materials in Radioactive Waste Repository Environment 93

concentrates in a thinner layer (smaller *Lshell*). Kurashige and Hironaga (2007) carried out a series of leaching experiments using OPC and low-heat portland cement in NaHCO3 solution, and found that the secondary calcite precipitation layer grew thinner as the NaHCO3 concentration increased. Based on their finding, it can be assumed that the thickness of the near-surface layer of calcite depends on the NaHCO3 concentration in the solution. This assumption is discussed by the sensitivity analysis of *Lshell*, as shown in Fig. 12. Figure 12 demonstrates that a smaller *Lshell* gives a larger reduction in the rate of calcium leaching. Table 6 gives the best-fit values of the near-surface layer obtained by

3.0x10-3

3.0x10-3

2.5

2.0

1.5

Leached Ca [mol cm-2

]

NaHCO3 [mol dm-3] Measured Modelling

 0 6e-5 1e-4 6e-4 1e-3 6e-3

1.0

0.5

0.0

2.5

2.0

1.5

Leached Ca [mol cm-2

(a) OPC

]

1.0

0.5

0.0

0 20 40 60 Time [weeks]

*Lshell* = 0.01 mm

*Lshell* = 0.09 mm

0 20 40 60 Time [weeks]

(b) FAC

Fig. 11. Results of calculation modelling alteration of cement monolith

The calculation also predicted qualitatively the calcium concentration in the solid matrix by comparing Fig. 11 with Fig. 6. The porosity in the altered region increased in distilled water, as shown in Fig. 11. The porosity decreased at the surface in NaHCO3 solutions and little alteration in the porosity of the solid was predicted for 1 × 10-3 and 6 × 10-3 mol dm-3 NaHCO3 solutions. In the case of FAC, a marked reduction in the porosity in the vicinity of the surface of the solid was predicted owing to calcite precipitation in the region.

#### **5. Discussion**

#### **5.1 Mechanism of clogging**

Figures 4 and 7 demonstrate that a higher NaHCO3 concentration in a solution induces a larger reduction in the rate of calcium leaching from a cement hydrate monolith due to calcite precipitation. However, in some cases, more calcite did not always precipitate at higher NaHCO3 concentrations; more calcite precipitated at the NaHCO3 concentrations of 6 × 10-4 and 1 × 10-3 mol dm-3 than at the NaHCO3 concentration of 6 × 10-3 mol dm-3 for OPC, as shown in Fig. 7 (a), and more calcite precipitated at the NaHCO3 concentration of 6 × 10-4 mol dm-3 than at the NaHCO3 concentrations of 1 × 10-3 and 6 × 10-3 mol dm-3 for FAC, as shown in Fig. 7 (b). These results also suggest that a denser calcite layer was formed at the higher NaHCO3 concentrations and its effect of inhibiting the leaching of calcium was more notable owing to a stronger restriction on mass transport at the interface between the solid and the solution.

The behaviour above can be described using the thickness of the near-surface layer (*Lshell*) as a parameter. In the proposed model, all calcite formed in the solution (in the HRL) precipitates on the near-surface layer so that a denser calcite layer formed as calcite

Concentration of calcite [mol/L-solid]

Porosity

(b) FAC

The calculation also predicted qualitatively the calcium concentration in the solid matrix by comparing Fig. 11 with Fig. 6. The porosity in the altered region increased in distilled water, as shown in Fig. 11. The porosity decreased at the surface in NaHCO3 solutions and little alteration in the porosity of the solid was predicted for 1 × 10-3 and 6 × 10-3 mol dm-3 NaHCO3 solutions. In the case of FAC, a marked reduction in the porosity in the vicinity of

Figures 4 and 7 demonstrate that a higher NaHCO3 concentration in a solution induces a larger reduction in the rate of calcium leaching from a cement hydrate monolith due to calcite precipitation. However, in some cases, more calcite did not always precipitate at higher NaHCO3 concentrations; more calcite precipitated at the NaHCO3 concentrations of 6 × 10-4 and 1 × 10-3 mol dm-3 than at the NaHCO3 concentration of 6 × 10-3 mol dm-3 for OPC, as shown in Fig. 7 (a), and more calcite precipitated at the NaHCO3 concentration of 6 × 10-4 mol dm-3 than at the NaHCO3 concentrations of 1 × 10-3 and 6 × 10-3 mol dm-3 for FAC, as shown in Fig. 7 (b). These results also suggest that a denser calcite layer was formed at the higher NaHCO3 concentrations and its effect of inhibiting the leaching of calcium was more notable owing to a stronger restriction on mass transport at the interface between the

The behaviour above can be described using the thickness of the near-surface layer (*Lshell*) as a parameter. In the proposed model, all calcite formed in the solution (in the HRL) precipitates on the near-surface layer so that a denser calcite layer formed as calcite

the surface of the solid was predicted owing to calcite precipitation in the region.

0.5 0.4 0.3 0.2 0.1 0.0  in Water in 6e-5M NaHCO3 in 1e-4M NaHCO3 in 6e-4M NaHCO3 in 1e-3M NaHCO3 in 6e-3M NaHCO3

5x10-3 0 1 2 3 4 Distance from surface [m]

after 76 weeks

20

 in Water in 6e-5M NaHCO3 in 1e-4M NaHCO3 in 6e-4M NaHCO3 in 1e-3M NaHCO3 in 6e-3M NaHCO3

15

10

Concentration of calcium [mol/L-solid]

5

0

**5. Discussion** 

**5.1 Mechanism of clogging** 

solid and the solution.

5x10-3 0 1 2 3 4 Distance from surface [m]

after 76 weeks

Fig. 11. Results of calculation modelling alteration of cement monolith

Ca in solid Calcite

concentrates in a thinner layer (smaller *Lshell*). Kurashige and Hironaga (2007) carried out a series of leaching experiments using OPC and low-heat portland cement in NaHCO3 solution, and found that the secondary calcite precipitation layer grew thinner as the NaHCO3 concentration increased. Based on their finding, it can be assumed that the thickness of the near-surface layer of calcite depends on the NaHCO3 concentration in the solution. This assumption is discussed by the sensitivity analysis of *Lshell*, as shown in Fig. 12. Figure 12 demonstrates that a smaller *Lshell* gives a larger reduction in the rate of calcium leaching. Table 6 gives the best-fit values of the near-surface layer obtained by sensitivity analysis.

(a) OPC

Modelling of Chemical

**repository** 

discuss the mechanism of clogging in more detail.

Table 6. Best-fit thickness of near-surface layer (*Lshell*)

on the basis of the low diffusivity in the FAC matrix.

repository environment will be effective in tackling this issue.

Alteration of Cement Materials in Radioactive Waste Repository Environment 95

was higher than that of OPC. The reason for the lower diffusion coefficients for FAC is still unclear, but it should be interpreted using the smaller pore size distribution in the FAC matrix (Chida and Sugiyama, 2007). Also, it could be suggested that the pore microstructure (tortuosity of the pore network) affected diffusivity (Promentilla, 2009). Further studies should be carried out to investigate the growth of the secondary precipitation layer to

NaHCO3 concentration [mol dm-3] *Lshell* for OPC [mm] *Lshell* for FAC [mm]

**5.2 Extrapolation of modelling to assess long-term alteration of cementitious** 

It has been demonstrated that the proposed model in this study has the capability of describing the leaching of calcium with the incongruent dissolution of C-S-H and the effect of the precipitation of the secondary less soluble phase (calcite) on the alteration of cement materials. To extrapolate the modelling for the assessment of the long-term alteration of a cementitious repository in the underground repository environment, some issues should be

Temporal and spatial discretizations should be carefully optimised in the modelling. A finer mesh size could give a better, more accurate prediction as examined in subsection 4.2; however, the numerical calculation would be instable with a very fine (too small) mesh size, as shown in Fig. 9. It would be impractical to choose very fine spatial and temporal resolutions when considering the computing time for the simulation of the long-term (sometimes up to million years) alteration of an engineering-scale repository (more than tens of meters). A deliberate sensitivity analysis should be carried out to provide a credible assessment. Also, it is suggested that the choice of material could improve the quality of assessment since FAC was less sensitive to the temporal and spatial discretizations, and also the size of HRL according to the modelling results in this study. This could be interpreted

Another important issue is the need to investigate the mechanism of clogging in more detail. In the underground environment, cementitious materials are placed in contact with groundwater through other engineering barrier materials or surrounding rocks. The rate of alteration could be moderated compared to the experiment in which the cement solid is placed in contact with the solution directly. Further experimentation is required on the alteration on the solid in an actual repository environment and enables us to establish optimised calculation conditions including spatial and temporal resolutions, the appropriate HRL, and the thickness of the near-surface layer (*Lshell*). In situ pilot experiments in the

6 × 10-5 0.18 0.01 1 × 10-4 0.18 0.01 6 × 10-4 0.18 0.01 1 × 10-3 0.09 0.01 6 × 10-3 0.01 0.01

discussed further to improve the reliability of the modelling calculation.

Fig. 12. Results of sensitivity calculation of thickness of near-surface layer

In the case of FAC, the near-surface layer appears to be insensitive to NaHCO3 concentration. It could be interpreted that the penetration of carbonate ions into the solid was restricted owing to the low diffusivity in the FAC matrix. It is noted that a lower diffusion coefficient was provided for FAC than for OPC even though the porosity of FAC (29%) was higher than that of OPC (12%). Similar observations were given by Sugiyama et al. (2008) and Chida and Sugiyama (2007, 2008): they observed that the effective diffusion coefficients of uranium (Sugiyama et al., 2008) , organic carbon (Chida and Sugiyama, 2008), caesium, strontium and iodine (Chida and Sugiyama, 2007) in the FAC matrix were more than an order of magnitude lower than those in the OPC matrix, though the porosity of FAC

NaHCO3 [mol dm-3

 0 6e-5 1e-4 6e-4 1e-3 6e-3

3.0x10-3

3.0x10-3

] Measured Modelling

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

2.5

2.0

1.5

Leached Ca [mol cm-2

(b) FAC

In the case of FAC, the near-surface layer appears to be insensitive to NaHCO3 concentration. It could be interpreted that the penetration of carbonate ions into the solid was restricted owing to the low diffusivity in the FAC matrix. It is noted that a lower diffusion coefficient was provided for FAC than for OPC even though the porosity of FAC (29%) was higher than that of OPC (12%). Similar observations were given by Sugiyama et al. (2008) and Chida and Sugiyama (2007, 2008): they observed that the effective diffusion coefficients of uranium (Sugiyama et al., 2008) , organic carbon (Chida and Sugiyama, 2008), caesium, strontium and iodine (Chida and Sugiyama, 2007) in the FAC matrix were more than an order of magnitude lower than those in the OPC matrix, though the porosity of FAC

Fig. 12. Results of sensitivity calculation of thickness of near-surface layer

]

3.0x10-3

3.0x10-3

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

2.5

2.0

1.5

Leached Ca [mol cm-2

]

1.0

0.5

0.0

0 20 40 60 Time [weeks]

*Lshell* = 0.03 mm

*Lshell* = 0.18 mm

0 20 40 60 Time [weeks]

1.0

0.5

0.0

0 20 40 60 Time [weeks]

*Lshell* = 0.01 mm

*Lshell* = 0.09 mm

0 20 40 60 Time [weeks]

was higher than that of OPC. The reason for the lower diffusion coefficients for FAC is still unclear, but it should be interpreted using the smaller pore size distribution in the FAC matrix (Chida and Sugiyama, 2007). Also, it could be suggested that the pore microstructure (tortuosity of the pore network) affected diffusivity (Promentilla, 2009). Further studies should be carried out to investigate the growth of the secondary precipitation layer to discuss the mechanism of clogging in more detail.


Table 6. Best-fit thickness of near-surface layer (*Lshell*)

#### **5.2 Extrapolation of modelling to assess long-term alteration of cementitious repository**

It has been demonstrated that the proposed model in this study has the capability of describing the leaching of calcium with the incongruent dissolution of C-S-H and the effect of the precipitation of the secondary less soluble phase (calcite) on the alteration of cement materials. To extrapolate the modelling for the assessment of the long-term alteration of a cementitious repository in the underground repository environment, some issues should be discussed further to improve the reliability of the modelling calculation.

Temporal and spatial discretizations should be carefully optimised in the modelling. A finer mesh size could give a better, more accurate prediction as examined in subsection 4.2; however, the numerical calculation would be instable with a very fine (too small) mesh size, as shown in Fig. 9. It would be impractical to choose very fine spatial and temporal resolutions when considering the computing time for the simulation of the long-term (sometimes up to million years) alteration of an engineering-scale repository (more than tens of meters). A deliberate sensitivity analysis should be carried out to provide a credible assessment. Also, it is suggested that the choice of material could improve the quality of assessment since FAC was less sensitive to the temporal and spatial discretizations, and also the size of HRL according to the modelling results in this study. This could be interpreted on the basis of the low diffusivity in the FAC matrix.

Another important issue is the need to investigate the mechanism of clogging in more detail. In the underground environment, cementitious materials are placed in contact with groundwater through other engineering barrier materials or surrounding rocks. The rate of alteration could be moderated compared to the experiment in which the cement solid is placed in contact with the solution directly. Further experimentation is required on the alteration on the solid in an actual repository environment and enables us to establish optimised calculation conditions including spatial and temporal resolutions, the appropriate HRL, and the thickness of the near-surface layer (*Lshell*). In situ pilot experiments in the repository environment will be effective in tackling this issue.

Modelling of Chemical

8846.

1998.

19780 EN, 2001.

Alteration of Cement Materials in Radioactive Waste Repository Environment 97

Glasser, F.P.; Adenot, F.; Bloem Bredy, P.J.C.; Fachinger, J.; Sneyers, A.; Marx, G.; Brodersen,

Haga, K.; Sutou, S.; Hironaga, M.; Tanaka, S. & Nagasaki, S. (2005). Effects of Porosity on

Harris, A.W.; Atkinson, A.; Balek, V.; Brodersen, K.; Cole, G.B.; Haworth, A.; Malek, Z.;

Hindmarsh, A.C. & Gresho, P.M. (1984). The Stability of Explicit Euler Time-Integration for

Kurashige, I. & Hironaga, M. (2007). *Mechanism of Leaching Inhibition of Cementitious Materials* 

Lagneau, V. & van der Lee, J. (2005). Simulation of Clogging Effects at the Interface between

Marty, N.C.M.; Tournassat, C.; Burnol, A.; Giffaut, E. & Gaucher, E.C. (2009). Influence of

Parkhurst D.L. et al. (1980). *PHREEQE - A Computer Program for Geochemical Calculations*, U.S.

Promentilla, M.A.B.; Sugiyama, T.; Hitomi, T. & Takeda, N. (2009). Quantification of

Sousa, E. (2003). The Controversial Stability Analysis, *Applied Mathematics and Computation*, Volume 145, Issues 2-3, (25 December 2003), pp. 777–794, ISSN 0096-3003. Sugiyama, D. (2008). Chemical Alteration of Calcium Silicate Hydrate (C–S–H) in Sodium

Sugiyama, D. & Fujita, T. (2006). A Thermodynamic Model of Dissolution and Precipitation

Sugiyama, D.; Fujita, T.; Chida, T. & Tsukamto, M. (2007). Alteration of Fractured

Sugiyama, D.; Chida, T. & Cowper, M. (2008). Laser Ablation Microprobe Inductively

Issue 9, (September 1984), pp. 853-897, ISSN 1097-0363.

Japanese with English Abstract], ISBN 4-86216-502-8.

Avignon, France, September 2005.

Geological Survey.

pp. 548-557, ISSN 0008-8846.

2008), pp. 1270-1275, ISSN 0008-8846.

2007), pp. 1257-1264, ISSN 0008-8846.

(February 2006), pp. 227-237, ISSN 0008-8846.

January 2009), pp. 58-72, ISSN 0022-1694.

K.; Cowper, M.; Tyrer, M. et al. (2001). *Barrier Performance of Cements and Concretes in Nuclear Waste Management*, Final Report for CEC Contract FI4W-CT96-0030, EUR

Leaching of Ca from Hardened Ordinary Portland Cement Paste, *Cement and Concrete Research*, Volume 35, Issue 9, (September 2005), pp. 1764-1775, ISSN 0008-

Nickerson, A.K.; Nilsson, K. & Smith, A.C. (1998). *The Performance of Cementitious Barriers in Repositories*, Final Report for CEC Contract FI2W-0040, EUR 16643 EN,

Certain Finite Difference Approximations of the Multi-Dimensional Advection-Diffusion Equation, *International Journal for Numerical Methods in Fluids*, Volume 4,

*due to Hydrogencarbonate Ion in Groundwater*, CRIEPI Report N06028, April 2007 [in

MX80-clay and Concrete in a Deep Radioactive Waste Repository, *Migration '05*,

Reaction Kinetics and Mesh Refinement on the Numerical Modelling of Concrete/Clay Interactions, *Journal of Hydrology*, Volume 364, Issues 1-2, (15

Tortuosity in Hardened Cement Pastes Using Synchrotron-based X-ray Computed Microtomography, *Cement and Concrete Research*, Volume 39, Issue 6, (June 2009),

Chloride Solution, *Cement and Concrete Research*, Volume 38, Issue 11, (November

of Calcium Silicate Hydrates, *Cement and Concrete Research*, Volume 36, Issue 2,

Cementitious Materials, *Cement and Concrete Research*, Volume 37, Issue 8, (August

Coupled Plasma Mass Spectrometry Study on Diffusion of Uranium into Cement

#### **6. Conclusions**

A reactive transport computational code, in which a geochemical model including the thermodynamic incongruent dissolution model of C-S-H is coupled with the advectiondiffusion/dispersion equation, was developed on the basis of a series of experiments on the alteration of hydrated cement monoliths in deionised water and in sodium bicarbonate solution. The code can describe the evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components and the clogging effect by insoluble secondary phase precipitation that inhibits the alteration of cement materials.

The precipitation of secondary calcite on the surface of cement solid induces a reduction in the rate of components leaching. The clogging behaviour depends on the feature of the nearsurface layer of precipitates, which is sensitive to the concentration of reactive ions in the solution and the mass transport property of the solid matrix. Further experimental studies are needed to analyse in detail the alteration at the interface between cementitious material and other barrier materials (e.g., bentonite and host rock) under actual underground repository conditions. FAC, in which a low diffusivity is given, is suggested to reduce the uncertainty in the long-term performance assessment.

#### **7. References**


A reactive transport computational code, in which a geochemical model including the thermodynamic incongruent dissolution model of C-S-H is coupled with the advectiondiffusion/dispersion equation, was developed on the basis of a series of experiments on the alteration of hydrated cement monoliths in deionised water and in sodium bicarbonate solution. The code can describe the evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components and the clogging effect by insoluble secondary phase precipitation that inhibits the alteration of cement materials.

The precipitation of secondary calcite on the surface of cement solid induces a reduction in the rate of components leaching. The clogging behaviour depends on the feature of the nearsurface layer of precipitates, which is sensitive to the concentration of reactive ions in the solution and the mass transport property of the solid matrix. Further experimental studies are needed to analyse in detail the alteration at the interface between cementitious material and other barrier materials (e.g., bentonite and host rock) under actual underground repository conditions. FAC, in which a low diffusivity is given, is suggested to reduce the

Atkinson, A. (1985). *The Time Dependence of pH within a Repository for Radioactive Waste* 

Atkinson, A.; Goult, D.J. & Hearne, J.A. (1985). An Assessment of the Long-term Durability

Bond, K.A.; Heath, T.G. & Tweed, C.J. (1997). *HATCHES: A Referenced Thermodynamic Database for Chemical Equilibrium Studies*, Nirex Report NSS/R379, 1997. Brodersen, K. (2003). *CRACK2 – Modelling Calcium Carbonate Deposition from Bicarbonate Solution in Cracks in Concrete*, Risø-R-1143(EN), ISBN 87-550-2612-5, 2003. Burnol, A.; Blanc, P.; Tournassat, C.; Lassin, A.; Xu, T. & Gaucher, E.C. (2005).

Concrete/Clay Interface, *Migration '05*, Avignon, France, September 2005. Chida, T. & Sugiyama, D. (2007). Observation of Diffusion Behavior of Trace Elements in

Chida, T. & Sugiyama, D. (2008). Diffusion Behavior of Organic Carbon and Iodine in Low-

Glasser, F.P.; Macphee, D.E. & Lachowski, E.E. (1987). Modelling Approach to the Prediction

585-592, ISBN 978-1-60511-079-0, Sheffield, UK, September 2007.

pp. 379-384, ISBN 978-1-60511-096-7, Boston, USA, December 2008.

of Concrete in Radioactive Waste Repositories, *Scientific Basis for Nuclear Waste Management IX (Material Research Society Symposium Proceedings volume 50)*, pp. 239-

Intercomparison of Reactive Transport Models Applied to Degradation of a

Hardened Cement Pastes by LA-ICP-MS, *Scientific Basis for Nuclear Waste Management XXXI (Material Research Society Symposium Proceedings volume 1107)*, pp.

heat Portland Cement Containing Fly Ash, *Scientific Basis for Nuclear Waste Management XXXII (Material Research Society Symposium Proceedings volume 1124)*,

of Equilibrium Phase Distribution in Slag-cement Blends and Their Solubility Properties, *Scientific Basis for Nuclear Waste Management XI (Material Research Society Symposium Proceedings volume 112)*, pp. 3-12, ISBN 0-931837-82-0, Boston, USA,

uncertainty in the long-term performance assessment.

*Disposal*, AERE R 11777, UKAEA, 1985.

246, Stockholm, Sweden, September 1985.

**6. Conclusions** 

**7. References** 

December 1987.


**5** 

*USA* 

**A New Generation of** 

Yifeng Wang, Huizhen Gao, Andy Miller and Phillip Pohl *Sandia National Laboratories* 

**Adsorbent Materials for Entrapping and** 

**Immobilizing Highly Mobile Radionuclides** 

The United States is now re-assessing its nuclear waste disposal policy and re-evaluating the option of moving away from the current once-through open fuel cycle to a closed fuel cycle. In a closed fuel cycle, used fuels will be reprocessed and useful components such as uranium or transuranics will be recovered for reuse (e.g., Bodansky, 2006). During this process, a variety of waste streams will be generated (NEA, 2006; Gombert, 2007). Immobilizing these waste streams into appropriate waste forms for either interim storage or long-term disposal is technically challenging (Peters and Ewing, 2007; Gombert, 2007). Highly volatile or soluble radionuclides such as iodine (129I) and technetium (99Tc) are particularly problematic, because both have long half-lives and can exist as gaseous or anionic species that are highly soluble and poorly sorbed by natural materials (Wang et al., 2003; Wang and Gao, 2006; Wang et al., 2007). Waste forms are probably the only engineered barrier to limit their release into a human-accessible environment after disposal. In addition, during the fuel reprocessing, a major fraction of volatile radionuclides will enter the gas phase and must be captured in the off-gas treatment. It is thus highly desirable to develop a material that can effectively capture these

In fuel reprocessing, spent fuels are first subjected to voloxidation and acid dissolution, during which 94% to 99% of iodine-129, together with other volatile radionuclides (14C, 3H, Kr and Xe), are released to the off-gas stream (Gombert, 2007). Various methods have been proposed for the recovery of volatile 129I, including scrubbing with caustic or acid solutions and chemisorption on impregnated adsorbents (Rovnyi et al., 2002; Kato et al., 2002; Gombert, 2007). The proposed adsorbents include both natural and artificial porous materials like zeolite (e.g., mordenite), alumina, and silica gels loaded with metals (e.g., Ag, Cd, Pb) that form low-solubility iodides or iodates. However, several issues have been recognized regarding the effectiveness of these materials (Gombert, 2007). For example, based on adsorption models, iodine diffusion inside the zeolite particles was found to be slow, only about 2x10-14 cm2/s, and likely to limit the adsorption process (Jubin, 1994, Gombert, 2007). The stability of silver-exchanged zeolite is also a concern. For example,

radionuclides and then be converted into a durable waste form.

**1. Introduction** 

Materials, *Radiochimica Acta*, Volume 96, Issue 9-11, Migration 2007, pp. 747-752, ISSN 0033-8230.

