Influence of activating solution and the addition
of NaF on metakaolin-based geopolymer
physico-chemical properties

J. Agullo<fnref n="1" />, B. Muzeau<fnref n="1" />*, D. Lambertin<fnref n="2" />, N. Trepy<fnref n="1" /> and V. L'Hostis<fnref n="1" />

**Abstract:**

The technology of geopolymerisation is attracting a lot of interest due to the wide variety of potential applications. Recently emerging and alternative cementitious systems based on geopolymers have been reported in literature. Geopolymer materials have been proposed as new binders for safe radioactive waste containment. Moreover, their good fluidity and fire resistance make them compatible with industrial processes for embedding wastes.

The purpose of this work was to investigate the physico-chemical properties of metakaolin-based geopolymer which could be used as mineral binder for encapsulation of nuclear wastes. To improve the knowledge of this binder, its microstructure and its interstitial solution have been characterized during hydration (up to 28 days). Interstitial solutions were extracted at different hydration time and subsequent analyses allow us to determine their chemical composition. pH measurements confirm the high alkalinity of the interstitial pore solution. The influence of the activating solution (NaOH or KOH) and the addition of NaF in the geopolymer composition were also studied. The nature of different crystalline phases was identified by X-ray diffraction. Water accessible porosity for these materials was estimated around 59%. The pore size distribution was determined by mercury intrusion porosity. Alkaline cations have proved to induce changes in the pore size distribution, these ones coming from whether the activating solution or the NaF addition.

<footnote n="1">¹ CEA, DEN, DPC, SECR, Laboratoire d'Etude du Comportement des Bétons et des Argiles, F-91191 Gif-sur-Yvette (France)</footnote>

<footnote n="2">² CEA, DEN, DTCD, SPDE, Laboratoire de Physico-Chimie des matériaux Cimentaires, F-30207 Bagnols sur Cèze Cedex (France)</footnote>

<footnote>* Corresponding author: Benoist.muzeau@cea.fr</footnote>
# 1. Introduction

The technology of geopolymerisation is attracting a lot of interest due to the wide variety of potential applications (Davidovits, 1991; Delatte, 1993; Komnitsas, 2007). Recently emerging and alternative cementitious systems based on geopolymers have been reported in literature (Duxson, 2007; Rashad, 2013). Mineral or organic geopolymer materials have been proposed as new binders for safe radioactive waste containment (Kunzel, 2010; Davidovits, 2011; Ojovan, 2011; Perera, 2004). Moreover, their good fluidity and fire resistance make them compatible with industrial processes for embedding wastes (Rooses, 2013). In this work, we are focused on metakaolin-based geopolymer which could be used as mineral binder for encapsulation of nuclear wastes.

In 1978 Joseph Davidovits coined the term geopolymer. In fact geopolymers can be associated to mineral polymers of geological origin (Davidovits, 1991). They are consisted essentially of mineral chemical compound or mixture of compounds with repeating units resulting from geopolymerization process. Although acidic medium synthesis route is possible the alkaline ones is the most used. In this work, metakaolin-based geopolymer pastes were elaborated from silico-aluminate source activated by alkaline solution at room temperature involving the formation of silico-aluminate units (-Si-O-Al-O-). Geopolymer material is a typical porous material. Pore structures and interfacial properties are influenced properties of ion transport behavior and durability. In this study pore size distribution was determined by employing mercury intrusion porosimetry (MIP), with metakaolin-based geopolymer made with two different activating solutions (NaOH and KOH) and with the addition of NaF in the formulation. To improve the knowledge of this type of binder, its mineralogy was determined by X-ray spectroscopy (XRD) and interstitial solutions were extracted and analysed by ionic chromatography. Physico-chemical characterizations were realized at different time of hydration (up to 28 days).

# 2. Materials and experimental

Four formulations of geopolymer pastes were studied:

- Two with KOH as activating solution with and without addition of NaF which are noted Na-Geo/NaF and Na-Geo, respectively;

- Two with KOH as activating solution with and without addition of NaF which are noted K-Geo/NaF and K-Geo, respectively.

Their chemical formula is indicated in table 1:
<table><caption>Table 1. Chemical formula of geopolymer pastes</caption><thead><tr><td>Sample</td><td>Chemical formula</td></tr></thead><tbody><tr><td>K-Geo</td><td>K<sub>2</sub>O-4.2SiO<sub>2</sub>-1.2Al<sub>2</sub>O<sub>3</sub>-14.8H<sub>2</sub>O</td></tr><tr><td>K-Geo/NaF</td><td>K<sub>2</sub>O-4.2SiO<sub>2</sub>-1.2Al<sub>2</sub>O<sub>3</sub>-14.8H<sub>2</sub>O +NaF 1.25M</td></tr><tr><td>Na-Geo</td><td>Na<sub>2</sub>O-4.2SiO<sub>2</sub>-1.2Al<sub>2</sub>O<sub>3</sub>-14.8H<sub>2</sub>O</td></tr><tr><td>Na-Geo/NaF</td><td>Na<sub>2</sub>O-4.2SiO<sub>2</sub>-1.2Al<sub>2</sub>O<sub>3</sub>-14.8H<sub>2</sub>O +NaF 1.25M</td></tr></tbody></table>

The geopolymer pastes are made by mixing metakaolin powder (Pieri Premix MK, Grace Constructions) with a sodium or potassium silicate solutions. Silicate solutions were prepared by dissolving NaOH or alkali hydroxide KOH (Prolabo, >98%) and amorphous silica (Tixosil 38, Rhodia) in Mili-Q water. When sodium fluoride was used, it was directly added to silicate solution. After mixing the geopolymer pastes were filled in cylinder plastic samples (φ=32 mm and h=70 mm) and were cured at 23°C and 98% HR.

After desired storage period the plastic samples were destroyed and the geopolymer pastes squeezed with high pressure under reproducible conditions. Around 5 mL of the pore solution was collected without any contact to air under gloves box. Extraction of pore solution was achieved with a squeezing apparatus manufactured by Enerpac and a steel mould. The nature and the concentration of the interstitial pore solution ions were determined by ionic chromatography from Thermo Fischer Dionex. Silica was dosed by Spectroquant test (Merck) and UV-vis spectrometer (Lambda 14-Perkin Elmer). pH were measured with a pH meter from MerterLab (PHM201) with a Hamilton minitrode pH electrode.

All solutions were prepared using Mili-Q deionized water (ρ > 18.2 MΩ cm⁻¹) and all the experiments were carried out at a room temperature.

The X-ray diffraction (XRD) diagrams were collected with a PANalytical X-Pert X-ray photometer operating with a CuKα radiation (λ=0.154 nm) generated at 45 kV and 40 mA from 5° to 90° (2θ). X-ray powder diffraction for curing period more than 7 days was recorded. For experimental purposes, X-ray diffraction were recorded on massif geopolymers for curing period less than 7 days and on powder with an angular step size of 2θ=0.017° and a rotating rate of 0.5 deg.s⁻¹ for curing period more than 7 days.

Water accessible porosity (φ<sub>w</sub>) was estimated by measuring the total water amount removed from water saturated samples after drying at 105 °C until stable mass loss. Water accessible porosity was calculated as follows:

$$ \theta_w = \frac{m_s - m_d}{\rho V} \qquad (1) $$

Where m<sub>s</sub> is the sample water saturated mass (kg), m<sub>d</sub> the sample dried mass (kg), V the sample volume (m³) and ρ the water density at 20°C (kg/m³).

The sample density can be defined by the following equation:

$$ d = \frac{m_s}{m_s - m_w} \qquad (2) $$
Where $m_s$ is the sample water saturated mass (kg), $m_w$ the sample mass in water measured by hydrostatic weighing (kg).

Mercury Intrusion Porosimetry (MIP) tests were performed with a Micromeritics Autopore IV 9500 mercury porosimeter with a maximum 413 MPa injection pressure. The minimum pore access diameter reached is about 3 nm. For these characterizations the freeze-drying technique was used. Samples were first frozen (-195°C) by immersion in liquid nitrogen during 5 min and then introduced in an freeze dryer (Lyovac GT2, Steris, D-Hürth) in which vacuum was kept to 2 Pa for 5 days.

# 3. Results and discussions

## 1.1. Mineralogy

Figures 1, 2 and 3 show X-ray diffraction patterns of Na and K-geopolymers with and without NaF. Na and K-geopolymers without NaF show a same XRD patterns so only Na-geopolymers is shown. These geopolymers derived from metakaolin and phases present in metakaolin, such as quartz or anatase for example, are detectable. Moreover, as metakaolin Pieri Premix MK an amorphous phase is detectable between 15 and 30° 20 as major geopolymer (Duxson, 2007). However, illite phase initially present in X-ray Pieri Premix diffractogram (not shown) disappears. All these phases detected by XRD are crystalline elements which are present in the raw-material and - do not take part in the geopolymerization (Davidovits, 2011).

For Na and K-geopolymers after 1 day of hydration, quartz, anatase and muscovite phases were identified (figure 1). After 28 days of hydration XRD patterns are similar (not shown) and no evolution was detected.
<figure><img src="image_1.png" /><figcaption>Fig. 1. X-ray diffractogram of Na-geopolymer after 1 day of hydration
(Q: quartz, A: anatase, M: muscovite).</figcaption></figure>

For Na-geopolymers with NaF (figure 2), X-ray diffractogram is a superposition
of Na-Geo and villiaumite (NaF) patterns and do not change with time. This indi-
cates that NaF is incorporated in the geopolymer matrix.

<figure><img src="image_2.png" /><figcaption>Fig. 2. X-ray diffractogram of Na-geopolymer with addition of NaF after 1 day of hydration
(Q: quartz, A: anatase, M: muscovite, V: villiaumite).</figcaption></figure>
Figure 3 shows that for K-geopolymers with NaF, an additional phase, hieratite (K₂SiF₆), appears in the first day in comparison to Na-Geo/NaF. X-ray diagram changes with the disappearance of villiaumite (NaF) phase with time in the first week. Hydration time undergoes a fluorine anions change phase. Solubility of K₂SiF₆ is lower than NaF (CRC Handbook of Chemistry and Physics, 2005). Fluorine ions are consumed by hieratite phase and equilibrium of NaF dissociation is displaced. Fluorine anion is limiting reagent and precipitates in K₂SiF₆ involving the NaF disappearance during the hydration time. Kinetic is relatively slow because NaF is still present after 3 days of hydration. At 7 days, the final composition is fixed with quartz, anatase, muscovite and hieratite phases. At 28 days XRD patterns are similar with no further evolutions (not shown). Solvation effects are possibly responsible for preferential incorporation of potassium than sodium in K-Geo/NaF (Duxson, 2005b). During geopolymerisation process, water molecules migrate into hydration spheres of sodium and potassium cations. The hydration sphere of potassium counts 8 water molecules which are less bounded than the 6-7 ones of hydration sphere of sodium (Toth, 1996). Then it is more energetically favorable for potassium to be incorporated into the binder and for sodium to adsorb the released water.

<figure><img src="image_3.png" /><figcaption>Fig. 3. X-ray diffractogram of K-geopolymer with addition of NaF after 1, 3 and 7 days of hydration (Q: quartz, A: anatase, M: muscovite, V: villiaumite, H: hieratite).</figcaption></figure>

In the absence of NaF, sodium or potassium ions in activating solutions do not impact the geopolymers crystalline phases. When NaF is added, sodium cation allows a stable incorporation of NaF in the structure whereas potassium cation involves the K₂SiF₆ phase formation and the disappearance of NaF in the matrix.
## 1.2. Microstructure

### 1.1.1. Water access porosity

Water accessible porosity values are given in Table 1. A low variation between 58.2 and 60.9 % is observed for water accessible porosity depending on cation of activating solution and the addition of NaF. The density mean values are around 1.65 g.cm⁻³. These values are in agreement with literature (Boher, 2012; Duxson, 2005a). No important effect of the nature of cation in the activating solution or addition of NaF was noticed for these parameters.

<table><caption>Table 2. Water accessible porosity and density of geopolymer materials values</caption><thead><tr><th>Sample</th><th>K-Geo</th><th>K-Geo/NaF</th><th>Na-Geo</th><th>Na-Geo/NaF</th></tr></thead><tbody><tr><td>Water accessible porosity (%)</td><td>58.2</td><td>58.5</td><td>58.8</td><td>60.9</td></tr><tr><td>Density (g.cm<sup>-3</sup>)</td><td>1.65</td><td>1.66</td><td>1.67</td><td>1.64</td></tr></tbody></table>

### 1.1.2. Pore size distribution

Mercury intrusion porosimetry (MIP) can be used to characterize the distribution of pore sizes in geopolymer materials (Kriven, 2008). The pore size distribution is essential to understand transport mechanisms. The effect of the activating solution cation and the addition of NaF on materials MIP spectrum was investigated. Figure 4 present the pore volume as the function of the pore access diameter for the 4 formulations after 28 days of hydration.
<figure><img src="image_4.png" /><figcaption>Fig. 4. Pore distribution of polymers after 28 days of hydration.</figcaption></figure>

Results from the MIP show that every geopolymer display a quasi-modal pore size distribution. However alkaline cations induce changes in the pore size distribution, these ones coming from whether the activating solution or the NaF addition. Pore dimension are less important for K-Geo, K-Geo/NaF, Na-Geo and Na-Geo/NaF, respectively. It is important to remind that size pore below about 3 nm cannot be detected by MIP. The first three have pore size distribution in mesopore range and the last one in macropore range. K-Geo and K-Geo/NaF exhibit pore size of from 3 to 77 nm and a maximum at 7 and 11 nm, respectively. Na-Geo presents a mesopores size distribution (5-95 nm) and a maximum at 32 nm. Addition of NaF in Na-Geo shifts the maximum at 150 nm and increase the pore size up to 582 nm. The trend is the same and no important evolution was noticed after 1 day of hydration (not shown). It is possible to adjust the pore size distribution changing alkaline cation coming from the activating solution or/and adding NaF in the geopolymer formulation. The increase of the mean and median pore size when NaOH is used in place of KOH is in good agreement with literature data (Boher, 2012; Kriven 2004, Steins 2014).

**1.3. Interstitial pore solution**

Interstitial pore solution were extracted from geopolymer pastes and analyzed after different time of hydration. The evolution of the identified ion concentration and pH values are plotted as a function of the hydration time for Na-Geo (Fig. 5), NaGeo/NaF (Fig. 6), K-Geo (Fig. 7) and K-Geo/NaF (Fig. 8). For geopolymers activated with NaOH, pH values are slightly higher with NaF. They reach 13.3 in
the first day and decrease to around 12.6 at 28 days. At 28 days of hydration time, the ions concentration is quite stable and slightly higher for Na-Geo/NaF than Na-Geo. The silica concentration declines from the first day and is lower in the case of Na-Geo. The excess of NaF added in the activating solution is seen in the interstitial pore solution. The concentrations of sodium and fluorine ions cap around 1 mol.L⁻¹ and 0.9 mol.L⁻¹. The presence of sulfate anions comes from impurities contained in raw material Tixosil 38 silica (Boher, 2012).

<figure><img src="image_5.png" /><figcaption>Fig. 5. Plots of the evolution of the ion concentration and silica contained in Na-Geo interstitial pore solution as a function of the hydration time.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 6. Plots of the evolution of the ion concentration and silica contained in Na-Geo with NaF interstitial pore solution as a function of the hydration time.</figcaption></figure>

For geopolymers activated with KOH, the addition of NaF involves a decrease of the pH values. At 28 days of hydration time, the ions concentrations are quite stable and slightly higher for K-Geo than for Na-Geo/NaF, except for F⁻, K⁺ and Na⁺. The concentration of Na⁺ and F⁻ is provided by the excess of NaF added in the activating solution at the beginning. The concentration of K⁺ and F⁻ is divided by 2 in 28 days. This can be explained by the formation of K₂SiF₆ (detected by XRD).

All these results allow us to determine the nature of the interstitial solution and can be related to XRD and MIP analyses. We can estimate the part of solubilized ions and the nature of the minerals phases in the geopolymer matrix.
<figure><img src="image_7.png" /><figcaption>Fig. 7. Plots of the evolution of the ion concentration and silica contained in K-Geo interstitial pore solution as a function of the hydration time.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 8. Plots of the evolution of the ion concentration and silica contained in K-Geo with NaF interstitial pore solution as a function of the hydration time.</figcaption></figure>
4. Conclusions

The purpose of this work was to investigate the physico-chemical proprieties of
metakaolin-based geopolymers which could be used as mineral binder for encapsu-
lation of nuclear wastes. XRD diagrams showed that in the absence of NaF, sodi-
um or potassium ions in activating solutions do not impact the geopolymers nature
crystalline phases. When NaF is added, sodium cation allows a stable incorpora-
tion of NaF in the structure whereas potassium cation involves the K₂SiF₆ phase
formation and the disappearance of NaF in the matrix. These results can be related
with the interstitial pore solutions analyses to determine the part of solubilized
ions. The interstitial solution pH of metakaolin-based geopolymers is very high. It
involves that only wastes which are inert under high pH conditions could be con-
siderate to be encapsulated. Water accessible porosity for these materials was es-
timated around 59%. It is possible to achieve a relative control of the pore size dis-
tribution by changing the activating solution and the quantity of NaF. In fact,
alkaline cations have proved to induce changes in the pore size distribution. All
these characterizations of the geopolymer matrix will be useful to ensure its ability
to be a good embedment of the radioactive wastes and its further behavior in the
geological disposal.

References

(2005) CRC Handbook of Chemistry and Physics, electronic version
http://www.hbcpnetbase.com. David R. Lide ed, CRC Press, Boca Raton, FL.

Boher, C. (2012) Etude expérimentale et modélisation de la diffusion gazeuse à travers des milieux poreux partiellement saturés en eau. Application aux verres Vycor, géopolymères et pâte de ciment CEM V, INSA de Toulouse, PhD.

Davidovits, J. 1991 Geopolymers -Inorganic Polymeric New Materials. Journal of Thermal Analysis 37, 1633-1656.

Davidovits, J. 2011 Geopolymer Chemistry and Applications - 3rd edition. Geopolymer Institute.

Delatte, J., Facy, F., sept. 1993 Des bétons antiques aux géopolymères, Arts et Métiers maga-
zine, p. 8.

Duxson, P., Fernandez-Jimenez, A., Provis, J.L., Lukey, G.C., Palomo, A., van Deventer, J.S.J. 2007 Geopolymer technology: the current state of the art. Journal of Materials Science 42, 2917-2933.

Duxson, P., Provis, J.L., Lukey, G.C., Mallicoat, S.W., Kriven, W.M., van Deventer, J.S.J. 2005a Understanding the relationship between geopolymer composition, microstructure and mechanical properties. Colloids and Surfaces a-Physicochemical and Engineering Aspects 269, 47-58.

Duxson, P., Lukey, G.C., Separovic, F., van Deventer, J.S.J. 2005b Effect of alkali cations on aluminum incorporation in geopolymeric gels. Industrial & Engineering Chemistry Research 44, 832-839.

Kommitsas, K., Zaharaki, D. 2007 Geopolymerisation: A review and prospects for the minerals industry. Minerals Engineering 20, 1261-1277.
Kriven, W.M., Bell, J.L. 2004 Effect of Alkali Choice on Geopolymer Properties. Ceramic Engineering and Science Proceedings 25, 99-104.

Kriven, W.M., Bell, J.L., Gordon, M., 2008 Microstructure and Nanoporosity of as-Set Geopolymers, Mechanical Properties and Performance of Engineering Ceramics II: Ceramic Engineering and Science Proceedings. John Wiley & Sons, Inc., pp. 491-503.

Künzel C., Vandeperre, L., Boccaccini, A.R., Cheeseman, C.R. 2010 Geopolymers for the encapsulation of solid nuclear waste. WP3 (Geopolymers and Cements), Project 3.2.1 - DIAMOND'10 Conference Decommissioning, Immobilisation and Management of Nuclear Waste for Disposal - Manchester, UK.

Ojovan, I. 2011 Handbook of Advanced Radioactive Waste Conditioning Technologies. Elsevier Science.

Perera, D.S., Blackford, M.G., Vance, E.R., Hanna, J.V., Finnie, K.S., Nicholson, C.L. 2004 Geopolymers for the Immobilization of Radioactive Waste. MRS Online Proceedings Library 824.

Rashad, A.M. 2013 Alkali-activated metakaolin: A short guide for civil Engineer - An overview. Construction and Building Materials 41, 751-765.

Rooses, A., Steins, P., Dannoux-Papin, A., Lambertin, D., Poulesquen, A., Frizon, F. 2013 Encapsulation of Mg-Zr alloy in metakaolin-based geopolymer. Applied Clay Science 73, 86-92.

Steins, P., Poulesquen, A., Frizon, F., Diat, O., Jestin, J., Causse, J., Lambertin, D., Rossignol, S. 2014 Effect of aging and alkali activator on the porous structure of a geopolymer. Journal of Applied Crystallography 47, 316-324

Toth, G. 1996 Ab initio pair potential parameter set for the interaction of a rigid and a flexible water model and the complete series of the halides and alkali cations. Journal of Chemical Physics 105, 5518-5524.