# pH and time dependent microstructure features of compacted bentonite-sand mixture

Yu Lei, Yujun Cui, Nadia Mokni

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Yu Lei, Yujun Cui, Nadia Mokni. pH and time dependent microstructure features of compacted bentonite-sand mixture. 5th European Conference on Unsaturated Soils and Biotechnology applied to Geotechnical Engineering, IST - Instituto Superior Técnico (Universidade de Lisboa), Sep 2025, Lisbonne, Portugal. pp.03029, 10.1051/e3sconf/202564203029. hal-05416701

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# pH and time dependent microstructure features of compacted bentonite-sand mixture

Yu Lei<fnref n="1" />*, Yujun Cui<fnref n="1" />, and Nadia Mokni<fnref n="2" />

**Abstract.** Compacted bentonite-sand mixture is considered as a potential sealing material for deep geological disposal of high-level radioactive waste. In the long run, the intruding alkaline plumes, as a result of concrete degradation, can alter the microstructure of the mixture, potentially affecting the sealing performance. In this study, Compacted bentonite-sand (40/60) mixture samples were hydrated by deionised water and alkaline solutions with varying pHs (8.8, 11.5, 13.5) for different durations under constant-volume conditions. The resulting microstructural changes were subsequently investigated by Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscope (SEM). Results showed that after hydration for one week, for the sample hydrated with deionised water, a tri-modal porosity was observed, defining different populations for intra-aggregate pores, inter-particle pores, and inter-aggregate pores. With the increase of pH, the intruded void ratio was significantly reduced, mainly due to the shrinkage of the diffuse double layer. The uniformity increased for the intra-aggregate pores while decreased for the meso and macro pores (inter-particle and inter-aggregate pores). Additionally, in the case of pH = 13.5, the intruded void ratio of the samples decreased over time, with further increase of void ratio and pore uniformity for the intra-aggregate pores and the decrease of size for the macropore population. These phenomena suggest that the alkaline attack resulted in montmorillonite dissolution, further modifying the macro-pores. This is confirmed by the SEM observations.

## 1 Introduction

In the concept of deep geological disposal for high-level and intermediate-level radioactive wastes, compacted bentonite and sand mixture with high sand content is being considered as one of sealing materials for their important radionuclide retardation capacities, high swelling capacity, low permeability, and potentially low gas breakthrough pressure, [1, 2].

In the long term, alkaline plumes originating from concrete structures may contaminate the pore water [3,4] and could gradually influence the microstructure of the mixture, potentially affecting its sealing performance. The alkaline porewater has a complex effect on the bentonite properties by altering the different swelling stages. Fig. 1 shows overall conceptual diagrams of hydration of compacted bentonite and sand mixture [5-8].

<figure><img src="image_2.png" /><figcaption>**Fig. 1** Conceptual diagrams of constant volume swelling process and microstructure evolution of compacted bentonite and sand mixture during alkaline hydration</figcaption></figure>

The alkaline solution plays different roles in the different stages. The crystalline swell (stages 1 to 2), is driven by

the adsorption of water molecules at clay surfaces [9], which is weakened by the replacement from sodium to

<footnote n="1">¹Ecole Nationale des Ponts et Chaussées, Laboratoire Navier, Cité Descartes, 8 Av. Blaise Pascal 6 et, 77420 Champs-sur-Marne, France</footnote>

<footnote n="2">²Autorité de sûreté nucléaire et de radioprotection (ASNR), PSE-ENV/SPDR/LETIS, F-92260, Fontenay-aux-Roses, France</footnote>

<footnote>* Corresponding author: yu.lei@enpc.fr</footnote>
potassium and rarely affected by hydroxide radicals [10-12]. Potassium ions exhibit stronger adsorption to the mineral lattice, reducing the accessibility of water molecules to interlayer spaces. This restricted hydration is reflected in a lower cation exchange capacity. Subsequently, the break-up of the quasicrystals stage occurs (stages 2 to 3), showing generation of new mesopore family in mercury intrusion porosimetry results [13, 14]. The swelling process is then dominated by diffuse double layer (DDL) swelling (stages 3 to 4) after the maximum number of hydrates is reached [7]. The capacity of surface sites governs DDL swelling by regulating ion adsorption and surface charge development. This swelling decreases with increasing solution concentration and potassium substitution, which reduce active adsorption sites and compress the double layer. Conversely, it increases with surface charge enhancement induced by hydroxide radicals, strengthening electrostatic repulsion between clay particles. [1] [8]. Moreover, the fabric re-arrangement (stage 5) occurs resulting from the dissolution of montmorillonite and silicate minerals [17], which is determined by the pH of solution and the dissolution time. In general, the mechanism of bentonite hydration has been widely investigated. However, the corresponding microstructure evolution is scarcely studied, especially for bentonite-sand mixture under varying pHs and dissolution time.

In this study, the mercury intrusion porosimetry was used for bentonite-sand mixture samples hydrated by different pH alkaline solutions and durations, to give an interpretation for microstructure. An additional concern focused on the evolution of different pore families and supported by the SEM observations.

# 2 Materials and methods

## 2.1 Materials

The MX80 bentonite and the quartz sand were used for this study with a proportion of 40/60 in dry mass. The bentonite was extracted from Wyoming, USA, which has been analyzed in many other studies [14, 18, 19]. It is a sodium bentonite with a high montmorilloinite content of 80%, a specific gravity of 2.77, and a cation exchange capacity (CEC) of 76 meq/100 g. The liquid limit is 575%, plastic limit is 53%. The bentonite grain size and deflocculated size distribution were separately analyzed by dry sieved [20] and hydrometer [21], as shown in Fig. 2.

Prior to compaction, the suction of bentonite was controlled by the water vapor method in 38 MPa suction, determined by the site conditions, the corresponding water content was around 18%.

The sand used in this study was from Eure and Loire (France), with a specific gravity of 2.65. The sand grain size distribution was also analyzed by dry sieved, and the results were characterized by a uniformity coefficient $C_u$ of 1.60 and a $D_{50}$ close to 0.6 mm. The sand grain size distribution curve is close to that of the bentonite grain.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Grain size distribution of the MX80 bentonite and sand.</figcaption></figure>

## 2.2 Sample preparation

The bentonite and sand are mixed to obtain a homogeneous mixture. The mixtures were then statically compacted at constant displacement rate of 0.1 mm/min, into a cylindrical mold (50 mm in diameter and 20 mm in height) to reach a dry density of 1.80 Mg/m³. The displacement and stress during compaction were also controlled to keep the same initial condition of the samples.

## 2.3 Alkaline solution

The pHs values of alkaline solution used in this study are 8.8, 11.5, and 13.5. The highest pH (13.5) solution, has the same chemical components and pH as the concrete pore water Table 1. The 11.5 pH solution was prepared by diluting the 13.5 pH one for 100 times, and the 8.8 pH solution was prepared by diluting the solution of 11.5 for 500 times [22, 23]. The pHs of three alkaline solutions were monitored by the pH meter.

<table><caption>Table 1. Chemical composition of concrete pore water presenting a pH of 13.5 [22, 23]</caption><thead><tr><td>Compound</td><td>Concentration (mol/L)</td></tr></thead><tbody><tr><td>NaOH</td><td>5.0×10<sup>-2</sup></td></tr><tr><td>KOH</td><td>2.7×10<sup>-1</sup></td></tr><tr><td>Ca(OH)<sub>2</sub></td><td>6.0×10<sup>-4</sup></td></tr></tbody></table>

## 2.4 Experimental methods

The samples were then placed into a constant-volume cell. To ensure consistent hydration conditions, the samples were placed in buckets connected to a pump and subjected to vacuum treatment for 2 hours. Afterward, the buckets were gradually filled with different alkaline
solutions, and the pressure was maintained for 12 hours. The bottles were then sealed with plastic film and lids and stored in a constant-temperature room for the planned duration. The pHs were monitored and renewed when a significant decrease was observed.

For the microstructure observation, subsamples were cut from the whole hydrated sample. All the subsamples were selected from the middled layer and central position uniformly. The subsamples were immediately freeze-dried [24, 25] after cutting to retain the microstructure.

# 3 Results

## 3.1 Mercury intrusion porosimetry (MIP)

The pore size distribution curves for as compacted sample and samples hydrated for one week with different pHs are shown in Fig. 3a and b. It can be observed from Fig. 3a that for the as-compacted sample, the final value of the intruded void ratio ($e_{min}$ = 0.44) is lower than the global soil void ratio (e = 0.50), due to the inaccessible pores. However, the void ratio values of hydrated samples are higher than the global one and decrease with the pH of alkaline solutions. The higher pH of the solution caused more significant shrinkage of the diffuse double layer, leading to the increase of inaccessible pore volume.

From the derivative curves Fig. 3b, a typical bimodal porosity is observed in as compacted sample, and two pore families can be defined: micropores and macropores, with the mean size of 0.02 and 50 µm respectively. The micropores correspond to the intra-aggregate pores, while the macropores correspond to the inter-aggregate pores and the pores between bentonite and sand. Upon hydration, both the macropores and micropores decreased, and the third pore family appeared with varying mean sizes from 0.4 to 1.3 µm, these evolutions result from the break-up of the quasicrystals. The break-up generates the smaller particles with fewer intra-aggregated pores, fills the macropores, and forms inter-particle mesopores (Fig. 1, stages 2 to 3).

As the pH of the alkaline solution increases, the micropore population also increases due to the higher potassium concentration. This leads to the replacement of interlayer cations from sodium to potassium, decreasing the cation exchange capacity, reducing the crystalline swelling of bentonite aggregates and decreasing the proportion of quasicrystal breakup. The reduction of break-up also leads to a decrease in the uniformity and population of the mesopores. Additionally, the replacement reduces the capacity of surface sites, should decrease the double layer swelling (DDL in Fig. 1, stage 3 to 4), and then increase the macropores' population and size, which is opposite from the results in Fig. 3b. This indicates that the macropores' evolution is dominated by the contribution from release stress. The higher pH solution causes a lower swelling pressure [8] and reduces the population of macropores.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Pore size distribution of the bentonite-sand mixture hydrated with different solutions in one week</figcaption></figure>

Fig. 4a depicts the pore size distribution curves for different hydration times by 13.5 pH alkaline solution. The values of the final intruded void ratio show a slight decrease over time. The explanation is that the hyper pH solution leads to a fabric re-arrangement ((Fig. 1, stage 5) with less capacity of surface sites, and then decreases the swelling pressure, resulting in a reduction of release stress and a decrease of void ratio.

As shown in the derivative curves in Fig. 4b, the fabric re-arrangement reduces the intra-aggregate pores (micropores). This re-arrangement promotes further filling of the macropores by quasicrystals, leading to an increase in inter-particle pores (mesopores) and a reduction in inter-aggregate pores (macropores). The remaining macropores may contributed by the pores between bentonite and sand skeleton.
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Pore size distribution of the bentonite-sand mixture hydrated with different times by 13.5 pH alkaline solution</figcaption></figure>

## 3.2 Scanning electron microscope

The Scanning Electron Microscope results (SEM) partly support the assumptions for MIP tests Fig. 5. The dispersion of laminar structures is clearly observed in 13.5 pH of alkaline conditions, and more pronounced with time. The presence of bentonite dispersion supports the fabric re-arrangement explanation.

Further photographs in low magnification are needed to obtain the interpretation of macropores and mesopores evolution.

## 4. Conclusion

In this study, the compacted bentonite-sand (40/60) mixture samples with a dry density of 1.80 Mg/m³ were hydrated by varying pH of alkaline solutions and durations. The pore size distributions, analyzed by MIP, reveal the evolution of different pore families in conjunction with the mechanism of bentonite swelling, further supported by the dispersion phenomenon from SEM. The main observations are shown as follows:

(1) The increase in pH of the alkaline solution decreases the final intruded void ratio of compacted

mixtures by weakening the release stress during demoulding and decreases their mesopore population by reducing the crystalline swelling of bentonite aggregates and break-up of quasicrystals.

(2) For the compacted samples hydrated over time with a 13.5 pH alkaline solution, fabric reorganization is supposed to explain the reduction in the population of micropores and macropores, the decrease in the mean macropore size, and the increase in both the population and mean size of mesopores.

(3) The dispersion of bentonite layers under 13.5 pH solution, observed by SEM, can partly support the fabric re-arrangement inside the bentonite aggregate. Some low magnification figures are expected for further investigations.

<figure><img src="image_6.png" /><figcaption>**Fig. 5.** SEM photographs of bentonite--sand mixture samples hydrated by different conditions in high magnification</figcaption></figure>

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