Scholarly article on topic 'Microfabric change of electro-osmotic stabilized bentonite'

Microfabric change of electro-osmotic stabilized bentonite Academic research paper on "Earth and related environmental sciences"

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{"Electro-osmotic stabilization" / "Sodium bentonite" / Microfabric / "Ion exchange" / "Geotechnical property"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Hui Wu, Liming Hu

Abstract Electro-osmotic stabilization has long been studied as a soft soil improvement technique, while the influence of an applied electrical field on the soil microfabric and minerals is always ignored. In this study, three laboratory experiments were conducted on sodium bentonite using copper, iron and graphite electrodes to investigate the microfabric and chemical composition change before and after electro-osmotic stabilization. The soil samples near the anode were identified using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). The microfabric of the sodium bentonite changed from flocculated fabric to aggregated fabric after electro-osmotic stabilization. Regular calcium sulfate tubes were generated near the copper and iron anodes. EDX tests showed that the content of sodium near the anode decreased, while the copper, iron and calcium presented substantial increase, indicating that the sodium ions were substituted by copper, iron, and calcium ions in copper, iron and graphite experiments respectively. The change of microfabric and the ion exchange reactions between sodium, copper, iron and calcium ions are the main reasons for the significant decrease of the plasticity index and free swelling ratio.

Academic research paper on topic "Microfabric change of electro-osmotic stabilized bentonite"

Microfabric change of electro-osmotic stabilized bentonite

Hui Wu1, Liming Hu *

State Key Laboratory of Hydro-Science and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, PR China

ARTICLE INFO ABSTRACT

Electro-osmotic stabilization has long been studied as a soft soil improvement technique, while the influence of an applied electrical field on the soil microfabric and minerals is always ignored. In this study, three laboratory experiments were conducted on sodium bentonite using copper, iron and graphite electrodes to investigate the microfabric and chemical composition change before and after electro-osmotic stabilization. The soil samples near the anode were identified using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). The microfabric of the sodium bentonite changed from flocculated fabric to aggregated fabric after electro-osmotic stabilization. Regular calcium sulfate tubes were generated near the copper and iron anodes. EDX tests showed that the content of sodium near the anode decreased, while the copper, iron and calcium presented substantial increase, indicating that the sodium ions were substituted by copper, iron, and calcium ions in copper, iron and graphite experiments respectively. The change of microfabric and the ion exchange reactions between sodium, copper, iron and calcium ions are the main reasons for the significant decrease of the plasticity index and free swelling ratio.

© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

CrossMark

Article history:

Received 28 March 2014

Received in revised form 2 September 2014

Accepted 11 September 2014

Available online 26 September 2014

Keywords:

Electro-osmotic stabilization Sodium bentonite Microfabric Ion exchange Geotechnical property

1. Introduction

Electro-osmotic stabilization is a widely studied method for soft soil improvement (Casagrande, 1948; Bjerrum et al., 1967; Esrig, 1968; Casagrande, 1983; Shang and Lo, 1997; Hu et al., 2012; Wu and Hu, 2013; Hu and Wu, 2014). The pore water is dragged from anode to cathode by the mobilizable cations under electrical field. Numerous laboratory tests and field experiments have been conducted to investigate the stabilization effect on soil mass in terms of water discharge, voltage distribution, current, pore water pressure, settlement and shear strength (Lo and Ho, 1991a; Micic et al., 2001; Lefebvre and Burnotte, 2002; Cherepy and Wildenschild, 2003; Burnotte et al., 2004; Reddy et al., 2011; Hu et al., 2013). The drainage and consolidation process were the major concerns in the above studies. Both the laboratory tests and field experiments illustrated the change of geotechnical properties, including Atterberg limit, swelling potential, and cation exchange capacity, after electro-osmotic stabilization (Bjerrum et al., 1967; Esrig and Gemeinhardt, 1967; Casagrande, 1983; Lo and Ho, 1991b; Bergado et al., 2000; Micic et al., 2001; Asavadorndeja and Glawe, 2005; Ou et al., 2009; Abdullah and Al-Abadi, 2010). Most of the previous studies simply attributed the change of soil properties to the increase of salinity. During the electro-osmotic stabilization process, complex chemical reactions occurred near the anode and cathode, including electrolysis of pore water, electrode corrosion, ion exchange and transport, and

* Corresponding author. Tel.: +86 10 62797416; fax: +86 10 62773576.

E-mail addresses: wuhui06.ts@gmail.com (H. Wu), gehu@tsinghua.edu.cn (L. Hu).

1 Tel.: +8610 62794154.

cementation (Acar and Alshawabkeh, 1993; Chien et al., 2009). Therefore, the microfabric and chemical compositions changed and induced the change of soil properties (Zimmie and Almaleh, 1976; Du et al., 1999; Santamarina et al., 2001; Dananaj et al., 2005; Mitchell and Soga, 2005; Al-Hamdan and Reddy, 2008; Yong et al., 2009; Karakaya et al., 2011; Zhang et al., 2012; Du et al., 2014). Dananaj et al. (2005) studied the influence of sodium and calcium contents on the coefficient of permeability and swelling parameters of bentonite. Du et al. (2014) reported the microfabric characteristics of cement-stabilized zinc-contaminated kaolin, which showed that the change of soil microfabric and zinc concentration had significant influence on soil properties including Atterberg limits and stress-strain characteristics.

The purpose of this paper is to study the change of soil microfabric and chemistry before and after electro-osmotic stabilization, accounting for the micro-mechanism of geotechnical property change. Scanning Electron Microscopy (SEM) was used to identify soil microfabric, and Energy Dispersive X-ray Spectroscopy (EDX) test was performed to analyze the change of chemical composition during the electro-osmotic stabilization process.

2. Material and experiments

Sodium bentonite with Na+ as predominant exchangeable cation was used for the electro-osmotic stabilization experiments. The benton-ite was obtained from Zhangjiakou, Hebei Province, China. The basic geotechnical properties and chemical composition of the sodium ben-tonite was summarized in Table 1. The Atterberg limits, free swelling ratio and cation exchange capacity of the sodium bentonite were

http://dx.doi.org/10.1016/j.clay.2014.09.014

0169-1317/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Table 1

Basic geotechnical properties and chemical composition of the sodium bentonite.

Properties Values

Geotechnical properties

Specific gravity, Gs 2.625

Initial water content, w (%) 1.33

Liquid limit, LL (%) 155

Plastic limit, PL (%) 31

Plasticity index, PI (%) 124

Specific surface area, S (m2/g) 33.89

Free swelling ratio, FSR (%) 540

Cation exchange capacity, CEC (meq/100 g soil) 40.03

pH (after saturated with deionized water) 9.4 Chemical composition (weight proportion, %)

SiC>2 68.2

Al2Ö3 15.1

CaC 4.2

MgC 3.8

Fe2>3 3.1

Na2O 2.9

K2O 1.6

SC3 0.4

TiO2 0.4

Cl 0.08

detected according to the Chinese Standard for Specification of Soils (SL237-1999), and the specific surface area was measured according to the ISO 9277-1995. The high values of plasticity index (124%) and free swelling ratio (540%) of the sodium bentonite indicate particularly high content of smectite. The presence of K2O indicates the existence of illite. The sodium bentonite contains 2.9% of Na2O and 1.6% of K2O and this implies that the bentonite possesses a great water adsorption capacity (Al-Mukhtar et al., 2010). This is also confirmed from the high plasticity index and free swelling ratio. A small amount of SO3 is identified in the bentonite. The cation exchange capacity was 40 meq/100 g, which means that strong ion exchange reaction may occur during the experiment.

Three soil column tests for electro-osmotic stabilization were conducted using copper, iron and graphite as anode respectively. The diameter and height of the bentonite samples are 90 mm and 200 mm, respectively. The anode was placed on the bottom, and the cathode was placed on the top of the bentonite sample, to induce an electro-osmotic flow from the bottom upwards. The bottom boundary was impervious and the top boundary was permeable for the drainage of pore water. After the fill and compaction of bentonite, the samples were saturated by deionized water with vacuum saturation method and the initial water content was 150%. A voltage of 20 V was applied to the soil samples and lasted for about 24 h.

Before and after the electro-osmosis treatment, the soil near the anode was carefully taken out and desiccated by freeze-drying method to maintain the microfabric. The SEM and EDX tests were then performed using the field emission scanning electron microscope (JSM 6301F,JEOL,Japan).

3. Results and discussion

3.1. Change of the microfabric

The results of SEM and EDX tests before and after electro-osmotic stabilization were presented in Fig. 1. Fig. 1(a) showed the SEM image of bentonite before electro-osmotic stabilization. The microfabric was characterized by large amounts of flocculus formed after the sample was immersed into the deionized water during the saturation period. These flocculi greatly increased the interaction between the soil and water. After electro-osmotic stabilization, the flocculated fabric changed

to aggregated fabric, and the soil aggregates were clearly visible in Fig. 1(b), Fig. 1(c) and Fig. 1(d). This phenomenon has also been observed by adding sodium nitrate electrolyte to bentonite (Stawinski et al., 1990). Compared to the flocculated fabric, the aggregated fabric is denser and possesses smaller void ratio (Mitchell and Soga, 2005), which is easier for water to enter into the clay with flocculated fabric than the clay with aggregated fabric. As a result, the water adsorption capacity of the bentonite with aggregated fabric is smaller than the bentonite with flocculated fabric.

Some well-formed regular tubes were found in the bentonite near the anode in the copper and iron experiments as shown in Fig. 2, while in the graphite experiment, a similar phenomenon was not observed. The chemical composition and formation of the tube was analyzed in the next section.

3.2. Change of chemical composition

Table 1 illustrated that the main elements in the bentonite were silicon, aluminium, oxygen with relatively less calcium, magnesium, iron, sodium, potassium, sulfur, titanium and chlorine. During the electro-osmotic stabilization process, complex oxidation reactions occur at the copper and iron anodes and induce the release of copper and iron ions from the anode into the bentonite. Therefore, EDX tests were performed to measure the content of elements that originally existed in the bentonite as well as elements that released from the anode. Fig. 1 displayed the full scale EDX results on the corresponding SEM images. The comparison of the element content before and after electro-osmotic stabilization was demonstrated in Fig. 1(e). The content of the predominant exchangeable cation, Na+, almost decreased to 0 in the copper and iron experiments and another cation, Ca2+, increased about 4 times near the copper anode and 1.5 times near the iron anode. In the graphite experiment, the content of Na+ only decreased about 15% while the Ca2+ content increased about 8.5 times. The content of other elements, such as Mg, Al, Si, and K, decreased while the content of S and Cl increased. The significant change of chemical elements was also reported by Acar and Alshawabkeh (1993), indicating that significant species transport processes under electric field consisted of mass fluxes generated by diffusion, electromigration and electro-osmosis, among which the ionic migration was the most significant component. Therefore the change of the content of Na, Mg, K, S and Cl was mainly induced by electromigration under the applied electric field.

Fig. 1(e) also illustrated that the content of copper increased from 0 to about 32.7% near the copper anode and the content of iron increased from 3.3% to about 22.4% near the iron anode. The significant increase of the copper and iron contents is due to the oxidation reaction near the anode, leading to the release of metal ions from the anode into the soil. The iron content in graphite experiment, nevertheless, decreased about 52% under the effect of electromigration.

Clay minerals have the characteristic of adsorbing cations and retaining them in an exchangeable state (Bradl, 2004). Replacement of cation may occur between different cations and the ease of the replacement depends mainly on the valence, hydrated radius of cation and relative abundance (Mitchell and Soga, 2005). Mitchell and Soga (2005) also reported that if other things were equal, divalent cations were held more tightly than monovalent cations and they showed a typical exchange series as Na+ < Li+

< K+ < Rb+ < Cs+ < Mg2 + < Ca2+ < Ba2 + < Cu2 + < Al3 +

< Fe3+ < Th4+. According to the exchange abilities of different cations, the sodium ion in the double layer has the priority to be exchanged by the entering copper and iron ions and original calcium ion. The change of the element content in Fig. 1(e) revealed that the original sodium ions in the bentonite were replaced by the copper and iron ions in the

Fig. 1. SEM images and chemical composition of the bentonite before and after electro-osmotic stabilization: (a) before electro-osmotic stabilization (magnification x2000), (b) after electro-osmotic stabilization by copper electrode (magnification x2000), (c) after electro-osmotic stabilization by iron electrode (magnification x2000), (d) after electro-osmotic stabilization by graphite electrode (magnification x1000), and (e) comparison of element content in the bentonite before and after electro-osmotic stabilization.

Fig. 2. SEM images of the regular tubes after electro-osmotic stabilization: (a) regular tubes in the bentonite stabilized by copper electrode (magnification x3000), and (b) regular tubes in the bentonite stabilized by iron electrode (magnification x3000).

copper and iron experiments, while in the graphite experiment, the sodium ions were partly substituted by calcium ions since graphite was inert and could not provide enough ions. As a result, the substitution of sodium ion was more complete in the copper and iron experiments than that in the graphite experiment. The exchanged sodium ions then entered the free pore water and were transported under the electrical field. The copper, iron and calcium ions, on the other hand, remained in the double layer and the ion species and concentration in the double layer was then changed, causing the shrink of the double layer characteristics.

33. Chemical composition and formation of the regular tube

As shown in Fig. 2, well-formed regular tubes were found after the copper and iron experiments. The comparison of the EDX results in Fig. 1(b)-(c) and Fig. 2(a)-(b) indicated that the content of calcium and sulfur in the regular tubes was high while the content of silicon, copper and iron was low. In order to identify the chemical composition of the tube, two points (one on the tube and one out of the tube) were chosen to do the EDX test. The result of the copper experiment was shown in Fig. 3(a)-(b). The EDX patterns illustrated that the chemical element of the tube was mainly sulfur, calcium and oxygen, while the chemical element of the point out of the tube mainly consisted of silicon, chlorine, copper and oxygen. Fig. 3(c)-(d) displayed the result of the iron experiment and illustrated that the main chemical elements of the tube were also sulfur, calcium and oxygen, while the chemical elements of the point out of the tube were mainly silicon, iron and oxygen. The atomic proportion of Ca:S:O on the tube found near the copper anode is 12.11:14.79: 65.4, which is close to the stoichiometry of 1:1:4. For the tube observed near the iron anode, the atomic proportion is 14.9:16.81:67.25 and is also close to the stoichiometry of 1:1:4. This indicated that the main substance of the tube must be CaSO4. Sulfur usually occurs in soil in the forms of sulfide and sulfate (Mitchell and Soga, 2005). Under the electrical field, the sulfide and sulfate ions moved to the anode and reacted with calcium to generate CaSO4 in the acid environment near the anode. The EDX patterns demonstrated that the chemical composition apart from the tube was most likely to be oxide and chloride of silicon and copper or iron.

The formation of CaSO4 tube requires an acid environment. In the three experiments, the pH of the bentonite near the anode after electro-osmotic stabilization was measured and the value was 7, 6.92 and 8.01 for copper, iron and graphite experiments respectively. In the graphite experiment, the bentonite near the anode was alkaline and the formation of calcium sulfate tube was restrained. Therefore, the CaSO4 tube was found only in the copper and iron experiments.

3.4. influence on the geotechnical properties

The liquid limit, plastic limit and free swelling ratio of the bentonite samples near the anode were measured to analyze the influence of electro-osmotic stabilization on the geotechnical properties of the sodium bentonite as shown in Table 2. After electro-osmotic stabilization, the liquid limit decreased while the plastic limit increased. The corresponding plasticity index encountered a remarkable decrease, which indicated that the water adsorption capacity of the bentonite was greatly reduced. The free swelling ratio also decreased especially for the bentonite stabilized with copper and iron electrodes. The results of the bentonite stabilized with copper and iron electrodes showed a decrease of about 46.2% and 43.38% for the liquid limit and an increase of about 28% and 20.8% for the plastic limit, while the change occurred to the bentonite stabilized with graphite electrode was much smaller, with a decrease of about 10.7% for the liquid limit and an increase of about 13.9% for the plastic limit. Correspondingly, the plasticity index decreased about 74.2%, 64.1% and 24.6% for the copper, iron and graphite experiments respectively. As for the free swelling ratio, the decrease was about 430%, 400% and 200% in the three experiments. Therefore, the electro-osmotic stabilization can reduce the plasticity index and free swelling ratio of the bentonite, especially when the copper and iron electrodes are used for this process. The effect on geotechnical properties of the graphite electrode is much smaller than that of the copper and iron electrode.

Cation type exerted a controlling influence on the geotechnical properties of soil (Mitchell and Soga, 2005). The theory about electrical double layer illustrated that the species and concentration of ions adsorbed in the double layer had significant influence on the thickness of the double layer and the potential distribution in it (Van Olphen,

Fig. 3. Element content of a point on and out of the regular tubes: (a) element content of a point on the tube (copper electrode), (b) element content of a point out of the tube (copper electrode), (c) element content of a point on the tube (iron electrode), and (d) element content of a point out of the tube (iron electrode).

Atterberg limits, plasticity index and free swelling ratio of the bentonite clay sample near the anode before and after electro-osmotic stabilization.

Type of electrode Geotechnical properties

Liquid limit (%) Plastic limit (%) Plasticity index (%) Free swelling ratio (%)

Before After Before After Before After Before After

Copper 155 108.8 31 59 124 49.8 540 110

Iron 155 111.7 31 51.8 124 59.9 540 140

Graphite 155 144.3 31 44.9 124 99.4 540 340

1977; Shangetal., 1993). The previous studies also confirmed the influence of microfabric and chemical compositions on the soil properties (Dananaj et al., 2005; Mitchell and Soga, 2005; Al-Hamdan and Reddy, 2008; Du et al., 2014). The polar water molecules can easily enter the space between the lattices and cause the clay minerals to expand since the repulsion force between the soil lattices is relatively high in the presence of sodium ions (Mitchell and Soga, 2005). This is why the sodium bentonite always presents a high plasticity index and high free swelling ratio. Under the electrical field, the calcium, copper and iron ions released from the anode gradually substituted the original sodium ions. The repulsion force decreases with the increasing cation valence, causing the decrease of the thickness of the double layer. The space between lattices is then greatly reduced and the water adsorption capacity also decreases. The expansion of the double layer in the presence of water was then reduced and the shrinkage and swelling characteristics were limited.

Both the change of soil microfabric and the ion exchange reactions near the anode decreased the water adsorption capacity of the bentonite, causing the decrease of the plasticity index and free swelling ratio as shown in Table 2. The decrease of the plasticity index and free swelling ratio was larger in the copper and iron experiments than that in the graphite experiment since the ion exchange was more complete. Therefore, the swelling and shrinkage capacities of the bentonite are significantly reduced after electro-osmotic stabilization, especially when copper or iron electrode is used. The influence of electro-osmotic stabilization on the geotechnical properties of bentonite implies that this technique can be used to improve the expansive soil sites, which has long been recognized as improper for constructions due to its severe swelling shrinkage characteristic. Both the consolidation effect and the change of microfabric and cation species can strengthen and improve the expansive soil. However, since the entrance of copper ions into the soil may cause environment problems, iron electrode may be more suitable for field application. The use of this technique in practice needs more research to deal with the cost and environment problems.

4. Conclusions

Three electro-osmotic stabilization tests were conducted with copper, iron and graphite electrodes on sodium bentonite. SEM and EDX tests were performed to study the change of microfabric and chemical compositions of the bentonite near the anode. The influence of electro-osmotic stabilization on the soil properties was then discussed from the aspect of microfabric and chemical compositions.

The SEM results showed that the microfabric of the sodium benton-ite changed from the flocculated fabric to the aggregated fabric after electro-osmotic stabilization in the three experiments and the water adsorption capacity was therefore reduced.

The EDX results indicated that the copper and iron ions released from the anode substituted the original sodium ions for copper and iron experiments, causing the decrease of the space between the lattice and the reduction of the water adsorption capacity. For the graphite experiment, the sodium ions were partly substituted by calcium ions. With the movement of sulfate ions from the cathode to the anode, calcium sulfate tube was generated in the presence of an acid environment near the copper and iron anodes.

Both of the plasticity index and free swelling ratio of the bentonite near the anode are greatly decreased after electro-osmotic stabilization, especially in the copper and iron experiments. The change of microfabric and the ion exchange reaction between sodium and calcium, copper and iron ions are the main reasons for the significant decrease. Since the replacement of sodium ion is much more complete in the copper and iron experiments than that in the graphite experiment, the decrease of plasticity index and free swelling ratio is larger in the copper and iron experiments than that in the graphite experiment. The electro-osmotic

stabilization can be a potential technique to improve bentonite or expansive soil sites.

Acknowledgements

Financial supports from the National Basic Research Program of China (Grant No. 2012CB719804), the National Natural Science Foundation of China (50978139, 51128901, 51323014), and the State Key Laboratory of Hydro-Science and Engineering (SKLHSE-2012-KY-01, SKLHSE-2013-D01) are gratefully acknowledged.

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