Impact of wetting–drying cycles on swelling behavior of clayey soils modified by silica fume

Applied Clay Science 52 (2011) 345–352

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Research Paper

Impact of wetting–drying cycles on swelling behavior of clayey soils modified by silica fume Ekrem Kalkan ⁎ Ataturk University, Oltu Earth Sciences Faculty, Geological Engineering Department, 25240 Erzurum, Turkey

a r t i c l e

i n f o

Article history: Received 28 June 2010 Received in revised form 22 March 2011 Accepted 23 March 2011 Available online 16 April 2011 Keywords: Clayey soil Silica fume Wetting–drying cycles Swelling characteristics

a b s t r a c t Expansive clayey soils contain silicate clay minerals that have the potential for swelling and shrinkage under changing moisture contents. Cyclic wetting–drying phenomena can cause progressive deformation of expansive clayey soils, which may affect building foundations, drainage channels, and liner and cover systems in waste containment facilities. To reduce the effects of cyclic wetting–drying phenomena, it is essential to modify these soils by stabilization techniques. For this purpose, expansive clayey soil samples have been modified using silica fume waste material and the effects of wetting and drying cycles on swelling behavior of modified expansive clayey soils have been investigated under laboratory conditions. A natural clayey soil and clayey soil–silica fume mixtures have been compacted at the optimum moisture content and all samples have been subjected to cyclic wetting–drying and swelling tests. The results show that silica fume decreases the progressive deformation of modified expansive clayey soils subjected to cyclic drying and wetting. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Clayey soils are generally classified as expansive soils. This means that some clays will tend to expand as they absorb water and will shrink as water is drawn away. These clayey soils contain clay minerals that have the potential for swelling and shrinkage under changing moisture contents. Clay minerals could originate from the weathering of shale, slate, sandstone, and limestone. Another source is the diversification of volcanic ash deposited under marine conditions during geologic times, settled alone or mixed with shale or limestone (Grim 1968; Kalkan and Bayraktutan, 2008). Expansive soils are known to cause severe damage to structures resting on them. However, these soils are very important in geology, construction, and for environmental applications, due to their wide usage as impermeable and containment barriers in landfill areas and other environmentally related applications (Erguler and Ulusay, 2003; Harvey and Murray, 1997; Kayabali, 1997; Keith and Murray 1994; Murray, 2000; Sabtan, 2005). Safe and economic designs of foundations on clayey soils and performance of compacted clayey soils for geotechnical purposes require the knowledge of swelling characteristics such as swelling pressure, swelling potential and swelling index. Cyclic drying and wetting phenomena can cause progressive deformation of expansive clayey soils, which may affect building foundations, drainage channels, buffers in radioactive waste disposals, etc. (Guney et al., 2007; Nowamooz and Masrouri, 2008; Rao et al., 2001).

⁎ Tel.: +90 442 816 62 66; fax: +90 442 816 33 32. E-mail address: [email protected]. 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.03.014

Several soil stabilization methods are available for stabilization of expansive clayey soils. These methods include the use of chemical additives, rewetting, soil replacement, compaction control, moisture control, surcharge loading, and thermal methods (Chen, 1988; Nelson and Miller, 1992; Yong and Ouhadi, 2007). Many investigators have studied natural, fabricated, and by-product materials and their use as additives for the stabilization of clayey soils. All these methods may have the disadvantages of being ineffective and expensive. Therefore, new methods are still being researched to increase the strength properties and to reduce the swell potential of expansive soils (Akbulut et al., 2007; Al-Rawas et al., 2005; Asavasipit et al., 2001; Bell, 1996; Cetin et al., 2006; Guney et al., 2007; Kalkan and Akbulut, 2004; Kolias et al., 2005; Miller and Azad, 2000; Moavenian and Yasrobi, 2008; Prabakar et al., 2003; Puppala and Musenda, 2002; Senol et al., 2006; Sezer et al., 2006). Silica fume is a by-product of silicon material or silicon alloy metal factories. Although the silica fume is a waste material of industrial applications, it has become the most valuable by-product among the pozzolanic materials due to its very active and high pozzolanic property (Atis et al., 2005). In previous studies, the effects of silica fume on the strength, permeability, and swelling characteristics of clayey soils were investigated. It was seen that silica fume improved these properties of clayey soils (Kalkan, 2009a,b; Kalkan and Akbulut, 2002, 2004). Recently, there have been many researchers investigating the influence of cyclic wetting and drying on the swelling behavior of natural clayey soils. Some researchers found out that swelling potential decreases when expansive clayey soils are repeatedly subjected to swell then allowed to dry to their initial water content (Al-Homoud et al., 1995; Basma et al., 1996; Day, 1994; Dif and

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E. Kalkan / Applied Clay Science 52 (2011) 345–352

Bluemel, 1991; Osipov et al., 1987; Rao et al., 2000; Rao and Revanasiddappa, 2006). On the other hand, several researchers have examined the influence of cyclic wetting and drying on the swelling behavior of expansive soils modified by lime. It is observed that the swelling potential of expansive soils modified by lime increases when it is subjected to the cyclic wetting and drying (Guney et al., 2007; Rao et al., 2000, 2001; Yong and Ouhadi, 2007). The basic objectives of this research are to investigate the modification of an expansive clayey soil using a by-product material and to evaluate the effect of drying and wetting cycles on the swelling characteristics of the modified expansive clayey soil. 2. Materials and methods 2.1. Clayey soil The clayey soil was supplied from the clay deposits of Oltu Oligocene sedimentary basin, Erzurum, Northeast Turkey. The disturbed expansive soil was collected by open excavation, from a depth of 0.5 m from the natural ground level of this sedimentary basin. This soil was placed in plastic bags and transported to a soil mechanics laboratory. These deposits are concentrated in two different stratigraphic horizons namely the Late Oligocene and the Early Miocene sequences. Clay-rich sedimentary units were deposited in shallow marine and lagoonar mixed environments. Their clay minerals originated from the alteration of Eocene calc-alkaline islandarc volcanics, primarily from pyroclastics (trachyte and andesite flows) which formed the basement for the Oltu depression area. Smectite group clay minerals are abundant in these deposits and they consist of montmorillonite and nontronite, while halloysite, palygorskite and hydrobiotite are also present. This clayey soil is overconsolidated and it has clayey-rock characteristics in natural conditions. It is defined as a high plasticity soil according to the Unified Soil Classification System (Akbulut, 1999; Kalkan, 2003; Kalkan and Bayraktutan, 2008). The grain-size distribution and X-ray diffraction (XRD) pattern of the clayey soil are shown in Figs. 1 and 2, respectively. Its chemical analysis and index properties are summarized in Tables 1, and 2, respectively. 2.2. Silica fume Silica fume used in this experimental study was supplied by the Ferro-Chromate Factory in Antalya (Turkey). Silica fume, a very fine solid material generated during silicon metal production, has historically been considered a waste product. It is a by-product of production of silicon metal or ferrosilicon alloys. Although the silica fume is a waste industrial material, it has become one of the most valuable by-product pozzolanic materials due to its very active and high pozzolanic property. One of the most beneficial uses of silica fume is in concrete. Because of its chemical and physical properties, it is a very reactive pozzolan (Atis et al., 2005; Kalkan and Akbulut, 2004). The grain size distribution of silica fume is shown in Fig. 1. Its chemical and index properties are summarized in Tables 1 and 2, respectively.

the materials were blended together in dry conditions. The mixtures of dry clayey soil and silica fume were then mixed with the required amount of water for optimum water content. All mixing was done manually, and proper care was taken to prepare homogeneous mixtures at each stage of mixing. The optimum water contents of clayey soil–silica fume mixtures were determined by Standard Proctor tests in accordance with ASTM D 698 (1995). The mixtures of clayey soil–silica fume were compacted at optimum water content to obtain the stabilized clayey soil samples. For cyclic swelling tests, cylindrical samples were prepared at optimum water content for the natural clayey soil and clayey soil–silica fume mixtures. Cylindrical metal molds of 70 mm in diameter and 20 mm high were used to prepare samples for each combination of clayey soil and clayey soil–silica fume mixtures for swelling tests. The swelling tests were carried out in accordance with ASTM D 4546 (1990). 2.4. Cation exchange capacity (CEC), specific surface area (SSA) and pH tests The CEC is the capacity of a soil for ion exchange of positively charged ions between the soil and the soil solution. It is an important concept for soil sciences. The SSA is a material property of solids which measures the total surface area per unit of mass, solid or bulk volume, or cross-sectional area. It has a particular importance in the cases of adsorption, heterogeneous catalysis, and reactions on surfaces. Soil pH is an indication of the acidity or alkalinity of soil. The clay soils containing montmorrillonitic minerals are subjected to alkalinity problems. The CEC and SSA of natural clayey soil and clayey soil–silica fume mixtures have been determined by methods using Ammonium Acetate (Chapman, 1965) and Ethylene Glycol Mono Ethylene Ether (Heilman et al., 1965), respectively. Furthermore, the pH values of natural clayey soil and clayey soil–silica fume mixtures were determined with a pH meter (Thermo scientific Orion 5 star plus multifunction). 2.5. Consistency limit tests The effect of silica fume on the consistency behavior of compacted clayey soil samples was determined by the evaluation of Atterberg limits. The natural and stabilized clayey soil samples were subjected to liquid limit and plastic limit tests in accordance with ASTM D 4318 (1995). 2.6. Compaction tests To determine the optimum water contents of natural and stabilized clayey soil samples and to prepare the samples for swelling and the wetting–drying cycle tests, Standard Proctor tests were carried out in accordance with ASTM D 698 (1995). The compaction curves were plotted and the values of optimum water content and maximum dry unit weight were determined from the compaction curves. The natural clayey soil and the clayey soil–silica fume mixtures were compacted at the optimum water content to prepare samples for the swelling pressure, swelling potential and wetting–drying cycle tests.

2.3. Preparation of samples for the tests 2.7. Swelling pressure tests The clayey soil was dried in an oven at approximately 105 °C before a grinding process. To prepare the mixtures of clayey soil and silica fume, first, the required amounts of clayey soil and silica fume were measured by a total dry weight of sample and mixed together in the dry state. The amounts of silica fume were selected to be 10%, 20%, 25% and 30% of the total dry weight of the clay soil–silica fume mixtures. The amounts of clayey soil and silica fume were weighed in accordance with the total weight of the composite samples and then

Swelling pressure tests were performed in the standard onedimensional oedometer aparatus in accordance with ASTM D 4546 (1990). The swelling pressure of each sample was directly measured from the surcharge, which loads the sample. The sample was confined in the consolidation ring of 74 mm diameter and 20 mm high, and water was allowed to flow into the sample. The samples were submerged in water. The deflection of the dial gauge was set to zero. As a result, when the samples showed no further tendency to swell,

E. Kalkan / Applied Clay Science 52 (2011) 345–352

Percentage passing, %

100

347

periodically until there were no further changes in swelling. The swelling percentage can be expressed as

80

SPT = HMS = HOT

60

where SPT is the swelling percentage, HMS is the axial expansion in mm, and HOT is the original thickness of the sample in mm.

ð2Þ

2.9. Wetting–drying cycle tests

40 Clayey soil Silica fume

20

0

0.000001

0.0001

0.01

100

1

Grain size, mm Fig. 1. The grain-size distributions of clayey soil and silica fume.

the maximum surcharge load, PMS, at that point was used for the calculation of the swelling pressure. The swelling pressure can be expressed as SPR = PMS = A

ð1Þ

where SPR is the swelling pressure in kPa, PMS is the maximum surcharge load on the sample in kN, and A is the area of sample in m2. The time required to reach a maximum value varies considerably, depending upon the percentage of clay (or silica fume). For the lowest percentage of clay, swelling stops in a day, but samples with the highest percentage of clay only stop swelling in 2 days or more.

The wetting–drying cycle test has the greatest advantage of being repeatable on the same sample (Al-Homoud et al., 1995; Guney et al., 2007; Rao et al., 2000). This test was performed to investigate the effect of silica fume on the swelling characteristics subjected to the wetting–drying cycles. All samples were submerged in tap water allowing the samples to swell fully over 48 h. Water was then drained and the consolidation cell with wetted samples were transferred from the odometer apparatus in to a test room. The wetted samples were then allowed to air-dry to their initial water contents at 22 °C. The drying of the samples required about five days. After all dried samples were carefully weighed, the dried natural and stabilized clayey soil samples within the consolidation cells were again placed in the odometer apparatus and these samples were wetted by allowing them to swell for 48 h. A wetting–drying cycle test was completed with the saturation of samples within the consolidation cell and their drying in an air-dry environment. In these tests, the natural and stabilized clayey soil samples within consolidation cells were subjected to 5 cycles of alternate wetting and drying. It was observed that there were no differences in the test results after five cycles of wetting–drying (Gangadhara, 1998; Rao et al., 2001). The wetting– drying procedure was terminated after five cycles of wetting–drying. 2.10. Images of samples

2.8. Swelling potential tests Swelling potential tests were carried out in a similar way as swelling pressure tests. However, the sample was allowed to swell under a small load. The samples were loaded to a static pressure of 0.7 kPa. The natural and stabilized clayey soil samples were submerged in pure water. The samples were allowed to swell under the initial seating load. The dial gauge readings were recorded

In order to evaluate the interaction between clayey soil and silica fume particles in modified samples, natural and modified samples were subjected to image analysis. Images of samples were magnified 5000 times by means of a SEM modeled Jeol 6400 SEM. After blending clayey soil and silica fume under dry conditions and mixing them with the required amount of water for optimum moisture content, all samples were allowed to air-dry to their initial water content at 22 °C.

Fig. 2. XRD pattern of clayey soil.

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E. Kalkan / Applied Clay Science 52 (2011) 345–352

Table 1 Chemical compositions of clayey soil and silica fume used in the study.

Table 3 The results of cation exchange capacity and specific surface area and pH tests.

Property

Clayey soil

Silica fume

No Samples

Compound Al2O3, % Fe2O3, % CaO3, % CaO, % MgO, % Na2O, % K2O, % SiO2, % SO3, % Heat loss, %

13.94 6.21 57.29 11.02 3.48 – – 41.59 0.12 12.45

1–3 0.5–1.0 – 0.8–1.2 1–2 – – 85–95 – 0.5–1

1 2

After curing natural and stabilized samples, all samples were subjected to imaging for microscopic analysis to determine the nature of the pores and the effect of varying the silica fume content on the pore structure.

3 4 5

Natural sample Stabilized sample 10% silica fume Stabilized sample 20% silica fume Stabilized sample 25% silica fume Stabilized sample 30% silica fume

Cation exchange capacity (cmol/kg)

Specific surface area (m2/g)

pH

52.7 with 24.1

243.8 168.2

9.72 9.90

with 14.2

135.0

10.10

with 10.3

124.9

10.17

with 10.1

121.4

10.21

al., 2000), the relative amount of silicate clay mineral in the samples (Schmitz et al., 2004), and the amount of low plastic material added to the soil (Attom and Al-Sharif, 1998). Due to this change in consistency limits, some of the composite samples with the high silica fume contents changed the soil groups from the high plasticity clay group to the low plasticity clay group as shown in Fig. 3.

3. Results and discussion 3.3. Effects of silica fume on the compaction parameters

3.1. Effects of silica fume on CEC, SSA and pH The effects of silica fume content on the CEC, SSA and pH of the natural clayey soil and clayey soil–silica fume mixtures are summarized in Table 3. Silica fume decreased the CEC and SSA values. It was found that the CEC value of composite samples containing 25% silica fume decreased to 10 cmol/kg from an initial value of 53 cmol/kg for the natural soil. The SSA values of composite samples containing 25% silica fume decreased from an initial value of 244 m2/g to 125 m2/g. Silica fume increased the pH values. It was found that the pH value of stabilized clayey soil samples containing 25% silica fume increased to 10.21 from an initial value of 9.72 for the natural soil. The decreases in CEC and SSA values (Kalkan, 2009b) and the increase in pH values are attributed to the decrease in the relative clay mineral contents in the composite samples.

The addition of silica fume affected the compaction parameters of clayey soil–silica fume mixtures. It was observed that the optimum water content values increased and the maximum dry unit weight values decreased with the addition of silica fume. The values of optimum water contents and maximum dry unit weights are summarized in Table 5. The increase in the optimum water content is due to the change in surface area of composite samples. The silica fume changed the particle size distribution and surface area of the stabilized fine-grained soil samples (Kalkan, 2006; Pera et al., 1997; Yarbasi et al., 2007). In the same way, the decrease in the maximum dry unit weights is due to the low density of silica fume, which filled the voids of the composite samples (Attom and Al-Sharif, 1998; Kalkan and Akbulut, 2004).

3.2. Effects of silica fume on the consistency limits

3.4. Effects of silica fume on the swelling pressure and swelling potential

The effects of silica fume content on liquid limit, plastic limit and plasticity index for the stabilized clayey soil samples are summarized in Table 4. The liquid limit and plasticity index values slightly decreased with increasing silica fume content up to 30%. However, the plastic limit slightly increased with increasing silica fume content up to 30%. This may depend upon the soil type (Bell, 1993), the cation exchange capacity (Kalkan, 2009b; Okagbue and Onyeobi, 1999; Sivapullaiah et

3.4.1. Before the drying and wetting cycles The effects of silica fume content on the swelling pressure and swelling potential values of stabilized clayey soil samples are presented in Figs. 4 and 5. The swelling pressure and swelling potential values steadily decreased with increasing silica fume content. The low values were reached at 25% and 30% silica fume contents. Although the swelling pressure and swelling potential values of all stabilized samples decreased with increasing silica fume content, it was observed that the swelling pressure and swelling potential values decreased rapidly as the silica fume contents increased from 0 to 20% and then only slightly as the silica fume content increased to 30%. With the addition of silica fume, the

Table 2 Engineering properties of clayey soil and silica fume used in this study. Property Density Density, (Mg/m3) Grain size Gravel (N2000 μm), % Sand (2000–75 μm), % Silt (2–75 μm), % Clay (b2 μm), % Atterberg limits Liquid limit, % Plastic limit, % Plasticity index, % Specific surface area Specific surface area, m2/g

Clayey soil 2.63

0 2 66 32

72 35 37

Silica fume 2–2.5

– – 20 80

– – –

Table 4 The results of consistency limit tests. No

1 2 3 4 5

243.6

20.12

Samples

Natural sample Stabilized sample silica fume Stabilized sample silica fume Stabilized sample silica fume Stabilized sample silica fume

Liquid limit

Plastic limit

Plasticity index

(%)

(%)

(%)

with 10%

65 61

35 34

30 27

with 20%

58

35

23

with 25%

58

36

22

with 30%

57

36

21

E. Kalkan / Applied Clay Science 52 (2011) 345–352

349

300

Swelling pressure, kPa

250 200 150

Clayey soil

100 50 0

0

5

10

15

20

25

30

Silica fume content, % Fig. 3. Plasticity chart for natural clayey soil and clayey soil–silica fume mixtures.

Fig. 4. Effect of silica fume on the swelling pressure.

swelling pressure values of stabilized samples containing 25% silica fume decreased from an initial value of 265 kPa for the natural clay to 16 kPa. Similarly, the swelling potential values of stabilized sample containing 25% silica fume decreased from an initial value of 54% for the natural soil to 5%. The decrease in the swelling pressure values of the stabilized samples is due to the addition of low-plastic materials and the interaction between clay and silica fume particles (Kalkan, 2006). Several studies indicate that a number of engineering properties of clayey soils are controlled by the CEC, SSA and pH (Eades, 1962; Erzin and Erol, 2007). There is a linear relationship between CEC and liquid limit. Similarly, there is a linear relationship between SSA and liquid limit (Churchman and Burke, 1991; Locat et al., 1984; Ohtsubo et al., 1983; Sridharan et al., 1988). The decrease in the CEC and SSA values due to the addition of silica fume decreased the liquid limit values (Kalkan, 2009b). When silica fume is added to natural clayey soil, the pH values increase. The high pH causes silica from the clay minerals to dissolve. The increase in pH values decreases the relative clay mineral contents in the clayey soil–silica fume mixtures (Eades, 1962; Kalkan, 2009b). The stabilized samples with low relative clay mineral contents and low liquid limit displayed low swelling pressure and swelling potential behaviors. To produce the modification of clayey expansive soils, the two important factors are the quality and quantity of silica fume added to the soil and the chemical composition of the clayey soil. The clayey soil used in this study has a significant quantity of calcium compounds (Table 1), which form Ca2+ ions and hydroxyl ions by reacting with water molecules. Silica fume is pozzolan that provides silica as a result of mineralogical breakdown in high pH clayey soil–silica fume mixtures. With addition of silica fume, the active silica reacts with calcium hydroxide and forms calcium silicate hydrate (C–S–H) gel. It

was found that the samples of the stabilized clayey soil material became stronger and more brittle (Bell, 1993; Kalkan and Akbulut, 2004, Sherwood, 1993) than the natural clayey soil samples due to this basic reaction of silica fume–calcium in the clayey soil.

No

1 2 3 4 5

Samples

Natural sample Stabilized sample with 10% silica fume Stabilized sample with 20% silica fume Stabilized sample with 25% silica fume Stabilized sample with 30% silica fume

Optimum water content

Maximum dry unit weight

(%)

(kN/m3)

26.2 27.3

14.5 14.1

28.1

13.7

28.8

13.6

29.2

13.4

60 50

Swelling potential, %

Table 5 The results of compaction tests.

3.4.2. Drying and wetting cycles The effects of wetting–drying cycles on the swelling pressure and swelling potential were determined by the wetting–drying cycle tests. The results of wetting–drying cycle tests are given in Figs. 6 and 7. From these figures, it can be seen that both the swelling pressure and swelling potential decrease with increasing wetting–drying cycles. The most reduction of swelling pressure and swelling potential is recorded within the first 3 cycles of wetting and drying. With the addition of silica fume, the most reduction of swelling pressure and swelling potential is observed after the first wetting–drying cycle for stabilized clayey soil samples containing 25% and 30% silica fume contents. It is observed that the effects of wetting–drying cycles on the swelling pressure and swelling potential depended on the amount of additive in the stabilized clayey soil samples. At the same time, it has been observed that the change in the swelling pressure and swelling potential of the natural clayey soil samples is different from that of stabilized clayey soil samples containing silica fume during the wetting–drying cycling process. The equilibrium is reached after the first cycle of wetting–drying for the stabilized clayey soil samples containing 25 and 30% silica fume. It is an important finding that with the addition of silica fume, both the swelling pressure and swelling potential decrease with increasing wetting–drying cycles. However,

40 30

Clayey soil

20 10 0

0

5

10

15

20

25

Silica fume content, % Fig. 5. Effect of silica fume on the swelling potential.

30

350

E. Kalkan / Applied Clay Science 52 (2011) 345–352

300

stabilized clayey soil samples is completed after the first wetting– drying cycle and the stabilized clayey soil samples become stronger and more brittle (Bell, 1993; Kalkan and Akbulut, 2004, Sherwood, 1993).

Swelling pressure, kPa

250 200

3.5. Image analysis

150

0% Silica fume 10% Silica fume 20% Silica fume 25% Silica fume 30% Silica fume

100 50 0 -50 0

1

2

3

4

5

Wetting-drying cycle Fig. 6. Effect of silica fume on the swelling pressure after drying–wetting cycles.

the swelling pressure and swelling potential increase when a clayey soil is modified by lime and is subjected to wetting–drying cycles (Guney et al., 2007; Rao et al., 2000; Rao et al., 2001; Yong and Ouhadi, 2007). Both the swelling pressure and swelling potential decrease with increasing wetting–drying cycles and they reached equilibrium at the fifth cycle of wetting–drying for the natural and modified clayey soil samples. The decrease in the swelling pressure and swelling potential is attributed to a gradual destruction of the matrix of the clay structure brought about by the cyclic swelling process. At the same time, the cyclic swelling process leads to a reconstruction and reorientation of the structure of the large microaggregates by disorientation of structural elements. As a result, all these phenomena change the expansive behavior of natural clayey soil samples with increasing number of swelling–shrinking cycles due to the wetting–drying cycles (Basma et al., 1996). The silica fume improves the wetting–drying durability and the swelling pressure while swelling potential decreases with increases in the wetting–drying cycles. One of the reasons for the low swelling pressure and swelling potential values of stabilized clayey soil samples is the chemical stabilization. This chemical stabilization occurring with the addition of the silica fume increases the optimum water content and decreases the maximum dry unit weight. The higher optimum water content and the lower maximum dry unit weight cause a decrease in the swelling pressure and swelling potential (Guney et al., 2007). With the addition of 25% and 30% silica fume contents, the equilibrium has been reached at the first wetting– drying cycle. The basic reaction of silica fume–calcium in the 60

Swelling potential, %

50

0% Silica fume 10% Silica fume 20% Silica fume 25% Silica fume 30% Silica fume

40 30 20 10 0 -10

0

1

2

3

4

5

Wetting-drying cycle Fig. 7. Effect of silica fume on the swelling potential after drying–wetting cycles.

Fig. 8 shows SEM micrographs of stabilized samples prepared with clayey soil and silica fume contents. Fig. 8a shows that large continuous pores among the fine-grained soil particles provide a large portion of the total void ratio while small connected pores exist among the micro-fine silica fume particles. It is seen from the images that silica fume particles cover the surrounding fine-grained soil particles and fill the voids of the stabilized clayey soil samples. In the samples with 10% silica fume content, grain surfaces are partly covered by silica fume particles and most of the pore spaces still contain air (Fig. 8b). In the samples with 20% silica fume content, grain surfaces are covered by silica fume particles in significant ratios (Fig. 8c). In the samples with 30% silica fume content, all grains are covered by relatively thick silica fume material (Fig. 8d). Silica fume particles settle in the pore spaces among the fine-grained soil particles, and then the settled silica fume particles react to form hydration products in the surrounding fine-grained soil particles (Fig. 8b and d). This textural event causes a significant improvement in swelling pressure and swelling potential. A detailed examination of each micrograph reveals that most of the flocculation products are deposited on the surfaces of the soil grains or at the contact points. These micrographs and the experimental results above have also revealed the possibility that a chemical reaction with silica fume and clay particles may occur (Kalkan and Akbulut, 2004). 4. Conclusion In this study, the effect of wetting–drying cycles on the swelling pressure and swelling potential behavior of expansive clayey soils stabilized with silica fume waste material was investigated and the following conclusions were drawn: ▪ Compared with the natural clayey soil sample, it was found that the CEC and SSA values of stabilized samples decreased and pH values of stabilized samples increased due to the increase of silica fume contents. ▪ The liquid limit and plasticity index values decreased with increasing silica fume contents. However, the plastic limit increased with increases in the silica fume contents. ▪ With the addition of silica fume, the maximum dry unit weight values decreased and the optimum water contents increased under the same compaction effort. ▪ The swelling pressure and swelling potential values steadily decreased with increasing silica fume content and the low values were finally reached in the stabilized samples with 25% and 30% silica fume contents. ▪ Both the swelling pressure and swelling potential of natural clayey soil decreased with increases in the wetting–drying cycles. The highest reduction of swelling pressure and swelling potential was recorded after the third wetting–drying cycle on the natural clayey soil samples. ▪ It is an important finding that with the addition of silica fume, both the swelling pressure and swelling potential decreased with increases in the wetting–drying cycles. With the addition of silica fume, the most reductions of swelling pressure and swelling potential of the stabilized clayey soil samples containing 25% and 30% silica fume contents were observed after the first wetting– drying cycle. ▪ SEM analysis showed that the structure of a material had a significant influence on its engineering properties such as swelling

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Fig. 8. Images of stabilized samples with (a) 0%, (b) 10%, (c) 20% and (d) 30% silica fume.

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