Reexamination of Lime Stabilization Mechanisms of Expansive Clay

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Reexamination of Lime Stabilization Mechanisms of Expansive Clay

Zhao Honghua1, Liu Jun2, Guo Jing3, Zhao Chunji4, Gong Bi-wei5,

1Lecturer, Department of Engineering Mechanics, Dalian University of Technology, Dalian,

China, 110624. E-mail: [email protected]

2Geotechnical Engineer, Key Laboratory of Geotechnical Mechanics & Engineering of MWR,

Changjiang River Scientific Research Institute, Wuhan, China, 430010

3Graduate student, Department of Engineering Mechanics, Dalian University of Technology,

Dalian, China, 110624

4Graduate student, Department of Engineering Mechanics, Dalian University of Technology,

Dalian, China, 110624

5Professor, Key Laboratory of Geotechnical Mechanics & Engineering of MWR, Changjiang

River Scientific Research Institute, Wuhan, China, 430010

Abstract

For the purpose of understanding the mechanism of lime treatment on expansive soil,

a series of tests were conducted on selected Nanyang expansive clay. X-ray diffraction

test indicated that the major clay minerals of Nanyang expansive clay are a mixture of

illite and smectite with some kaolinite and chlorite. The scanning electron microscope

images obtained at different curing times were used to interpret the microstructure

change and the formation of new products for the lime treated samples. Atomic

absorption method was adopted to test the concentration of Ca2+ and K+ in the pore

water and exchange complex of lime treated Nanyang expansive clay. The results

indicate that: (1) cation exchange, ions crowding, formation of Ca(OH)2.nH2O,

agglomeration, broken of clay particles in high pH environment are the dominant

reactions at early stage for lime treatment on expansive clay; (2) pozzolanic reaction

and carbonation happened at late stage after lime treatment (90 days in this study) .

Journal of Materials in Civil Engineering. Submitted October 20, 2013; accepted January 24, 2014; posted ahead of print January 27, 2014. doi:10.1061/(ASCE)MT.1943-5533.0001040

Copyright 2014 by the American Society of Civil Engineers

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Keywords: Lime, Nanyang Expansive Clay, X-Ray Diffraction Analysis, Cation

Exchange, Scanning Electronic Microscopy, Atomic Absorption

Introduction

For centuries, lime has been applied to clay soils to improve its strength and stiffness

properties and to reduce the swell-shrink potential of expansive soils (Holtz 1969;

Bell 1988b; Chaddock 1996; Snedker 1996; Rao 2005; Rao and Thyagaraj 2003).

Lime can be applied either in a form of quicklime or as hydrated lime. Lime increases

strength and stiffness of soils through pozzolanic reaction, cementation and

carbonation (Bell 1996; Rajasekaran and Rao 2000; Consoli et al. 2011; Dash and

Hussain 2012). Curing at high temperature also speeds up the increase of strength for

lime treated clay (Toohey et al. 2013; Rao and Shivananda 2005; Boardman et al.

2001). The cementation is mostly brought by pozzolanic reactions and can

significantly improve the long-term performance of the stabilized soils (Rogers et al.

2006; Khattab et al. 2007). Lime precipitation technique in stabilizing expansive soil

can promote lime-soil pozzolanic reactions and increase the unconfined compressive

strength of expansive soils (Thyagaraj et al. 2012). However, excessive lime will

cause strength decrease of treated soil (Dash and Hussain 2012). Lime can reduce

liquid limit and the plasticity of soils (Bell 1988; Herrin and Mitchell 1961; Barker et

al. 2006). Expandable clays tend to react readily with lime, losing plasticity

immediately (Bell and Coulthard 1990). Lime reduces the swell-shrink potential of



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expansive soils through physico-chemical modifications (Mateos 1964; Bhasin et al.

1978; Akbulut and Arasan 2010). Lime treatment on the expansive clay is a very

complicated process. Several mechanisms were proposed to explain this process.

Cation exchange is believed to be the first important reaction when adding lime to

expansive soil (Bell 1996; Prusinski and Bhattacharja 1999; Boardman et al. 2001). A

large amount of Ca2+ generated by lime dissolved in water can exchange other

cations absorbed by smectite clay. This can reduce the swelling of the smectite clay.

Flocculation and agglomeration are the second phenomena that change the

microstructure of clay. High ions concentration and high pH environment reduce the

thickness of the electrical double layer and cause the flocculation of the clay particles

(Mitchell & Soga 2005). The flocculated clay particles attach to each other and form

large clusters or blocks, which is the agglomeration (Bell 1996; Boardman et al. 2001;

Cuisinier et al. 2011; Al-Mukhtar et al. 2012). Pozzolanic reaction is considered to be

another important reaction for the lime reacting with expansive clay. Ca2+, SiO32-,

and Al2O32- from dissolved clay particles in high pH environment form calcium

silicate hydrate (CSH) and calcium aluminium silicate hydrate (CAH) (Diamong and

Kinter 1965; Boardman et al. 2001). This reaction is a long term process, contributing

to the later strength increase of the expansive clay (Bell 1996; Rao et al. 2005).

Earlier studies indicate that curing periods>7days (at room temperature) facilitate a

substantial degree of pozzolanic reactions (Prakash et al. 1989; Bell 1988a)

Ca(OH)2 and Mg(OH)2 continuously react with CO2 in the air and form stiff solid

particles with high strength such as CaCO3 and MgCO3. The crystallization reaction



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of forming Ca(OH)2∙nH2O is also an important process during lime treatment of

expansive clay. Scanning electron microscope (SEM) and x-ray diffraction studies

were conducted to observe the physicochemical and cementitious reactions process

during lime treatment on soil (Wilkinson et al. 2010). SEM is also used investigate the

microstructure of lime treated expansive soils (Shi et al. 2007). This paper reported a

study conducted on lime treatment of Nanyang expansive clay. The above

mechanisms were reexamined in the study.

Nanyang Expansive Clay

Nanyang expansive clay is used in this testing program. This clay is widely

distributed in Nanyang of Henan Province, located in the central middle of China.

Nanyang expansive clay is a yellowish brown and very stiff soil. It is an alluvial

deposit formed in the mid Pleistocene epoch (Huang 1991). Table 1 lists the basic

physical properties of Nanyang expansive clay. Its residual water content is 7.28%

and specific gravity is 2.48. Its liquid limit and plastic limit are found to be 62 and 23,

respectively. A hydrometer analysis conducted on this type of clay gives an 82.7% of

clay and silt particles passing 0.075 mm sieve. The activity of this clay is 1.35. The

1D swelling test value is 4.20% according to ASTMD4546. The cation exchange

capacity of this clay is found to be 41.37 cmol/kg. X-ray diffraction analyses were

conducted using SHIMADZU XRD-6000 apparatus. X-ray diffraction pattern for

Nanyang expansive clay is shown in Fig. 1. The X-ray diffraction analyses indicate

that the major clay minerals are a mixture of illite and smectite, with some kaolinite



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and chlorite.

The lime used in this study was bought from a commercial manufacturer. An X-ray

fluorescence spectrometer analysis was conducted on the lime sample using

SHIMADZU XRF-1800 device. The testing results are listed in Table 2. The testing

data indicated that CaO content is over 90%, with 3.69% of MgO. The optimum lime

content was determined through pH value according to ASTM D 6276-99a to be 3.8%

for Nanyang expansive clay.

1D Swelling and Triaxial Test of Lime Treated Nanyang Expansive Clay

In order to examine the improvement of swelling properties of lime treated Nanyang

expansive clay, one dimensional swelling test was conducted at different curing times

(immediately after compaction(0h), 1h, 4h, 24h, 7d, 28d, 90d). To prepare the

specimen, deionized water was added to the air dried soil (passing 0.5 mm sieve) to

optimum water content, mixed uniformly and then sealed in a plastic bag cured for 24

hrs in a moisture chamber at a humidity above 90% and a temperature about 23 C.

After 24 hrs, the mixture was compacted in the consolidometer ring according to the

maximum dry unit weight. It was then used for 1D swell test. For the lime treated

specimens, 3.84% dry soil mass of lime powerder will be added to the moist soil

mixture and mixed again. The lime mass added was determined by performing pH test

according to ASTM D6276. The lime-soil mixture was then compacted in the

consolidometer ring according to the maximum dry unit weight of Nanyang expansive

clay. Then, they were put into sealed plastic bags and cured in a moisture chamber

with humidity above 90% and temperature around 23 C. At the specified curing time,



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they were taken out from the moisture chamber for 1D swell test. Testing results are

summarized in Table 3. It is noticed that 1D swell of Nanyang expansive clay dropped

from 4.20 %( untreated) to 3.24% (tested immediately after compaction). With curing

time continuing, 1D swell of treated specimen kept decreasing. At 28 days of curing

time, 1D swell of the treated clay is very low now, not showing noticeable swelling.

The most significant reduction in swelling happened within the first 4 hrs after

compaction for the case studied.

In order to obtain the improvement of strength properties of Nanyang clay after

lime treatment, triaxial test was conducted on the prepared specimen after 28 days

curing. The specimen preparation procedure is the same as that for 1D swell test

except that the soil specimen was compacted in a split triaxial mold. The dry unit

weight of the specimens is controlled as the maximum dry unit weight. The specimens

were sheared at the optimum water content at different confining pressures. Fig. 2 and

Fig. 3 showed the triaxial test results of Nanyang clay before and after lime treatment,

respectively. It was seen from the triaxial test results that the stress-strain curve of

natural Nanyang clay is softened at low confining pressure(less than 200 kPa).

However, the stress-strain curve of lime treated Nanyang clay is stiffer compared to

the untreated specimen. The friction angle and cohesion for the untreated Nanyang

clay are 12.5 and 86.7kPa, respectively. The friction angle and cohesion for the lime

treated Nanyang clay are 14.9 and 95.1kPa, which increases slightly.

SEM Analyses

To observe the microstructure change and reaction of lime and Nanyang expansive



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clay, a series of SEM (scanning electronic microscopy) analyses were conducted. The

specimen preparation is the similar as that for 1D swell test triaxial test except that

the specimen was compacted according to the standard proctor compaction method

(ASTM D698-12) in a compaction mold. After compaction, the 4-inch diameter

specimen was sealed in a plastic bag and cured in the moisture chamber at a humidity

above 90% and a temperature about 23 C. At the specified curing time, a small block

of soil mass was taken from the compaction sample and air dried for SEM testing.

The SEM micrograph on natural Nanyang expansive clay is shown in Fig. 4(a). Some

clay particles have a flat thin surface which is a feature of semctite. Other particles

have a fibrous structure at the edge, which is a feature of illite. SEM image indicated

that major clay minerals in Nanyang expansive clay are a mixture of illite and

smectite with some kaolinite (Fig. 4 (a)), which is consistent with the testing results of

X-ray diffraction analysis.

The microstructure of lime treated Nanyang expansive clay compacted without curing

is shown in Fig. 4(b). It is clear that the original microstructure of clay was destroyed.

The large particles were broken into many small particles. The small particles were

agglomerated together. There formed polygonal shaped crystal sheets in the clay

matrix, which are Ca(OH)2.nH2O crystals.

The SEM micrograph of lime treated Nanyang expansive clay at 4 hrs curing after

compaction is shown in Fig. 5(a). It is clear that the clay particles were further broken

into smaller sized particles. We can see the eroded edges of the original clay particles.



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Ca(OH)2.nH2O crystals were also observed in the SEM images. At 24 hrs curing time

after compaction, SEM image for the lime treated Nanyang expansive clay is shown

in Fig. 5(b). This image indicates that the reaction was continuing and the clay

particles were further eroded by lime.

Fig. 6 (a) is the SEM micrograph for lime treated Nanyang expansive clay at 7 days

curing time after compaction. In this image, we almost cannot recognize the original

structure of clay. There are large flat polygonal thick sheets formed from small to

large size. This structure formation is possibly due to not well-crystallized

Ca(OH)2•2H2O particles. Many single of them linked together to form the large sheet

structure. However, this structure is only locally distributed in the whole clay

specimen because of the non-uniform distribution of lime and water available

surrounding clay particles. This micro morphology for the lime treated expansive clay

was rarely observed by previous researchers. Fig. 6(b) was the SEM image of lime

treated Nanyang expansive clay at 28 days curing time after compaction. This image

displays the severely broken structure of Nanyang expansive clay after lime treatment.

Comparing with the microstructures obtained at earlier time, the particle size was

larger, indicating that the more small particles agglomerate together with increasing

curing time.

Fig. 7 is the SEM micrograph of lime treated expansive clay at 90 days curing time

after compaction. From this image, we can observe the polygonal sheet structure,

broken clay particles, and the fibrous structure in the treated clay. The fibrous

structure indicated that there are Ca(OH)2.nH2O and pozzolanic reaction products



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(CAH and CSH) in the late curing period for lime treated Nanyang expansive clay.

Quantitative Analyses of Ca2+ and K+

Once lime was mixing with the clay, it firstly dissolved in the soil pore water and

further Ca2+ reacted with cations in the clay. In order to analyze the mechanism of

cations reaction during the lime treatment of the expansive clay, a series of chemical

tests were conducted on the lime treated Nanyang expansive clay at different curing

time after lime treatment. The concentration of Ca2+ and K+ in the soil pore water

and in the exchange complex of clay was examined, respectively.

The sample preparing procedure was the same as that for SEM analyses. Using the

same compacted sample, after different curing time, a soil mass was obtained for the

chemical analyses. It was then air dried quickly with a wind dryer and broken into

particles with size less than 0.075 mm. The pore water extraction was conducted

according to the following steps: 1) A 25 g of air dried soil was put into a clean 200

ml plastic centrifuge bottle; 2) A 100 ml of deionized water was added into the bottle;

3) The bottle was placed on a high speed shaker for 15 minutes and the bottle was

centrifuged for 15 minutes at 2500 revolution per minutes (rpm); 4) The supernatant

from the sample was removed with a pipette and was placed into a clean

polypropylene bottle. 0.5% of concentrated nitric acid was added into the bottle to

lower the pH to less than 2.0. After the extraction, major cations’ concentration in the

extracted solutions was tested using an atomic absorption spectrometer (Solaar969).

Fig. 8 shows that after adding lime (curing for one day), the concentration of Ca2+



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increased from 198.7 meq/L to 326.3 meq/L. This increase was due to free Ca2+

released by the lime dissolved in the soil pore water. This is ions crowding

phenomenon as observed by previous studies (Osula 1991; Little 1995). The

dissolution of lime in the soil pore water finished rather quickly within 24 hrs. At 7

days, we did not observe any significant increase of the concentration for Ca2+ in the

soil pore water. The concentration of Ca2+ tested after 28 day and 90 day curings

decreased significantly as shown in Fig. 8. This indicated that there was a great

consumption of Ca2+ during this period. One possible consumption source is the

pozzolanic reaction. After 90 days curing, there were the pozzolanic reaction products

observed in SEM studies, which also consumed Ca2+ in the pore water. The

formation of CaCO3 was also possible after a long curing period. This is another

consuming source of Ca2+ in the pore water.

The same chemical analyses were done on K+ in the soil pore water. Testing results

are summarized in Fig. 9. Immediately after compaction, concentration of K+ slightly

decreased in the pore water. A significant increase in the concentration of K+ tested at

7 days of curing was observed. Then, the concentration of K+ started to reduce after 7

days of curing. The concentration of K+ was reduced to 0.05 and 0.04 meq/L at

curing time of 28 days and 90 days, respectively. The first increase of concentration

for K+ is due to the cation exchange by Ca2+. The reduction of K+ is possibly due to

formation of K2SiO3 with SiO32- by the clay particles dissolved in the high pH

environment or other low soluble potassium compounds.

In order to further examine the cations change phenomena in Nanyang expansive clay,



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the Ca2+ concentration in the exchange complex of clay was also investigated.

Cations in the exchange complex of Nanyang expansive clay were extracted as

follows:

1) Prepare a solution of ammonium acetate by placing 77.1 g of crystal ammonium

acetate in a container and bring the volume up to one liter with deionized water; 2)

Put 4 g of air dried soil (passing 0.075 mm sieve) into a 50 ml clean plastic centrifuge

bottle; 3) Add 33 ml of the ammonium acetate solution and shake until no dry soil

seen at the bottom; 4) Place the bottle on a high speed shaker for 15 minutes, and

centrifuge the mixture at 2500 rpm for 15 minutes; 5) Remove the supernatant with a

pipette; 6) Repeat step 2 through 5, two more times on the same soil sample; and 7)

Place all collected supernatant in a clean polypropylene bottle and add 0.5%

concentrated nitric acid, lowering the pH to below 2.0.

After this extraction was completed, the concentration of Cations was measured using

atomic absorption (AA ) method. The testing data for the concentration of Ca2+ was

shown in Fig. 10. The concentration of Ca2+ in the exchange complex of clay

increased from 230 meq/100g before treated by Lime to 280 meq/100g after treated

with Lime immediately after compaction. This indicated a strong cations exchange

reaction happened between Ca2+ in the pore water and other cations in the exchange

complex of clay. This reaction continued to 28 days curing time after compaction. The

Ca2+ concentration in the exchange complex keeps increasing to 28 days. The

concentration of Ca2+ tested at 90 days decreased greatly. This decrease was due to

the broken of clay particles in the high pH environment and the pozzolanic reaction



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happened in the lime treated expansive clay.

To examine the behavior of other cations during the lime treatment, a series of

chemical analyses were also conducted on K+ in the exchange complex of Nanyang

expansive clay. Testing results are summarized in Fig. 9.

As illustrated in Fig. 11, immediately after compaction, the concentration of K+

increased greatly. This increase was due to the exchange reaction between K+ in the

pore water and other cations (Ca2+, Na+ etc.) in the exchange complex of clay. 7 days

after curing, the concentration of K+ in the exchange complex of clay decreased

greatly. The concentrations of K+ tested in the pore water and in the exchange

complex of the clay decreased. This sounds contradictory. This was possibly because

there are many more complex reactions ongoing during lime treatment on the clay.

After long curing time, part of the K+ in the exchange complex of clay might not be

extracted in the AA testing. Some clay particles may be bound by carbonation

products and pozzolanic products. Further research needs to explore into this topic.

Discussions

This first step change in the microstructure of Nanyang expansive clay by lime

reflected in Fig. 2 (b). The clay structure became more flocculated and granulated

which reduced its contacting surface with water, which caused the reduction of its

swelling potential. This observed flocculation and agglomeration was also noticed by

other investigations (Bell 1996; Boardman et al 2001; Cuisinier et al 2011;

Al-Mukhtar et al 2012) .This was observed immediately after compaction after

mellowing lime and moist soil for 24 hrs. Meanwhile, as seen from Fig. 8 through Fig.



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11, the concentration of Ca2+ increased in the pore water, which further depresses the

double layer’s thickness of clay particles; the concentration of Ca2+ and K+ increased

in the exchange complex of clay, which further draws clay particles more firmly

together. The swelling potential of clay also decreases with the depressed double layer

and the firmly connected clay particles. These changes in microstructure and chemical

properties caused the decrease of 1D swelling value (from 4.20% to 3.24%). This

initial and immediate change caused by cation exchange was also point out by other

researchers (Bell 1996; Bell and Coulthard 1990; Boardman et al. 2001; Al-Mukhtar

et al 2012). At curing time of 4 hrs, the further broken down of the clay particles by

lime caused a more coarse structure, which further reduced 1D vertical swelling to

0.11%. While at 24 hrs of curing, the microstructure of clay showed no significant

difference from that of at 4 hrs, which only caused a slight reduction of 1D swelling.

Most of the effect of reducing swelling of the clay by lime finished in the first 7 days

as observed from this study. The flocculated and coarse microstructure of lime treated

Nanyang clay increases its friction angle; the formation of Ca(OH)2.nH2O and other

cohesive products (from lime-soil pozzolanic reaction) made the lime treated clay

increasing its cohesion. These were proved with the triaxial testing results (Fig. 2 and

Fig. 3). The SEM image of treated clay at 90 days of curing (Fig. 7) indicates that the

pozzolanic reaction is more significant. The concentration drop of Ca2+ and K+ also

supported this conclusion. The observation of pozzolanic products at 90 days curing

contributed to the later strength of lime treated clay, which was also pointed out by

previous studies (Bell 1996; Boardman et al. 2001). The lime-clay reaction can be



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described as a progressive dissolution of large clay particles followed by formation of

CSH and CAH (Al-Mukhtar et al 2012). The increase of friction angle and cohesion

at 28 days were not very significant for Nanyang clay can also be explained by few

pozzolanic products observed in the SEM testing (Fig. 6(b)).

Conclusions

Lime generally improves the engineering properties of soils. The mechanism of lime

on expansive clay was reexamined in this paper through testing on Nanyang

expansive clay. Through SEM analyses and AA cations analyses, the following

conclusions are drawn.

The major clay minerals of Nanyang expansive clay is mixture of illite and smectite,

kaolinite and chlorite. It is a type of medium expansive clay.

At early stage of lime treatment, cations crowding happened in the soil pore water as

indicated by the rise of Ca2+ and K+ in the soil pore water. There was a formation of

Ca(OH)2. nH2O crystals in the treatment as shown in SEM results. Cation exchange

also happened during the early stage of lime treatment. Cations crowding and high pH

environment caused flocculation and agglomeration of clay particles. These reactions

happened rather quickly. Most of reactions finished within one week for this study.

These early reactions caused most of the reduction of swelling for the tested clay

within one week after lime treatment. Thus, the dominant reactions are cations

crowding, cation exchange, formation of Ca (OH)2. nH2O crystals, broken of clay

particles in high pH environment, and agglomeration. There was an increase of

friction angle and cohesion for the lime treated Nanyang expansive clay after curing



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15

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for 28 days. Pozzolanic reaction happened more significantly at late stage during lime

treatment (90 days in this study). Carbonation also possibly happened at late stage of

lime treatment.

The lime treated Nanyang expansive clay is a mixture of many different materials at

late stage, such as unreacted clay particles, Ca(OH)2. nH2O crystals, CSH and CAH,

carbonation products etc. The great decrease of Ca2+ and K+ in both pore water and

exchange complex of clay at late stage of lime treatment needs more researches to

explore this phenomenon.

Acknowledgement

The financial support of National Natural Science Foundation of China (No.

51308091) is greatly appreciated. This research is part of the National Key

Technology R&D Program in the 12th Five-Year Plan of China (No. 2011BAB10B05)

Additionally, the project is also sponsored by SRF for ROCS, SEM.

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Fig. 1 X-ray diffraction pattern of Nanyang expansive clay

Fig. 2 Stress-strain curves of untreated Nanyang clay

Fig. 3 Stress-strain curves of Lime treated Nanyang clay

Fig. 4 (a) SEM image of compacted Nanyang expansive clay; (b) SEM image of lime

treated Nanyang expansive clay immediately after compaction

Fig. 5 SEM image of lime treated Nanyang expansive clay after compaction (a) at 4

hrs curing time, (b) at 24 hrs curing

Fig. 6 SEM image of lime treated Nanyang expansive clay after compaction (a) at 7

days curing time, (b) at 28 days curing time

Fig. 7 SEM image of lime treated Nanyang expansive clay at 90 days curing time

after compaction

Fig. 8 Concentration of Ca2+ in soil pore water

Fig. 9 Concentration of K+ in soil pore water

Fig. 10 Concentration of Ca2+ in exchange complex of Nanyang expansive clay

Fig. 11 Concentration of K+ in exchange complex of Nanyang expansive clay



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Table 1 Basic Physical Properties of Nanyang Expansive Clay

w r

(%)Gs PL LL PI

w opt

(%)dmax

(kN/m3)

1D Swell

%

Activity(PI/Clay%

<0.002mm)

7.27 2.48 23 62 39 25.0 15.3 4.20 1.35

Accepted Manuscript Not Copyedited



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Table 2 X-ray fluorescence spectrometer analysis on Lime

Oxide CaO MgO SiO2 Al2O3 Fe2O3 SO3 K2O TiO2 SrO other

% 91.35 3.69 2.70 0.74 0.66 0.53 0.15 0.11 0.06 0.02




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Table 3 1D swell of lime treated Nanyang expansive clay at different curing times

Curing time 0 hr 1 hr 4 hrs 24hrs 7 days 28 days 90 days

Lime treatedNanyang clay

3.24 3.09 0.11 0.09 0.06 0.005 0.004




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Documents

Reexamination of Lime Stabilization Mechanisms of Expansive Clay