Geotechnical properties of lime-treated gypseous soils

Applied Clay Science 88–89 (2014) 39–48

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Research paper

Geotechnical properties of lime-treated gypseous soils A. Aldaood a,b, M. Bouasker a, M. Al-Mukhtar a,⁎ a b

Research Center on Divided Matter – CRMD, FRE CNRS 3520 1b rue de la Férollerie, 45100 Orléans Cedex 2, France Civil Engineering Department, College of Engineering, Mosul University, Iraq

a r t i c l e

i n f o

Article history: Received 25 July 2013 Received in revised form 12 December 2013 Accepted 15 December 2013 Available online 8 January 2014 Keywords: Gypseous soil Lime stabilization Unconfined compressive strength Swelling potential Texture and microstructure

a b s t r a c t This paper presents the results of an experimental research study on the behavior of gypseous soils treated with lime. The aim of this work was to investigate the effect of a wide range of gypsum content and curing conditions on two important geotechnical properties: the mechanical strength and swell potential of lime-treated finegrained soil. For this purpose, soil samples were prepared with different gypsum content (0%, 5%, 15% and 25%), treated with lime and assessed at different curing times and temperatures. The results for untreated gypseous soil showed that the unconfined compressive strength increases and the swell potential enhances with the presence of gypsum. The geotechnical properties of the lime-treated gypseous soil depend not only on the gypsum content but also on the curing conditions. An approximate doubling of the unconfined compressive strength of soil samples cured for 28 days was observed when the curing temperature was increased from 20°C to 40°C. Moreover, the swell potential increased with curing times and decreased with curing temperature in the lime-treated gypseous soil. The tests carried out at the microscopic level showed the formation of calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH), which are responsible for strength development in the treated soil samples. Further, the ettringite mineral, which induces swelling, was also observed in the treated soils. © 2014 Elsevier B.V. All rights reserved.

1. Introduction and scientific background Gypsum (CaSO4.2H2O) is one of the most soluble of the common minerals that are found in soils. Gypseous soils are widespread in many areas of the world (FAO, 1993), and the gypsum content varies widely from low (less than 5%) to very high (about 50%) (Adams et al., 2008; Al-Dabbas et al., 2012). These soils are encountered in construction and civil engineering works, often in the road pavement layers and subgrades due to the lack of economic alternative materials (Adams et al., 2008; Ahmed, 1985; Aibn et al., 1998; Hunter, 1988; Sultan, 1995; Wang et al., 2003). Smith and Roberson (1962, as cited in FAO, 1990) found that a gypsum content of less than 10% does not significantly affect the soil characteristics (structure, texture and water retention). In contrast, an increase in the gypsum content (more than 10%) causes significant distress on the geotechnical properties of soil samples (Ahmed, 1985; Al-Dabbas et al., 2012). Gypseous soils show a rapid and significant settlement (collapse) when water is added, because the loose structure of their particles is cemented by soluble minerals and/or by small amounts of clay. Unfortunately, civil engineers have encountered numerous problems with gypseous soils: hydraulic structure failures, residential building collapse and cracking, and the settlement of pavement layers (Aibn et al., 1998; Cooper, 1998; James and Lupton, 1978; Taha, 1979; Van Alphen and Romero, 1971).

⁎ Corresponding author. Tel.: +33 2 38 25 78 81; fax: +33 2 38 25 53 76. E-mail address: [email protected] (M. Al-Mukhtar). 0169-1317/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.12.015

The geotechnical properties of gypseous soils can be improved by various methods, such as chemical stabilization to avoid damage due to gypsum dissolution. Improvement with lime treatment takes place through two basic chemical reactions: short-term and long-term reactions. The short-term reactions include the cation exchange reaction, flocculation/aggregation and carbonation, which result in a decrease in the plasticity of the soil and an increase in its workability. The longterm reactions include the pozzolanic reaction, in which calcium from the lime reacts with the alumina and silica from the clay to produce stable calcium silicate hydrate (CSH), calcium aluminate hydrates (CAH) and calcium alumino-silicate hydrates (CASH), which are responsible for the gain in strength, improvements in compressibility and volume change properties of the soil (Al-Mukhtar et al., 2010a; Bell, 1996; Ingles and Metcalf, 1972; Little, 1995). Immediately after treatment, the stabilized gypseous soil exhibits an acceptable range of engineering properties and behaviors. Unfortunately, with time and in the presence of water, gypseous soil becomes less durable and new problems can be initiated. These problems relate to the formation of expansive minerals, which in turn causes soil heave and then the cracking of pavement structures; these problems induce a reduction in stability and loss of bearing capacity (Hunter, 1988; Little et al., 2010; Mitchell and Dermatas, 1992; Puppala et al., 2005; Yong and Ouhadi, 2007). The current study explored the influence of lime treatment and gypsum content on the strength and free swell behavior of finely-grained clayey soil. In this laboratory study, the gypseous soil samples (artificial samples) were obtained by adding different amounts of gypsum (5%, 15% and 25%) to the natural soil. The natural and gypseous soil

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samples were treated with 3% lime (based on the pH test method suggested by Eades and Grim, 1966). The lime-treated gypseous soil samples were cured for 2, 7 and 28 days at 20°C and 40°C. The second curing temperature of 40°C was considered for this research work in order to study the geotechnical behavior for periods of longer than 28 days, since numerous studies have shown that increasing the curing temperature accelerates reactions in the lime-treated soils (Al-Mukhtar et al., 2010b; Mooney and Toohey, 2010; Thompson, 1967). Finally, the evolution of the microstructure was also studied using electronic scanning microscopy, X-ray diffraction and mercury porosimetry analysis. 2. Materials

Table 2 Chemical analysis of natural soil. Compound

Value (%)

SiO2 Al2O Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 PF Total

68.77 8.35 3.53 0.05 0.76 5.86 0.61 1.46 0.73 0.08 8.62 98.82

2.1. Soil The soil used in this study was obtained from a site near the Jossigny region, to the east of Paris, France. The soil samples were collected at depths varying between 1.5 to 2.0 m below the ground surface and the natural in situ water content was determined to be approximately 18.5%. The liquid limit was 29% and its plasticity index was equal to 8%. The main index and the physical and chemical properties of the natural soil are summarized in Tables 1 and 2. The grain size distribution analysis was 17% sand, 64% silt and 19% clay. The X-ray diffraction analysis showed that the soil contained mainly quartz and calcite, while the clay mineral was predominantly composed of kaolinite. Based on the Atterberg Limits and according to the Unified Soil Classification System (USCS), the soil was classified as low plasticity clay (CL). In the remainder of this paper this soil, which does not contain gypsum, is termed natural soil. 2.2. Hydrated lime and gypsum The quicklime used in this study, supplied by the French company LHOIST, is a very fine lime which can pass through an 80-μm sieve opening. The activity level of the lime used was 94% (Al-Mukhtar et al., 2010a). The gypsum used in this study, supplied by the German company Merck KGaA, is very fine gypsum which can pass through an 80-μm sieve opening, and with a purity of more than 99%. 2.3. Sample preparation and compaction test In order to conduct a precise parametric study, different gypsum contents were added to natural soil (5%, 15% and 25%). The natural and the prepared gypseous soil samples were treated with 3% lime. The quick test suggested by Eades and Grim (1966) was used to determine the amount of lime required for the stabilization of the natural soil.

Table 1 Some physical and index properties of natural soil. Properties

Values

Liquid limit (%) Plasticity index (%) pH Electrical conductivity (μS/cm) Natural moisture content in situ (%) Specific gravity Gs Standard compaction Max. dry density (kN/m3) Optimum moisture content OMC (%) Modified compaction Max. dry density (kN/m3) Optimum moisture content OMC (%) Grain size distribution Sand (%) Silt (%) Clay (%) USCS Group symbol Group name

29 8 8.01 400 18.5 2.66 17.7 11 19.5 10 17 64 19 CL Sandy Lean Clay

This lime content was used for the treatment of all soil samples in this study. To prepare the soil sample, the natural soil was firstly ovendried for two days at 60°C; then it was pulverized and sieved (4 mm), after which it was mixed with a predetermined amount of gypsum or gypsum and lime, and thoroughly mixed in a dry state until the mixture had a homogeneous and uniform appearance. In order to investigate only the effect of gypsum content, all the materials were added using the complementary substitution method. For example, to prepare the soil samples with 5% gypsum content, 95% of soil was mixed with 5% of gypsum. In the same way, to prepare the lime-treated soil samples, 92% of soil was mixed with 5% and 3% of gypsum and lime, respectively. After that, the required amount of water was added to the mixture which was then remixed thoroughly. The mixing continued until the final mixture gained a uniform moisture distribution. The wet mixture was then kept in plastic bags and the untreated soil was left for 24 hours and the lime-treated soil for one hour, to allow it time for mellowing. Finally, the soil samples were compacted at optimum moisture content and maximum dry unit weight represented by the standard compaction curve of natural soil (Table 1) according to an ASTM D-698 procedure. 3. Testing methods 3.1. Unconfined compression and wave velocity tests To evaluate the strength characteristics of the lime-treated and untreated soil, an unconfined compression test was conducted using cylindrical samples (50 mm diameter × 100 mm height). The cylindrical soil samples were prepared by static compaction at a rate of 1 mm/min. The soil samples were compacted at the moisture content and dry unit weight that represented the optimum moisture content of the standard compaction curve of natural soil, corresponding to the maximum dry unit weight. All prepared samples containing lime were immediately wrapped with cling film and coated with paraffin wax to reduce moisture loss. The soil samples were cured for 2, 7 and 28 days at 20°C and 40°C. At the end of each curing period, the wave velocity of the soil samples was determined. A PUNDIT instrument and two transducers (a transmitter and a receiver) with a frequency of 82 kHz were used. The direct transmission method, which is more sensitive than other methods, was preferred for measuring the wave velocity of the soil samples. The wave velocity was calculated from the ratio of the travel distance to travel time of the wave through the soil sample. The unconfined compressive strength was then determined according to the ASTM D-5102 procedure (ASTM, 1994), using a Universal Testing Machine (UTM-INSTRON 4485) at a strain rate of 0.1 mm/min. 3.2. One-dimensional free swell test To evaluate the free swell potential of the untreated and limetreated soil, a free swell test was performed using a standard one-

A. Aldaood et al. / Applied Clay Science 88–89 (2014) 39–48

dimensional oedometer device in accordance with the ASTM standard (D-4546). The soil samples were statically compacted at a strain rate of 1 mm/min in rigid stainless steel rings that were 71 mm in diameter and 20 mm in height. The lime-stabilized soil samples with different gypsum contents were subjected to 2, 7 and 28 days of curing at 20°C and 40°C, as mentioned previously in Section 3.1. At the end of each curing period, the compacted soil samples in the oedometer rings were placed in a consolidation cell between two dried porous stones, and a sensitive dial gauge was fixed on top of the consolidation cell to measure the vertical soil displacement. After calibration with an initial vertical pressure equal to 2.75 kPa (representing the weight of the loading plate which is part of the consolidation cell), an initial reading was taken to estimate the swell potential, following which the soil samples were soaked with tap water and allowed to swell under the vertical pressure (2.75 kPa) at a constant laboratory temperature (25°C ± 2°C). After that, time–swell readings were continuously noted during the process. The final reading of the dial gauge (which represents the highest reading) was used to calculate the free swell potential together with the initial height of the soil samples, using the equation below: Swell potential ¼

ΔH  100 Hi

ð1Þ

where ΔH is the vertical displacement in mm, which represents the difference between initial and final readings of the dial gauge, and Hi is the initial height of soil sample in mm. The time required to reach the final reading of the dial gauge (the maximum value of vertical displacement) depends on the gypsum content, curing time and curing temperature. Thus, the swell test was continued until the dial gauge reading had stabilized for at least 24 h to allow for any residual swelling of the soil samples. 3.3. Mineralogical and micro-structural analysis The mineralogical and micro-structural aspect of the lime-treated soil samples was studied using a scanning electron microscope (SEM), X-Ray diffraction (XRD) and porosity tests. The main objective of the mineralogical and micro-structural investigations was to determine changes due to lime treatment and to find out about the formation of cementitious materials and ettringite mineral. These tests were conducted on the soil samples at the end of the 28 days of curing at 20°C and 40°C. For the scanning electron microscopy test (SEM), the soil samples (volume 1 cm3) were injected by epoxy fix resin, polished, goldcoated and then scanned by a high resolution scanning electron microscope (PHILIPS XL 40 ESEM). Several digital images at different magnifications were recorded in order to examine the formation of ettringite and cementitious materials. It is worth noting that all the soil samples were prepared in the same manner. For the X-Ray diffraction test (XRD), fractured samples produced after the unconfined compression test were powdered and sieved through a 400-μm sieve to serve as samples for the XRD test. Before testing, the soil sample was dried for 24 hours at 40°C. A PHILIPS PW3020 diffractometer was used for XRD analysis. The diffraction patterns were determined using Cu–Kα radiation with a Bragg angle (2θ) range of 4°–60° running at a speed of 0.025/6 sec. A pore size distribution assessment was carried out to determine the fabric of the soil samples using a Pore Seizer Porosimeter (9320), in which the mercury pressure is raised continuously to reach more than 210 MPa and measure the apparent pore diameter in the range 3.6 nm–350 μm. In a mercury intrusion porosimetry test, the mercury is forced into the soil samples; the applied mercury pressure and the intruded volume of mercury are monitored during the test. Mercury intrusion porosimetry tests were conducted on the soil samples after 28 days of curing at 20°C and 40°C for all soil mixtures containing

41

various amounts of gypsum. Soil samples were lyophilized using an ALPHA 1–2 LD plus – GmbH apparatus before applying mercury tests to minimize micro-fabric changes (Al-Mukhtar et al., 1996). 4. Results and analysis 4.1. Compaction characteristics Table 3 summarizes the compaction characteristics of untreated and lime-treated soil samples. The analysis of the test results depended on the same energy of compaction (the same standard compaction effort), which gave different initial structures of gypseous soil samples due to the gypsum addition. The results show that, as the gypsum content increased, the Proctor compaction curves trended towards a slightly lower maximum dry unit weight (γdmax) but higher water content. The reduction in the γdmax was due to the lower density value of gypsum particles compared with soil particles. Conversely, the optimum moisture content (OMC) increased with the addition of gypsum. The value of OMC increased from 11% for natural soil to 14.4%, 15.6% and 16.4% for the soil samples treated with 5%, 15% and 25% gypsum, respectively. This was due to the large specific surface area of gypsum, meaning that more water absorption is required for gypsum than for soil particles. The introduction of a small amount of gypsum tends to reduce particle separation (i.e. it causes more dispersion) due to the exchange of Ca++ ions between the soil and gypsum, and this results in a lowering of the γdmax and raising of the OMC. For lime-treated soil samples, laboratory experiments showed an increase in the OMC and a decrease in γdmax with lime addition. This behavior can be attributed to the tendency of lime to absorb water in order to complete its hydration, causing an increase in the OMC. Further, the immediate reactions represented by flocculation and agglomeration resulted in an increase of the OMC and decrease of the γdmax. These variations depended on the mix proportions: the maximum OMC and the minimum γdmax were recorded with the soil samples of 25% gypsum content. In summary, the compaction properties of the gypseous soil revealed that as gypsum content increases, the γdmax reduces and the optimum moisture content increases. 4.2. Mineralogical and micro-structural changes Mineralogical and micro-structural studies were carried out to investigate the mineralogy and micro-structure of the cured samples. These specifically focused on determining whether cementitious minerals such as calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH) were present within the samples, and on analyzing the action of these materials on the texture and structure of soil samples. Additionally, attention was paid to searching for ettringite, which is known for its potential to cause swell. XRD analyses of natural untreated and lime-treated soil samples cured for 28 days at 20ºC and 40ºC are shown in Fig. 1. The XRD pattern of the natural (untreated) soil samples indicated that the soil was composed mainly of kaolinite and illite clay minerals, and contained quartz, calcite and feldspars. Identification revealed that, after the treatment with lime, the formation of CSH and CAH were

Table 3 Compaction characteristics of untreated and lime treated soil samples. Gypsum content (%)

Maximum dry unit weight (kN/m3) Without lime

With lime

Without lime

With lime

0 5 15 25

17.7 17.73 17.2 16.5

16.9 17.2 16.6 16.2

11 14.4 15.6 16.4

13 15.5 17 18

Optimum moisture content (%)

A. Aldaood et al. / Applied Clay Science 88–89 (2014) 39–48

E

E

A

G

25% gypsum

CAH

600

C

L

400

15% gypsum

I 004

L

CSH

Intensity ( counts/s )

G

CAH

G G

CSH + G

800

I 003 + Q101

42

5% gypsum

K 001

0% gypsum

I 002

I 001

200

Q

K

K

F

C

Q

C

Natural soil

0 0

10

20

30

40

50

60

25% gypsum

CAH

600

C

L

400

15% gypsum

I 004

L

CSH

Intensity ( counts/s )

E

E

E

B

G

CAH

G

G

CSH

G

CSH + G

800

I 003 + Q101

2θ (°)

5% gypsum

K 001

0% gypsum

I 002

I 001

200

Q

K

K

F

C

C

Q

Natural soil

0 0

10

20

30

40

50

60

2θ (°) Fig. 1. XRD patterns of the soil samples after 28 days of curing (A) at 20 °C (B) at 40 °C (G: Gypsum; L: Lime; E: Ettringite; Q: Quartz; K: Kaolinite; I: Illite. C: Calcite; F: Feldspar).

observed. Ettringite was also found in all the gypseous soil samples. Ettringite is a calcium aluminum sulfate hydrate (CASH) type of mineral which is responsible for the early strength gain. The high intensity of the ettringite mineral was observed within the soil samples having 5% gypsum content (especially for the soil samples cured at 40ºC). The presence of ettringite within gypseous soil samples can cause structural distress. However, the ettringite may not have weakened the soil by swelling, but instead may have strengthened it (Wild et al., 1998). Fig. 2 illustrates the effect of curing temperatures and gypsum content on the pore size distribution of lime-treated soil samples. It can be seen that, the soil samples without gypsum content exhibited a bimodal pore size distribution, with a less pronounced reflexion of pore size centered on 1 μm when cured at 20ºC; they exhibited a tri-modal distribution, with a less pronounced reflexion of pore size centered on 2 μm when cured at 40ºC. In addition, micropores of less than 0.01 μm seem not to have been significantly affected by the curing temperatures. Further, the amount of micro- and macropores reduced with curing temperature due to the pozzolanic reaction products during curing periods. The pozzolanic products (CSH and CAH) not only enhanced the inter-cluster bonding strength but also filled the pore space. As a result, the unconfined compressive strength of the soil samples significantly

increased with an increasing curing temperature, as will be discussed in the next section. Gypsum addition affects the pore size distribution of soil samples by increasing the amount of micropores with a diameter of (0.04–0.05 μm), which increased with high gypsum content (25%) and when cured at 20ºC; while the amount of macropores with a diameter of (2–3 μm) did not seem to be significantly affected by this addition. The gypseous soil samples showed a new reflexion centered on 6 μm which was not present in soil samples without gypsum. This reflexion may be due to the ettringite formations, as shown in Fig. 1. Moreover, ettringite formations influence the microstructural properties of lime-treated gypseous soil. Such an influence is of great interest since the effect of ettringite on the geotechnical properties of soil is a function of its microstructural changes. Porosimetry measurements provided some additional insight into the gypsum addition effects, as exhibited by soil samples treated with lime. It was found that the gypseous soil samples exhibited the same pore size distribution (tri-modal) with gypsum addition at all curing temperatures, as shown in Fig. 2. Moreover, the mineral ettringite fills the pores within the soil matrix, thus leading to a decrease in the void ratio of the gypseous soil samples. This assumption is in agreement with the results of a scanning electron microscope (SEM) analysis

Cumultative intrusion (mL/g)

Incrimental Intrusion (mL/g)

A. Aldaood et al. / Applied Clay Science 88–89 (2014) 39–48

0.025 0% G

0.02

5% G

Curing at 20°C

15% G

0.015

25% G

0.01 0.005 0 0.001

0.01

0.1

1

10

100

0.25

Cumultative intrusion (mL/g)

Incrimental Intrusion (mL/g)

0% G 5% G 15% G

0.015

25% G

0.01 0.005 0 0.001

0.01

0.1

1

10

0% G 5% G 15% G

0.15

25% G

0.1 0.05 0 0.001

0.1

10

Entrance Diameter (µm)

0.025 0.02

Curing at 20°C

0.2

Entrance Diameter (µm)

Curing at 40°C

43

100

Entrance Diameter (µm)

0.25

Curing at 40°C

0.2

0% G 5% G 15% G

0.15

25% G

0.1 0.05 0 0.001

0.01

0.1

1

10

100

Entrance Diameter (µm)

Fig. 2. Incremental and cumulative pore size distribution of the soil samples cured for 28 days at 20°C and 40°C.

performed on soil samples cured for 28 days at 20°C and 40°C, as shown in Fig. 3. Ettringite was observed to have formed and precipitated in the pores of the soil matrix, especially of the soil samples with a higher amount of gypsum content. It can be noted from Fig. 2 that the soil samples cured at 20°C and 40°C showed a higher proportion of macropores centered at around 2 μm. The soil samples with a gypsum content of 5% had a macropore reflexion that was slightly higher than for other types of soil. This reflexion is at 3 μm and 5 μm for a temperature of 20°C and 40°C respectively. The same figure also shows that at 20°C the micropore reflexion is more pronounced for the soil containing gypsum: it is centered at around 4 nm. At 40°C, this pore mode is more flattened, meaning that the micropores are present in the range 4–20 nm. 4.3. Unconfined compressive strength properties The unconfined compressive strengths of natural and gypseous soil samples compacted at OMC and γdmax were measured. The soil samples were found to have unconfined compressive strengths of 0.19, 0.23, 0.27 and 0.32 MPa for 0%, 5%, 15% and 25% gypsum, respectively. The increase in strength values with gypsum content is explained by the reduction in the void ratio of the soil samples, as shown in Table 4. Indeed, gypsum particles not exceeding 80 μm in diameter will fill the pore space between soil particles, therefore increasing the compactness of the matrix and consequently the unconfined compressive strength of the soil samples. Further, decreasing the void ratio in tested soil samples with the same moisture content (i.e. 11%) may introduce increased suction pressure which results in increased unconfined compressive strength. The results for the unconfined compressive strength of the limetreated soil samples with various amounts of gypsum content and subjected to different curing conditions are displayed in Figs. 4 and 5. The unconfined compressive strength of all mixes, with or without gypsum, increased with increased curing temperature. This behavior is attributed to the acceleration of chemical reactions in the mixtures due to the increase in temperature. In fact, a higher temperature promotes the pozzolanic reaction within the mixture and the formation of calcium

silicate hydrate (CSH) and calcium aluminate hydrates (CAH), which act as cementitious materials (Fig. 1) so that they in turn contribute to the strength development of the soil samples. The unconfined compressive strength values of soil samples containing gypsum were higher than those of the zero gypsum samples. This behavior can be attributed to the faster hydration rate (greater reaction between soil, lime and gypsum) and the formation of ettringite by the reaction of sulfate ions with CSH or CAH, as shown in Fig. 1. It is known that gypsum improves soil strength by altering the course of the hydration of calcium silicate, which is predominantly formed in the early stages of hydration. The addition of gypsum leads to the release of sulfate ions which react with the alumina phase in the soil. Gypsum accelerates the chemical reaction between soil and lime (Holm et al., 1977; Kujala and Nieminen, 1977). Silicate hydrate is formed as well as calcium aluminate hydrate, which favors stronger soil samples. Gypsum has a significant activation effect on soil-lime, with the main hydration products being CSH, CAH and ettringite. Furthermore, the formation of CSH and CAH are not the only factors contributing to the higher strength of the gypseous soil samples, as ettringite also plays an important role. The effect of the gypsum content was most pronounced in the soil samples treated with 5% gypsum, which demonstrated the largest unconfined compressive strength (Fig. 4). The soil samples treated with 15% gypsum were the next strongest, while the soil samples treated with 25% gypsum exhibited the lowest strength. The presence of unreacted gypsum particles, which represent light-weight materials with a specific gravity equal to 2.3, was the cause of this reduction in the unconfined compressive strength of the soil samples. Also, the inclusion of gypsum content affects the texture of the soil samples: as the gypsum content increased the size of the macropore increased (Fig. 2) resulting in a reduction of the unconfined compressive strength. Added to this, the reactions forming the cementitious materials are also affected by gypsum content, causing these changes in the unconfined compressive strength. The unconfined compressive strength of the soil samples increased with curing time and curing temperature (Fig. 4). The increased

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A. Aldaood et al. / Applied Clay Science 88–89 (2014) 39–48

0% G

Cementing Materials

Cementing Materials

5% G

Cementing Materials

Cementing Materials

15% G

Ettringite

Ettringite

25% G

Ettringite

Ettringite

Fig. 3. Cementing materials development during 28 day of curing at 20°C at left and 40°C at right.

Table 4 Variation of specific gravity and void ratio with gypsum addition. Gypsum (%)

Specific gravity (Gs)

Void ratio (%)

0 5 15 25

2.66 2.6 2.56 2.49

0.5 0.46 0.44 0.4

strength ranged from 2 to 6 and 2 to 10 times the original natural strength for the soil samples cured at 20°C and 40°C, respectively. Moreover, the results showed an approximate doubling in the unconfined compressive strength of the soil samples cured for 28 days when the curing temperature was increased from 20°C to 40°C. Fig. 4 makes it clear that the trend in the rate of gain in the strength of the soil samples cured at 40°C was sharper than that for the soil samples cured at 20°C. The continuous reaction between soil, lime and gypsum with increased temperature, as well as the formation of CSH,

A. Aldaood et al. / Applied Clay Science 88–89 (2014) 39–48

2.5

4.4. Wave velocity measurements

UCS (MPa)

2

1.5

1

0.5

0%G at 40°° C

0%G at 20°° C

5%G at 40°° C

5%G at 20°° C

15%G at 40°° C

15%G at 20°° C

25%G at 40°° C

25%G at 20°° C

0 0

10

20

30

Curing Period (day) Fig. 4. Unconfined compressive strength variation Vs curing time.

CAH and ettringite minerals, caused the soil samples cured at 40°C to consistently grow more quickly. Additionally, the refinement of the pore structure and decrease of the porosity of the soil samples with increased curing temperature contributed to the higher rate of increase in the unconfined compressive strength of the soil samples when compared to those samples cured at 20°C, because of the higher amount of hydration products that were formed. This argument is consistent with the results of the mercury intrusion porosimetry test, as shown in Fig. 2. Finally, many attempts were made to directly correlate the unconfined compressive strength values with the water content after the curing (residual water content) of the soil samples, as shown in Table 5. Residual water content is the ratio of the weight of water to the weight of the solid after curing periods. The residual water content was observed to decrease progressively with the length of curing period and curing temperatures, and it also decreased with increasing gypsum content. As expected, the lower value of residual water content was related to the higher gypsum contents. This further reinforces the fact that the addition of gypsum had a greater influence on the residual water content than the curing period, due to the high affinity of these materials to water. The reduction in the residual water content is attributed to the hydration process of lime. Also, as the curing period increased, the amount of pozzolanic compounds increased, resulting in the increase of the weight of solids per unit volume. Further, ettringite formation leads to a further decrease in residual water content due to the chemical consumption of water during ettringite nucleation and subsequent growth. The reduction of water content and the formation of ettringite minerals negatively influenced the strength properties of the soil samples, especially the soil samples with higher gypsum content. 2.5

2 days at 40° C 7 days at 40° C 28 days at 40° C 2 days at 20° C 7 days at 20° C 28 days at 20° C

2

UCS (MPa)

45

1.5

1

0.5

0 0

10

20

Gypsum Content (%) Fig. 5. Unconfined compressive strength variation Vs gypsum content.

30

The wave velocity test method is one of the non-destructive test methods used to evaluate the stiffness properties of materials. A wave velocity test was conducted to evaluate the untreated and limetreated soil mixtures and to determine the variation and correlation of the wave velocity with gypsum content and curing conditions. The wave velocity of the soil samples was measured before performing the unconfined compression test. The wave velocity values of natural and gypseous soil confirmed the results for unconfined compressive strength: 617, 642, 664 and 755 m/sec for 0%, 5%, 15% and 25% gypsum, respectively. The increase in the wave velocity values with the addition of gypsum was due to the reduction in the void ratio of soil samples, as discussed previously. Moreover, in a three-phase system (solid, liquid and gas) such as compacted soil, wave transmission occurs through all the phases. Generally, wave velocities in solids are higher than velocities in liquids, which in turn are higher than velocities in gases. Therefore, a lower amount of voids in the soil samples gives higher wave velocity values compared to the other samples. The results of the wave velocities of lime-treated soil samples are presented in Figs. 6 and 7. The results show similar trends to the results for the unconfined compressive strength. For all levels of gypsum content, the data demonstrates that the wave velocity essentially increased linearly with the curing period (Fig. 6) and for both curing temperatures. After 28 days, the soil samples treated with 0%, 5%, 15% and 25% gypsum attained maximum wave velocity values of the order of 1483, 1609, 1533 and 118 m/sec for curing temperatures of 20 °C, and 1602, 1716, 1643 and 1225 m/sec for curing temperatures of 40 °C, which gave for the same curing temperatures an improvement ratio of 2.4, 2.5, 2.3 and 1.4, and 2.5, 2.6, 2.4 and 1.6 times the wave velocity of the natural soil (950 m/sec), respectively. The variation of the wave velocity with the unconfined compressive strength values for all soil mixtures is illustrated in Fig. 8. Generally, the wave velocity values increased with unconfined compressive strength values and the relationship was linear with the coefficient of determination (R2 = 0.65). However, a significant amount of scatter was observed in the data, which could be attributed to the unconfined compressive strength of the soil being the property of the material that is most affected by its composition, including cementing material, voids filled with air and water and other parameters (fissures, cracks etc.). Thus, the unconfined compressive strength of the soil samples is not directly related to the velocity of the wave propagation; although, in general, a higher unconfined compressive strength can be expected to be associated with a higher velocity. 4.5. One-dimensional free swell properties The swell potentials of the natural and gypseous soil samples, compacted at OMC and γdmax, were measured. The soil samples had swell potentials of 1.9, 1.4, 0.8 and 0.4 for 0%, 5%, 15% and 25% gypsum content, respectively. The decrease in the swell potential values of the soil samples was due to the addition of non-expansive materials (gypsum) and the reduction in the soil matrix, especially clay content (due to the complementary substitution method used in the samples' preparation). Also, the void ratio decreased with gypsum content, as presented in Table 4, since gypsum acts mainly as an inert filler and tends to reduce the swell of the soil samples. Similar observations were reported by Yilmaz and Civelekoglu (2009) during their study on the effect of gypsum addition on the behavior of expansive soil (bentonite soil). The swell potential decrement was larger when the addition of gypsum was larger, and the reduction in value of the swell potential reached 70% for the soil samples with 25% gypsum content. Figs. 9 and 10 illustrate the effects of the curing conditions and gypsum content on the free swell potential of lime-treated soil samples. From these two figures it can be seen that the free swell potential of

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Table 5 Residual water content and unconfined compressive strength of soil samples. Curing temperature (°C)

Gypsum (%)

Curing period (day) 2

20

0 5 15 25 0 5 15 25

40

7

28

w/c (%)

UCS (MPa)

w/c (%)

UCS (MPa)

w/c (%)

UCS (MPa)

10.3 9.8 9.6 9.5 9.8 9.4 9.2 9.1

0.42 0.79 0.75 0.57 0.6 1.16 0.96 0.78

9.6 9.3 9.1 9 9.5 9.1 9 8.8

0.68 1.26 1.12 0.93 0.74 1.77 1.54 1.26

8.6 8 7.5 7.2 7.7 7.1 6.9 6.1

0.76 1.4 0.98 0.89 1.45 2.44 1.8 1.47

lime-stabilized soil samples without gypsum decreased with increased curing time, and that a greater reduction was observed for soil samples cured at 40°C compared to samples cured at 20°C. The free swell potential became suppressed at seven days for soil samples cured at 40°C and at 28 days for samples cured at 20°C. This reduction is attributed to the addition of lime, which is known to be an effective stabilizer for reducing the swell potential of clay soils. When lime is added to the soil, there is an immediate effect on the soil properties caused by cation exchange, which results in a reduction in the diffuse double layer and soil plasticity, thus causing flocculation and agglomeration. The other reaction is the pozzolanic reaction, which is time- and temperature-dependent (Al-Mukhtar et al., 2010a,b; Little, 1995), and which results in cementitious materials (Fig. 2) that bind the soil particles together. Thus the magnitude of free swell is decreased. For the soil samples with added gypsum, the free swell potential increased with curing time (Fig. 9). The free swell potential values increased from 0% for soil samples without gypsum to 0.85%, 0.5% and 0.35% for soil samples cured for 28 days at 20°C, and to 0.75%, 0.45% and 0.25% for soil samples cured for 28 days at 40°C, with 5%, 15% and 25% gypsum content, respectively. During two days of curing at both temperature values, the swell potential of the soil samples decreased with the addition of gypsum (Fig. 9). This behavior was due to the presence of unreacted lime and gypsum particles, which act as filler materials and tend to reduce the swell potential. After two days of curing, the swell potential of the soil samples increased up to 5% gypsum content, and then decreased with more swelling occurring for soil samples cured for 28 days. In the case of the lime-treated soil samples with 5% gypsum content, the increase in the swell potential can be explained by the chemical reactions caused by the presence of gypsum, alumina and lime. These reactions lead to

the formation of ettringite nuclei which have the ability to grow, as the necessary chemical compounds for this are available. Moreover, although no crystalline ettringite was observed in the soil samples with 5% gypsum content (Fig. 3), it could that the presence of an amorphous ettringite product causes the swell, as mentioned by Wild et al. (1999). Further, X-ray diffraction test results emphasized the ettringite formation of the soil samples with 5% gypsum content, and the highest intensities of ettringite mineral were found in these samples. At higher levels of gypsum content (more than 5%), the soil samples exhibited less swelling in comparison to samples with a 5% gypsum content, even though there were ettringite crystals, as shown in Fig. 3. This was due to unreacted gypsum particles, which act as inert filler materials and restrict the swell. Also, these samples appeared to contribute less to ettringite formation than did the pozzolanic reactions, and a small swell potential was therefore observed with an increased gypsum content. Finally, increasing the temperature from 20°C to 40°C led to a greater reduction in swell potential. This may be attributed to the greater formation of cementitious materials during curing at 40°C than at 20°C. In addition, soil samples cured at 20°C show a higher proportion of macropores, a coarser pore structure and higher porosity values, when compared with soil samples cured at 40°C. Thus the small pores and lower porosity give little liberty, if any, for the clay particles to expand because of the cementitious materials which formed during the curing process. Moreover, smaller pore size and lower porosity decreases the soil's ability to retain further water upon wetting, and this in turn reduces the swell potential. 5. Conclusions The aim of this work was to investigate the effect of gypsum content and curing conditions on two important geotechnical properties: the

1750

1500

Wave Velocity (m/sec)

Wave Velocity (m/sec)

1750

1250

1000

750

0%G at 40° C

0%G at 20° C

5%G at 40° C

5%G at 20° C

15%G at 40° C

15%G at 20° C

25%G at 40° C

25%G at 20° C

1500

1250 2 days at 40° C

1000

7 days at 40° C 28 days at 40° C 2 days at 20° C

750

7 days at 20° C 28 days at 20° C

500

500 0

10

20

Curing Period (day) Fig. 6. Wave velocity variation Vs. curing conditions.

30

0

10

20

Gypsum Content (%) Fig. 7. Wave velocity variation Vs. gypsum content.

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A. Aldaood et al. / Applied Clay Science 88–89 (2014) 39–48

2.5

1 2 days at 40° C

2

2 days at 40°° C

y = 0.0017x - 1.0543 R² = 0.65

7 days at 40° C

7 days at 40° C 28 days at 40°° C

28 days at 40° C

0.75

Free Swell (%)

UCS (MPa)

2 days at 20° C

7 days at 20° C

1.5

28 days at 20° C

1

2 days at 20°° C 7 days at 20°° C 28 days at 20°° C

0.5

0.25

0.5

0 750

1000

1250

1500

1750

0 0

Wave Velocity (m/sec)

mechanical strength and swell potential of lime-treated fine-grained clayey soil. In order to conduct a precise parametric study, all tested samples were prepared in the laboratory. Moreover, studies in the literature have shown that the gypsum content in nature varies widely. Thus, to control the exact gypsum content in the different samples tested and in order to study an important range of gypsum soil behavior, samples were prepared with three gypsum contents: 5%, 15% and 25% by dry mass of soil. The following conclusions can be drawn from this study: - The presence of gypsum enhanced the mechanical properties but the swelling potential decreased as the gypsum content increased, with a more pronounced effect with a higher gypsum content. - It is necessary to determine the gypsum content before lime stabilization in order to investigate the negative or positive impact on the effectiveness of soil stabilization. - For the tested soil treated with 3% lime, the presence of gypsum enhanced the strength properties of lime-treated soil up to a certain limit (5% gypsum), and as the gypsum content further increased the strength properties decreased for all curing conditions. - The contribution of lime to stabilization was found to be more prominent for the soil samples with lower gypsum content (5%). This implies that lime stabilization plays an important role in the formation of a compact matrix for mixes with a lower gypsum content. - The beneficial effect of lime stabilization in controlling the swell potential of gypseous soil is partially lost due to the ettringite formation, especially when the soil is subject to long curing times. - Lime treatment can be effective in stabilizing gypseous soils and in

1

0%G at 40°° C

0%G at 20°° C

5%G at 40°° C

5%G at 20°° C

15%G at 40°° C

15%G at 20°° C

25%G at 40°° C

25%G at 20°° C

0.75

0.5

0.25

0 0

10

20

Curing Period (day) Fig. 9. Free swell potential vs. curing conditions.

10

20

30

Gypsum Content (%)

Fig. 8. Unconfined compressive strength and wave velocity relationship of all soil samples.

Free Swell (%)

47

30

Fig. 10. Free swell potential vs. gypsum content.

enhancing their mechanical properties, but the amount of lime added must be adjusted according to the mineralogical composition and the gypsum content of the soil. - Tests at the microscopic level (porosimetry, XRD and SEM) are the key to better insight into the lime stabilization reactions of gypseous soils and the evolution of the geotechnical properties of these soils. Analyses of lime-treated gypseous soils revealed the formation of new hydrates (CSH) and (CAH) and also ettringite. The high intensities of the new minerals were observed within the soil samples cured at 40ºC. An understanding of the changes occurring in the mineralogy and at the microstructural levels (porosities and pore size distribution) is essential for a better interpretation of the modifications in the geotechnical properties of the lime-treated gypseous soil.

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