Review on the effect of gypsum content on soil behavior

Transportation Geotechnics xxx (2015) xxx–xxx

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Review on the effect of gypsum content on soil behavior Dina Kuttah a,⇑, Kenichi Sato b a b

Researcher at the Swedish National Road and Transport Research Institute, Linköping, Sweden Faculty of Engineering, Fukuoka University, Japan

a r t i c l e

i n f o

Article history: Received 9 June 2014 Revised 18 June 2015 Accepted 19 June 2015 Available online xxxx Keywords: Gypsum Sulfate bearing soil Subgrade soil Problematic soil Soil improvement and treatment

a b s t r a c t Increasing the utilization of urban soil usage brings along very important problems to be addressed at the international level regarding the use of sulfate bearing soils as construction materials. After briefly exploring current research perspective, this paper captures the current state of the art in the field of sulfate bearing soils used as construction materials through a detailed discussion of different studies that pave the way to the possible treatment of such soils to be used in road construction. Additionally, the purpose of this paper is to acquaint geotechnical and pavement engineers with the present state of the art of the physical and chemical properties of gypsum and hence its effect on the subgrade soil performance. On the other hand, this paper discussed an opposite action in which some researchers have mixed recycled waste gypsum components with soil in order to stabilize it. In other words, a number of open research issues are highlighted with the intension of inspiring new conditions and developments in stabilizing problematic gypsiferous soil as well as adding gypsum to stabilize non gypsiferous soils of weak performance. Ó 2015 Elsevier Ltd. All rights reserved.

Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Some physical and chemical characteristics of gypsum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors affecting gypsum solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Roads problems encountered in gypsiferous soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of gypsum content on soil properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of gypsum content on soil compaction characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of gypsum content on soil permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of gypsum content on soil swelling and heaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect of gypsum content on soil strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stabilization of gypsiferous and non-gypsiferous soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stabilization of naturally occurring sulfate-bearing soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physical stabilization of sulfate-bearing soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical stabilization of sulfate-bearing soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stabilization of non-gypsiferous soil by adding gypsum components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion and conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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⇑ Corresponding author. http://dx.doi.org/10.1016/j.trgeo.2015.06.003 2214-3912/Ó 2015 Elsevier Ltd. All rights reserved.

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Notations CP collapse potential CaO calcium oxide CaSO4 anhydrite CaSO4
2H2O gypsum CaSO4
½H2O bassanite (hemihydrate) CBR California bearing ratio Cc compression index FGD Flue gas desulfurization HCl hydrochloric acid

Introduction It is very common that the soil at a site to be developed is not ideal from the viewpoint of geotechnical engineering. An attractive approach which is usually used to avoid many of the settlement and stability problems associated with soft foundation soils is soil improvement and treatment. In its broadest sense, soil improvement is the alteration of any property of a soil to enhance its engineering performance (Sherard et al., 1963; Chen, 2000). Since pavements are designed to distribute stresses imposed by traffic to the subgrade, the subgrade conditions have a significant influence on the choice and thickness of pavement structure and the way it is designed. Depending on the existing soils and project design, the properties of the subgrade may need to be improved, either mechanically, chemically, or both, to provide a platform for the construction of subsequent layers and to provide adequate support for the pavement over its design life (Jones et al., 2010). In most regions of the world, especially in the Middle East, natural soils and aggregates contain varying quantities of soluble salts (Blight, 1976; Fookes, 1976, 1978; Fookes and French, 1977; Tomlinson, 1978). Gypsum is one of the soluble salts that can have a detrimental effect on subgrade soils, buildings and earth structures if it is presented in high quantities in the soil (Subhi, 1987; Obika et al., 1989; Razouki et al., 1994; Razouki and Kuttah, 2004, 2006). According to Klein and Hurlbut (1985), gypsum (CaSO4
2H2O) contains 32.6% calcium oxide (CaO), 46.5% sulfur trioxide (SO3) and 20.9% combined water (H2O). As a result of dehydration of gypsum, the first 1½ molecules of (H2O) in gypsum are lost relatively continuously between 0 °C and about 65 °C, perhaps with only slight changes in the gypsum structure, leading to bassanite (CaSO4
½H2O). At about 70 °C, the remaining (½H2O) molecule in bassanite (hemihydrate) is still retained relatively strongly but at about 95 °C it is lost and the structure transforms to that of anhydrite (CaSO4). Both, anhydrite and hemihydrate have several different forms with different properties, but in general, if anhydrite or hemihydrate are mixed with water, they will hydrate to gypsum (Claisse and Ganjian, 2006).

HNO3 K ML Ø OMC SM SO3 TxDOT

cdmax

nitric acid dissolution rate constant sandy silt low plasticity soil the angle of internal friction optimum moisture content silty sand soil sulfur trioxide Texas Department of Transportation the maximum dry unit weight

The presence of gypsum in subgrade soil could be naturally or artificially added gypsum as follows: 1. Naturally occurring gypsum, in which hydrate and anhydrate gypsum are considered as part of the soil components. Soil science has paid little attention to gypsiferous soils, and this limited knowledge is reflected in the direct loan of customary terms of soil science that can lead to misconceptions on the composition and behavior of soils with large proportions of gypsum. Both geological and climatic reasons cause gypsumrich soils to occur in dry lands (Herrero and Porta, 2000). Such soil can be found in the Middle East (especially in Iraq, Syria & Iran), Europe especially in Spain, former USSR (Siberia, Georgia, Transcaucasia, Azerbaidzhan), north Africa (Algeria, Tunisia), south east of Somalia, southern central Australia and in former inland lakes in western USA (Van Alphen and Romero, 1971). Soils containing gypsum in Cardiff area of Wales in the United Kingdom were reported by Hawkins and Pinches (1987). The presence of natural occurring gypsum in subgrade soil usually found in high quantities and therefore using such soils as subgrade materials may lead to detrimental effects of roads structures as reported by many authors (Razouki et al., 2011, 2012a,b). The problems in using naturally occurring gypsiferous soil as road construction materials are usually faced when these soils subjected to long term soaking and leaching (Razouki and Kuttah, 2006; Razouki et al., 2011; Aldaood et al., 2014) as well as cyclic drying and wetting as studied by Razouki and Salem (2014). 2. Artificially added gypsum, when gypsum and/or bassanite (virgin or recycled) are added to non-gypsiferous subgrade soil in control quantities to improve the mechanical properties of the subgrade soil (due to the cementation action of gypsum) and/or to minimize the landfilling of waste products involve gypsum, since the use of secondary (recycled) instead of primary (virgin) materials in roads construction helps easing landfill pressures and reducing demand of extraction (Huang et al., 2007). During the three stages of production, construction and demolition of plasterboards, approximately 15 million tons of gypsum waste plasterboard is generated annually in the world (Ahmed et al., 2011). Production of Flue gas

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desulfurization (FGD) gypsum doubled from 12 to 25 million tons between 2004 an Jones et al., 2010, according to the U.S. Geological Survey, and it’s projected to reach 40 million tons by 2020 (Fisher, 2011). Due to these facts, recently, by product plasterboards gypsum has been mixed with subgrade soil in order to reduce the quantities of waste boards sent to land filling. Many researchers have carried out investigations on the effect of adding by-product gypsum content on the properties of soil used as construction materials. In practice, especially in Japan, new specifications controlling the mixing percentage of gypsum with soil has been modified (Kyokai, 2007) to accept as much as possible higher mixing percentages of gypsum with soil because the priority now in Japan is to reduce the environmental effect of waste plasterboards (gypsum) by mixing it with soil used as a construction materials (Karami et al., 2007; Ganjian et al., 2008; Rao et al., 2011; Sato et al., 2012; Kamei et al., 2013). In summary, the soil used as a construction materials may contain gypsum components naturally and refer to as (gypseous, gypsiferous, or sulfate bearing soils) or the gypsum may be added to the non gypsiferous soil in small quantities either to improve its properties or to minimize the landfilling of waste products involve gypsum. Therefore, the presence of gypsum in soils used as construction materials and its effect on the different soil properties has been studied independently by many researchers all over the world. Correspondingly, this paper highlights the efforts done in this subject and discusses the main findings obtain up to now in this research area. Some physical and chemical characteristics of gypsum The structure of gypsum consists of parallel layers of (SO4) 2 groups strongly bonded to (Ca)+2. These layers are separated by sheets of (H2O) molecules with weak bonds existing between the H2O molecules in neighboring sheets (Klein and Hurlbut, 1985). Klein and Hurlbut (1985) pointed out that gypsum is either colorless or it may be white, gray, red, brown or having various shades of yellow resulting from impurities. The hardness of gypsum is 2, so that it can easily be scratched by the finger nail. It is also important to note that the hot dilute (HCl) is capable to dissolve gypsum (Klein and Hurlbut, 1985). Following Kuznetsova and Lomovskii (1986), the initial monoclinic structure of gypsum single crystals could be changed into hexagonal and then into orthorhombic during thermal decomposition (<200 °C) in a vacuum up to 1.33 Pa pressure. Regarding the specific gravity, Klein and Hurlbut (1985); Horta (1989) reported that gypsum has a specific gravity of 2.32. Factors affecting gypsum solubility Gypsum, whether in massive or particulate form, dissolves producing caverns and/or progressive settlements;

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accelerating seepage flows and accompanying deteriorations of foundation unlikely if provision is made to keep the initial seepage flow rates to low value (James and Lupton, 1978). Due to the fact that various factors can affect the solubility of gypsum in water, which in turn may affect the soil engineering properties, the influence of different factors on the gypsum solubility have been studied and reported by different authors as discussed below. Van Alphen and Romero (1971) reported that the solubility of gypsum is 2.6 gm/l (although the solubility varies somewhat with the concentration and the composition of soil solution). They found out that 1:1 soil:water ratio would dissolve only about 0.25% weight of gypsum in a soil sample. The soil:water ratio should therefore be very dilute when high gypsum percentages are involved, e.g. for 40% gypsum, the ratio soil:water ratio should be at least 1:160 to dissolve the whole gypsum. James and Lupton (1978) found out that the dissolution rate constant (K) of gypsum increased 3.25 times when the temperature increased from 5 °C to 23 °C. Akili and Torrance (1981) pointed out that the chemical composition of water can have a significant influence on the dissolution of gypsum. The pH value, the particle size distribution and the applied pressure are all factors affecting the gypsum solubility. Subhi (1987) reported that the effect of pH is important in that acids increase the solubility of most common substances. However, Shlash and Al-Rawi (1994) found out a reduction in the percentage of many salts such as CaSO4 when a gypsiferous soil is treated with nitric acid (HNO3) and hydrochloric acid (HCl) of different concentrations. Khan (1994) reported that the solubility of gypsum is conversely proportional to the particle size (i.e. its solubility increases linearly with increasing mesh number). Freyer and Voigt (2003) reported that the solubility of all CaSO4 phases (i.e. gypsum, hemihydrate and anhydrite) increases with increasing the applied pressure. According to the Environment and Raw Material Committee (2010), the solubility of gypsum and anhydrite in water at 25 °C is 2.6 and 2.1 g/l respectively. Although the solubility varies somewhat with the concentration and the composition of soil solution).

Roads problems encountered in gypsiferous soils Physical salt damage may occurs when soluble salts contained in the aggregate of the base or in underlying material are moved upwards by evaporation of water through the surfacing and they crystallized out beneath the surfacing layer to blister and crack the surface. Such pavement damage was noticed in USA by Blight (1976) and in Arabian Gulf by Fookes (1976). According to Fookes and French (1977), the soluble salts damage in the road occurs when the soluble salts, particularly sulfate, are found in the ground, ground water, and road mineral aggregate. They added that the magnitude of the damage is influenced by traffic load and pavement thickness.

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Fookes and French (1977) reported that disintegration of thin surface course due to salt attack in road crossing a salina was noticed in Bahrain and reflective cracking in surface course largely from disintegration and settlement of sub-base attacked by salt was noticed in United Arab Republic. Sulfate-induced heave has been a recognized problem in the United States since it was reported by Mitchell (1986) and later by Hunter (1988). Due to the fact that large amount of money is spent in the construction of highways, the use of available materials becomes one of the essential requirements in minimizing their construction cost. Subhi (1987) reported that salt-bearing soils have been used extensively in road construction both as general fill for embankments as well as subbase materials. Accordingly, it is very important to study the characteristics of gypsiferous soils and their soluble mineral constituents in order to understand their behavior under different field conditions. Hunter (1988) reported sulfate-induced heaves in Stewart Avenue and Owens Street in Las Vegas, and his investigations found distress in areas where the soil had as little as 10 percent clay fraction. Evidence of distress appeared within 6 months of project completion and resulted in severe damage to the pavement within 2 years. Pavement heaves rose as high as 300 mm and were parallel to the roadway. Large cracks 25–150 mm wide were measured on the surface. Moreover, heaves were also observed in areas where soluble sulfate concentrations were as low as 700 ppm but located adjacent to a major water source (Hunter, 1988). Highly gypsiferous soils which are permeable or containing fissures suffer from the formation of cavities due to leaching out of gypsum from regions surrounding the waterways (Cooper, 1989). These cavities become large and large until a sudden collapse of overlying strata takes place. These cavities can be very dangerous if the soil is used as a foundation for any type of structures. During the construction of a runway in the Turks and Caicos Island, British West Indies, damage to the pavement surfacing from soluble salt occurred in the form of blistering & fluffing of the bituminous prime coat (Obika et al., 1989). Razouki et al. (1994) reported many of the problems encountered in highly gypsiferous soils. Such problems include non-homogeneity, great losses in strength upon wetting, sudden increase in compressibility upon wetting, continuation of deformation and collapse upon leaching due to water movement, existence of cracks due to seasonal changes, existence of holes due to local dissolution of gypsum. Cooper and Saunders (2002) reported that gypsum karst problems in the Permian and Triassic sequences of England had caused difficult conditions for bridge and road construction. In Northern England, the Ripon Bypass crosses Permian strata affected by active gypsum karst and severe subsidence problems (Cooper and Saunders, 2002). Several roads, airfield pavements, and parking lots in Texas and other states in the western United States have suffered severe pavement damage due to expansive

minerals formed from the reactions of calcium based materials used to stabilize sulfate-bearing soils. Remediation costs for projects that suffer sulfate-induced heave damage are very high, because often the entire pavement may have to be removed and reconstructed (Kota et al., 2007). The need for constructing new highways in areas having high gypsum content is faced with the problem of soluble salts in the subgrade soil as well as in fill material for the embankments. From economic point of view, it is necessary to study in depth the possibility of using gypsiferous soils for subgrade and embankment purposes. All of the aforementioned problems has been encountered due to the high presence or formation of gypsum in soil used as a construction material. Therefore, it is very important to shed light on the effect of gypsum content on the different engineering soil properties as discussed in the following paragraph. Effect of gypsum content on soil properties The presence of crystalline gypsum in the soil, possessing special physicochemical properties, which differ sharply from the properties of other minerals of the soil, complicates the determination of the water content, the specific gravity of the particles, the grain – size distribution, and consequently other characteristics connected with them (Arakelyan, 1986). Depending on the existing soils and project design, the properties of the subgrade need to be evaluated, to provide a platform for the construction of subsequent layers and to provide adequate support for the pavement over its design life (Jones et al., 2010). It is well known that gypsum affects the properties of soils used as a construction materials. However, some researchers used natural gypsum and other used recycled gypsum to investigate the effect gypsum on soil performance. The authors illustrate and discuss the research done to investigate the effect of natural gypsum as well as recycled gypsum on different soil properties as shown below. Effect of gypsum content on soil compaction characteristics Subhi (1987) studied the compaction characteristics of the sandy silty clay soil. She found out that the addition of gypsum <63 lm in size or gypsum particles between 250 and 355 lm tends to decrease the maximum dry density (cdmax) and to increase the optimum moisture content (OMC) of the soil. In the case of the addition of gypsum in the size fraction between 850 and 1000 lm a decrease in both the maximum dry density and optimum moisture content was obtained. This compaction behavior occurs as a result of both the specific gravity and the grain size of the gypsum. The decrease in the maximum dry density may be attributed to the loss of some compactive energy in breaking the cementatious bonds which may form between clay and gypsum particles. Subhi (1987) pointed out that the mixing of gypsum with the sandy silty clay may involve cation exchange. It may also produce flocculation and agglomeration of the soil which will decrease the

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plasticity index and should therefore decrease the optimum moisture content, because of the decrease in the surface area due to the increase in edge to face contacts of the particles. Kamei et al. (2012) reported that the dry unit weight increased, and moisture content decreased with the increase of the recycled bassanite content (CaSO4
½H2O) in the very soft clay soil mixture. The increase in dry unit weight when the amount of bassanite increases is attributed to the potential of bassanite for absorbing the water from the test soil. Furthermore, the developed hardening between soil particles prevents or reduces the penetration of water inside the soil sample and then no more or little water can be further absorbed by the sample (Kamei et al., 2012). Ahmed (2013) tested eight sand-gypsum mixtures using standard Proctor effort to evaluate the effect of gypsum content on the compaction characteristics of sandy soil. The mixtures had 0%, 10%, 20%, 30%, 40%, 50%, 65%, and 80% gypsum content by weight. The tests results showed that at low gypsum contents (i.e., gypsum content ranging from zero to about 30% by weight) there was a slight increase in the maximum dry density associated with a slight decrease in the optimum water content when gypsum content increased up to 15%. On the contrary, when gypsum content increased more than 30%, the maximum dry density started to decrease noticeably associated with a clear increase in the optimum water content. Ahmed (2013) attributed this behavior to two influence factors. The first factor was the role of gypsum particles as a filling material to the intergranular voids of the soil matrix, while the second factor was the decrease in the overall specific gravity of the soil mixture associated with the increase in gypsum content, since the specific gravity of the sandy soil used is 2.65 while it is 2.33 for gypsum (Ahmed, 2013). Effect of gypsum content on soil permeability Keren et al. (1980) observed that gypsum only reduces the hydraulic conductivity when it is very fine (<44 lm) and close the macropores of fine textured soils. In soils with coarser textures, by-pass flow is less important and the fine-gypsum infillings have less influence on the saturated hydraulic conductivity. The presence of large gypsum particles in macropores does not reduce the saturated hydraulic conductivity because the resulting packing pores are still not large enough to restrict flow (Keren et al., 1980). Subhi (1987) investigated the effect of addition of gypsum to the sandy silty clay from Baghdad with 2.2% initial gypsum content. Her tests indicated that the permeability of soil compacted at the optimum moisture content increased with the increasing amount of gypsum of 850–1000 lm size fraction and decreased with the addition of the less than 63 lm size fraction. She added that many factors can affect the permeability of a compacted soil. These include non-uniform saturation, the migration of fines during testing and variation of void ratio. Al-Dabbagh et al. (1990) reported that the permeability increased with increasing gypsum content of a compacted

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clayey loam and sandy loam of Mosul city. This is attributed to the increase in void ratio with increasing gypsum content and this occur due to the dissolution of gypsum in the soil which leads to enlarging cavities between soil particles and forming channels that ease water flow. Effect of gypsum content on soil swelling and heaving Lutenegger et al. (1979) reported that the estimated expansion pressure from gypsum is generally less than that from expanding montmorillonitic clay and is probably greater than that from capillary frost heave. Hawkins and Pinches (1987) reported that hydration of CaSO4 could obviously cause some expansion resulting from void filling by water, while the primary cause of the expansion was the growth of gypsum. They added that gypsum crystallization can occur in several forms. The most easily recognizable are the prismatic crystals which may also be clustered together in disc shaped rosettes, or the growth may be as thin acicular crystal form that can exert most force at its growing end, thus being primarily responsible for heave due to gypsum growth. On the other hand, Ameta et al. (2008) reported that the problems associated with expansive soil are related to bearing capacity and cracking, breaking up of pavements, and various other building foundation problems. The effect of gypsum on swelling pressure is studied and it is found that swelling pressure decreases with addition of gypsum. Adding of 6% gypsum to expansive soil in India, caused more than 60% reduction in the swelling pressure of the tested soil according to Ameta et al. (2008). Yilmaz and Civelekoglu (2009) reported that gypsum can be used as a stabilizing agent for expansive clay soils, effectively. They found that the swell percent obtained from carrying out free swell test on expansive clay that the swell percent of the clay decreased from about 65% for untreated clay to 20% for clay samples mixed with 10% gypsum by mass. Frost heave property throughout capillary rise test were investigated to determine the behavior of treated soil with recycled gypsum (Ahmed et al., 2011). Ahmed et al. (2011) reported that the increase of recycled gypsum content are associated with reducing of capillary rise. Thus, the time required for water to rise within the soil sample is increased. It is followed by reducing the formation of ice lenses and then the effect of frost heave will be minimized (Ahmed et al., 2011). Effect of gypsum content on soil strength Salas et al. (1973) reported that the angle of internal friction (Ø) of low plasticity gypseous clay increased with increasing gypsum content. Ramiah (1982) studied the effect of adding different gypsum contents (0%, 3% and 10%) on shear strength of a silty clay brought from a site in Baghdad city. The test results showed that the unconfined compressive strength increased as the gypsum content increased for unsoaked specimens. Petrukhin and Arakelyan (1985) examined the character of variation in the strength of the soils as a function

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of initial (natural) gypsum content. They found that in clayey soils, the specific cohesion increased sharply with increasing initial content of gypsum up to 15% due to the formation of crystals in the pores of the soil, which reduces the porosity and hence increases the cohesion. At 15% gypsum content, the strength of the clayey soils attains a critical value, after which the specific cohesion decreases due to the failure of crystal bonds. The angle of internal friction increased steadily with increasing the gypsum content up to about 20% since the friction between the gypsum particles is greater than that between the mineral components of the soil. After 20% gypsum content, the angle of internal friction decreases and no reason for this phenomenon was reported. For sandy loam, the angle of internal friction Ø increased with increasing gypsum content up to 25% then decreased. This is due to the fact that the mineral friction increases with increasing gypsum content, then the porosity increase causes a reduction in the angle of internal friction Ø for more than 25% gypsum content (Petrukhin and Arakelyan, 1985). AL-Ani et al. (1991) studied the effect of adding gypsum (0.6%, 5%, 10%, 15% and 20%) to an A-7-6 (24) silty clay subgrade soil on the soaked CBR for 4 days soaking period. They reported a soaked CBR of (3.6%, 4.6%, 8.8% and 6.5%) for (0.6%, 5%, 10%, 15% and 20%) gypsum added respectively. This mean that the soaked CBR value increased with increasing gypsum content up to 15% and then decreased. Ahmed and Ugai (2011) reported that the compressive strength values for poorly graded sandy soil samples stabilized with recycled gypsum increased from 14.42 kPa to 25.43, 81.99 and 331.18 kPa due to adding 5%, 10% and 20% content of recycled gypsum, respectively. This can be explained by the addition of recycled gypsum to the soil causing cementation or hardening of soil particles; thus cohesion strength between soil particles is developed (Ahmed and Ugai, 2011). Kamei et al. (2012) investigated the effect of recycled bassanite content (of 0%, 5%, 10% and 20%) on the ultimate compressive strength for different investigated clayey samples mixed with 5% furnace slag cement and subjected to five freeze–thaw cycles. They found that the presence and increase in the recycled bassanite content in the soil mixture has a significant effect on the improvement of strength of samples subjected to freeze–thaw cycles. Kamei et al. (2012) found that for the all five freeze–thaw cycles, the maximum ultimate compressive strength reached was for samples with 20% recycled bassanite. They added that the role of bassanite in increasing the strength of very soft clay soil is more significant in the case of samples exposed to freeze–thaw cycles compared to those not exposed to freeze–thaw cycles. Kobayashi et al. (2013) studied the effect of different percentages of recycled basanite content (namely 0%, 5%, 10% and 15%) on both compressive and splitting tensile strengths of two types of cohesion-less soil. They found that both compressive and splitting tensile strengths enhanced with the additives of recycled bassanite. The increase of bassanite content had a more significant effect on the compressive strength compared with the effect on tensile strength according to Kobayashi et al. (2013).

They added that the use of recycled bassanite to enhance the strength of sandy soil had a more significant effect compared with silty soil. For silty soil, increasing the basanite content from 0% to 15% caused an increase of 2.5-fold and 6.5-fold in the splitting tensile and compressive strength respectively. Stabilization of gypsiferous and non-gypsiferous soils As discussed above, it is well known that gypsum has different positive and negative impact on the engineering properties of subgrade soil depending on the type of the soil, the quantity of gypsum, and its degree of hydration, added additives, environmental circumstances, and other factors. At the same time, it is important to distinguish between two issues highlighted in this paper, namely, stabilization of naturally occurring sulfate bearing soil (gypsiferous soil) and non-gypsiferous soil stabilized by adding one of gypsum components. The following sections highlights these two different subjects separately. Section 6.1 discusses the recent methods used to improve gypsiferous soils in which gypsum is presented naturally, while Section 6.2 deals with stabilization of non-gypsiferous soil to which gypsum is added as a stabilizing material. Stabilization of naturally occurring sulfate-bearing soils Gypsiferous soils are one of the most complex materials that challenge the geotechnical engineers, but due to the fact that many of gypsiferous soil regions are opened up to industrial development, it has become essential to study in depth the properties, behavior and the possible stabilization methodology of such soils under different conditions. As shown previously, the abundant amount of data obtained from the lengthy research programs all over the word revealed in some cases contradicting results due to the complexity of the gypsiferous soils. However, in spite of this contradiction, the scientists have agreed about the importance of improving further the performance of sulfate bearing soils (e.g. gypsiferous soils) used as a construction materials and therefore rigid actions have been considered to stabilize gypsiferous soils as discussed below. Physical stabilization of sulfate-bearing soils The physical stabilization of soil means that the soil properties are improved using mechanical methods, such as compaction, soil reinforcement, pre-wetting, and others. In practice, it is expected to deal with compaction of soil in every civil engineering project. In connection with gypsiferous soils, the importance of compaction increases as the dissolution of gypsum depends on the permeability of the soil and hence on the relative compaction of the soil. For these reasons, the present subject is devoted to the effect of relative compaction on the behavior of clayey gypsiferous soil. Thus, the physical, chemical, mineralogical, microscopical and engineering properties of the clayey gypsiferous soil are studied. In an attempt to stabilize gypsiferous soil, Razouki and Kuttah (2004) used physical stabilization method instead

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of adding chemicals to stabilize the gypsiferous soil. This physical stabilization is achieved by compacting the gypsiferous subgrade soil at high compaction efforts. Razouki et al. (2005) reported that increasing the compaction effort from 12 to 56 blows/layer causes an increase of 2.3-fold and 6.5-fold in the estimated ultimate bearing capacity for the unsoaked and soaked CBR gypsiferous soil samples for 120 days respectively, indicating that the effect of compaction effort becomes more pronounced as the soaking period increases for highly gypsiferous soil (of about 33% gypsum content). Razouki et al. (2012a), reported that for gypsiferous CBR soil samples compacted at different compaction efforts and soaked up to 120 days under controlled soaking conditions, increasing the compaction effort 25% above the modified Proctor compaction effort, caused 11% increase in soil strength (in terms of CBR) for unsoaked samples. For gypsiferous CBR soil samples soaked for 4 days and 120 days, increasing the compaction effort 25% above the modified Proctor compaction effort, caused an increase of 10.5% and 9% in the CBR value of subgrade soil tested respectively. Even though, compacting the subgrade soil above the modified Proctor compaction is not common in practice, but recent studies has showed that improving the subgrade strength and swelling characteristics of sulfate bearing soil by increase compaction has resulted in simple, adequate and environmental friendly method to stabilize even highly gypsiferous subgrade soil instead of replacing it. Moreover, Fattah et al. (2012) carried out laboratory tests to study the geotechnical properties and the behavior of three gypseous soils of different gypsum contents; 60.5%, 41.1% and 27%. The tests included compaction characteristics, compressibility, and collapsibility tests for samples tested before and after treatment by both standard compaction tests and dynamic compaction process under different number of blows, falling weights and heights of falling weights. Fattah et al. (2012) concluded that the best improvement in compressibility is achieved when the samples are subjected to 20 drops, this conclusion is based on the improvement of compression index of soaked samples obtained after treatment. In addition, as the height of drop increases from 35 to 65 cm, the compression index Cc decreases. This effect increases with the increase in the gypsum content. As the gypsum content increases, the dynamic compaction has greater effect on improvement of compressibility of the soil. In samples subjected to dynamic compaction, the change in void ratio upon soaking becomes smaller than that of untreated samples which means that the collapse potential decreases (Fattah et al., 2012). On the other hand, Najah et al. (2013a) studded the effect of replacing partly the highly gypseous soil by other types of non-gypseous soils in order to reduce the negative effect of high gypsum content and hence stabilize the gypseous soils by a physical approach. Najah et al. (2013a) presented the results of experimental studies on the collapsibility and compressibility of gypseous soil and showed the effect of mixing other soils with gypseous soil on this property. Three types of soils: gypseous, (SM) and

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(ML) soils are used in this study. Seven percentages of (SM) and (ML) soils namely: (5%, 10%, 15%, 50%, 85%, 90% and 95%) from dry weight of Gypseous soil were mixed. A series of Oedometer tests under maximum dry density and optimum moisture content were performed and the collapse potential (CP) evaluated. Regarding the soil collapsibility, results of the study showed that the soils and mixture from soils are classified as a slightly problematic in term of collapsibility. The collapse potential (CP) of the gypseous soil decrease when added (SM) soil but the high value at 50%, while for (ML) soil the height value for (CP) at 10% (Najah et al., 2013a). In terms of soil compressibility, the results of the study showed that the compression index of the gypseous soil increases with added (SM) soil, after that when the content of (SM) increases the (Cc) value decreases. For (ML) soil, the value of compression index has small value at 50% (Najah et al., 2013a). In terms of soil dry unit weight, Najah et al. (2013b) evaluated the compaction properties of highly gypseous soil after mixing with non-gypseous soil (i.e. SM-silty sand soil and ML-sandy silt low plasticity soil) at 5%, 10%, 15%, 50%, 85%, 90% and 95% by weight of the dry gypseous soil. The results obtained showed that the maximum dry density increased at 85% of (SM) mixing with gypseous soil while for (ML) soil no significant changes had been noticed (Najah et al., 2013b). Chemical stabilization of sulfate-bearing soils The chemical treatment means that the soil properties are improved with some chemical additives, such as lime, cement, bituminous, bentonite, dehydrate calcium chloride, etc. According to Harris et al. (2005), the Texas Department of Transportation (TxDOT) has seen an increase in pavement failures during and immediately after construction on roads designed to last 20 years or more. Harris et al. (2005) contributed the cause of many of these failures is sulfate-induced heave where an expansive mineral called ettringite is formed from a calcium-based stabilizer (lime or cement) reacting with clay and sulfate minerals (usually gypsum) in the soil. According to Harris et al. (2005), TxDOT has removed and replaced soils with more than 2000 ppm sulfates. Earlier in this research project, lime was identified as a plausible stabilizer in soils bearing sulfate concentrations up to 7000 ppm. As a result, Harris et al. (2005) carried out research to investigate if anything can be used to stabilize soils (reduce swell and increase strength) with sulfate concentrations above 7000 ppm. Three-dimensional swell was measured on laboratory prepared specimens with sulfate concentrations of 0, 10,000, and 20,000 ppm. Twelve stabilizers were selected for the 3-D swell testing based upon positive results obtained by other researchers. Stabilizers that significantly reduced swell in the high-sulfate soils were then subjected to unconfined compressive strength testing. According to Harris et al. (2005), three stabilizers (Claystar 7, ground granulated blast furnace slag + lime, and class F fly ash + lime) provided significant swell reduction (10–12 percent) over the untreated soil; two of the stabilizers were selected for strength testing. The fly ash swell test results were obtained too late to include in strength testing. The

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Claystar7 showed an improvement of 41 lb/in2 (282 kPa) over the untreated sample for retained strength in the unconfined compressive strength after 10 days capillary rise. The ground granulated blast furnace slag showed a 79 lb/in2 (544 kPa) retained strength. This project showed that soils with sulfate concentrations up to 20,000 ppm can be treated in a timely manner without having to remove the high-sulfate soil and replace it with a select material (Harris et al., 2005). According to Little and Nair (2009), damage in sulfate-bearing soils and aggregate systems stabilized with additives containing lime, including lime and Portland cement, has drawn considerable attention over the past two decades. Researchers and practitioners have made considerable contributions to the understanding of the problem, including the mechanisms involved in the formation of the two minerals, ettringite and thaumasite, that are most often associated with this damage. Similarly, Nair and Little (2011) carried out a study focused on identifying alternative, probable mechanisms of swelling when sulfate laden soils are stabilized with lime. The research addressed the hypothesis that swelling in sulfate-bearing fine-grained soils is due to one or a combination of three separate mechanisms: (1) volumetric expansion during ettringite formation, (2) water movement triggered by a high osmotic suction caused by sulfate salts, and (3) the ability of the ettringite mineral to absorb water and contribute to the swelling process. On the other hand, Aziz and Ma (2011) investigate the suitability of fuel oil in improving gypseous soil. A detailed laboratory tests were carried-out on two soils (with 51.6% and 26.55% gypsum content). The testing program included tests on permeability and compressibility of the soil and their collapse properties. The results showed that fuel oil is a good material to modify the basic properties of the gypseous soil of collapsibility and permeability, which are the main problems of this soil. Aziz and Ma (2011) added that the permeability were decreased due to the effect of reducing void ratio of the treated soil by increasing the lubrications between the soil particles and maintain rearrangement and reducing the voids. Moreover, treatment of the gypseous soil with fuel oil decreased the collapsibility and compressibility. This happened by the coating of the soil particles by the fuel oil including the gypsum and leading to reduce the dissolution of gypsum and preventing the collapse. According to Aziz and Ma (2011), using of 4% fuel oil for sandy soils and 3% fuel oil for clayey soils is the suitable solution for treatment the gypseous soil from the collapsibility. Stabilization of non-gypsiferous soil by adding gypsum components In contrast of the research carried out to reduce the negative impact of naturally available gypsum in the soil, several researchers have used gypsum as a soil stabilizing agent for weak or swelling non- gypsiferous soils. Ganjian et al. (2008) reported that gypsum can be used to stabilize subgrade soil by mixing it with other construction products. Ganjian et al. (2008) added that a mix of 15% gypsum, 5% cement bypass dust and 80% basic oxygen slag

was the optimum combination of a novel cementitious blend used successfully in site trials to stabilize subgrade soil in Nottinghamshire. According to Kamei et al. (2012), the presence and increase in the bassanite content in the soil mixture has a significant effect on the improvement of strength, volume change and durability of samples subjected to freeze–thaw. Kamei et al. (2012) added that the role of bassanite in increasing the strength and durability of very soft clay soil is more significant in the case of samples exposed to freeze–thaw cycles compared to those not exposed to freeze–thaw cycles. Ahmed et al. (2012), investigates the use of recycled gypsum, produced from gypsum wastes, as a stabilizer material to enhance the strength of organic, very soft clay soil taken in consideration environmental impacts. According to Ahmed et al. (2012), recycled gypsum was mixed with lime in different ratios and different contents of this admixture were used to improve both mechanical and environmental properties of the tested soil. Lime was used as a solidification agent for gypsum-soil mixture since gypsum is a soluble material. The test results show that the use of this admixture improved the strength and mechanical properties of tested soil. The strength increased with increasing both content and ratio of gypsum-lime admixture. Sato et al. (2012) investigated the effect of adding 10% and 15% recycled basanite on the strength improvement of decomposed granite soil and dewatering cake generated from construction sits in Japan. They found that the unconfined compressive strength of the two tested soils increased with increasing the recycled basanite content.

Discussion and conclusion In spite of the variation in the research findings with respect to the effect of gypsum content on different soil properties, the main findings can be discussed point by point as follows. Most of the researchers accept the fact that there is an optimal gypsum content in the soil which lead to the best performance of that soil, but this percentage differ from one soil to another depending on many factors such as the type of the soil and its particle size distribution, the type of gypsum component and its fineness, the presence of other salts in the soil, the drying and soaking conditions, . . .etc. These factors have played a large role in the agreement and the contradiction among the research findings in this field. For example, with respect to the effect of gypsum content on compaction characteristics of soil, some researchers have showed that adding gypsum to soil will increase the maximum dry unit weight of the soil and decrease the optimum moisture content as reported by Kamei et al. (2012). Ahmed (2013) agreed completely with Kamei et al. (2012), but only for gypsum content ranging between 0% to 15%. For gypsum components content of more than 15%, Ahmed (2013) observed a decrease in the maximum dry unit weight of the soil and an increase in the optimum moisture something which is in full agreement with Subhi (1987) findings for high gypsum content

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of particle size less than 63 lm and gypsum particles between 250 and 355 lm. Therefore, based on the available research it can be noticed that the size of added gypsum particles with respect to the size of soil particles play the major role in the influence of gypsum content on the maximum dry unit weight and the optimum moisture content of the soil. Similarly, with respect to the effect of gypsum content on soil permeability, researches have also shown that the soil permeability affected mainly by the amount and size of gypsum particles with respect to the soil particles. In other words, the soil permeability increases with increasing the gypsum content when the added gypsum consists of particles larger than the soil particles due to the gypsum solubility, and decreased when the gypsum particles are smaller than the soil particles and hence gypsum block the flow paths. Concerning the effect of gypsum content on soil swelling and heaving, generally, the free swell pressure of clay decreases with increasing the amount of gypsum added to the soil. However, this observation does not include the swell pressure resulting from natural crystallization and formation of gypsum inside the soil due to chemical reactions caused by weathering and chemical reactions between the soil components. With respect to frost heave, most of the findings show that the frost heave decreases with increasing the amount of gypsum added to the soil. In conclusion, adding gypsum to the soil will influence the swelling and heave characteristics of the soil based on the presence of other minerals and chemicals in the soil. In other words the soil chemical composition play the major role in controlling the heave and swelling characteristics of the soil when gypsum is added while the size of the added gypsum particles have a limited effect on this soil property. In general, according to the authors’ point of view based on the research presented in this topic, adding of 6–10% gypsum to expansive clays will reduce the swelling and heave problems of these soils. Regarding the effect of gypsum content on soil strength, it can be noticed that the role of gypsum in increasing the strength of soil is clear and significant. Based on the research findings illustrated in this overview, the authors concluded that the best soil strength performance can be achieved by adding 15% to 20% gypsum to sandy soils and 20% to 25% gypsum or basanite to clayey soils. Note that these percentages of added gypsum and/or bassanite go well with those recommended to get maximum dry unit weight and minimum optimum moisture content of the stabilized soil by Ahmed (2013). Here, it is important to mention that even though, the authors have recommended percentages of added gypsum or bassanite to optimizes some soil properties, it is advised that geotechnical engineers must investigate each case separately depending on the chemical composition of the soil as well as the particle size distribution of the soil and the added gypsum component and other influencing factors. As mentioned previously in this paper that gypsum may be added to weak non-gypsiferous soil in recommended quantities to improve its engineering performance, or on the other hand, gypsum may present naturally in the soil

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in high quantities and cause different structural problems. These types of gypsiferous soils requires special stabilization techniques as mentioned previously in this paper. In conclusion, the physical stabilization of gypsiferous soil by increase compaction can be considered as a firm solution to improve the performance of problematic gypsiferous soils as it is cheaper than replacing fully or partly the sulfate bearing soil with a sulfate free soil in the site. In addition, the physical stabilization has no negative environmental effects as that resulting from using of fuel oil as a stabilizing agent. However, even if the chemical agents used to stabilize gypsiferous soil have a minor negative environmental effects, its reaction with gypsum and hence the effect of this reaction on the behavior and performance of gypsiferous soils is still considered as unknown factor. Therefore, it is advised that geotechnical engineers must investigate each case separately depending on the type of structure, characteristics of site, environmental conditions coupled with the engineering judgment of the consultant taking into account the research findings in this topic.

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Please cite this article in press as: Kuttah D, Sato K. Review on the effect of gypsum content on soil behavior. Transport Geotech (2015), http://dx.doi.org/10.1016/j.trgeo.2015.06.003