Abstract: The stabilization of different types of problematic soils such as expansive soil, soft soil, contaminated soils etc. by using various chemical stabilizers such as lime and cement is well known techniques in the field of geotechnical engineering and ground improvement and has been practiced since last several decades. Lime and cement which are used to bind problematic soil particles are found to be ineffective in soils containing significant amount of sulfate in them due to alteration of cementitious compounds. Further, stabilization of soils with lime is not seemed to be an environmental friendly. Hence, efforts have been made to utilize the waste by–products such as fly ash, ground granulated blast furnace slag etc. in large scale to stabilize soils; proper utilization of waste materials reduces the higher cost of calcium based additives, and is fruitful to embrace sustainable development and safe ecosystem for environmental friendly construction operations. These stabilization methods can be effectively used for various geotechnical applications such as base course for flexible pavements, construction of embankments, as cushion between expansive soils and foundation, improve properties of marine soils, foundations on expansive soils and waste dumps, as liner applications for waste disposal facilities, stabilization of slopes, etc. Hence, the aim of present paper is to review pozzolanic agents, such as fly ash, GGBS, alone or in combination with various chemical additives such as lime, cement and gypsum, in order to assess their effectiveness in improving the properties of soils. It has been observed that utilization of waste binders along with minimal use of chemical lime and gypsum lead to improve the strength behaviour of soils with curing periods. Use of the waste materials not conserve the natural resources but also have several technical advantages. It can suppress the negative effect of sulphate, can enhance the properties beyond optimum lime content.
Several ground improvement techniques such as mechanical stabilization, chemical stabilization, columnar injection, preloading, asphalt stabilization and use of reinforcing materials are practiced frequently to modify and stabilize the soils for suitable construction purpose. Vibro-compaction which increases the density of the soil by using powerful depth vibrators, vacuum consolidation which can improve soft soils by using a vacuum pump, preloading which can be used to remove pore water over time, heating to form a crystalline or glass product by electric current, ground freezing which converts pore water to ice to increase their combined strength and make them impervious, vibro replacement stone columns improve the bearing capacity of soil, vibro displacement method which displaces the soil, electro osmosis which makes water flow through fine grained soils, electro kinetic stabilization by the application of electro osmosis, reinforced soil steel for retaining structures, sloping walls, dams etc., seismic loading for construction in seismically active regions have been used to improve the properties of soils. Mechanically stabilized earth structures create a reinforced soil mass. Other methods such as geo-methods (use of geosynthetics, geogrid, geotextiles etc.), soil nailing, grouting, micro-pile are also been practiced to increase the shear strength, rigidity of the in-situ soil and to restrain its displacement. However, soil stabilization, particularly by using chemical and waste stabilizers, is different from all these ground improvement technique in which the soil properties are improved by blending of soil particles by mixing other materials. Soil stabilization depends mainly on chemical reactions between stabilizer and soil minerals (pozzolanic stabilization) to achieve the desired effect. The purpose of stabilization is to improve the strength and volume stability but may be to enhance or reduce permeability of soils. This facilitates construction of pavements, embankments, reinforced earth structures, railways, bulk fill applications, housing and industrial units on soils where they were not technically feasible or not economically viable. Further, stabilization can often reduce significantly the time taken to complete a project and reduce tipping or need to import. The process also enables wet ground to be dried and strengthened for immediate use. Soil stabilization has transferred into the most cost effective and viable method for preparation of sites for all most all construction projects. Several types of techniques and stabilizers such as traditional ones (hydrated lime, Portland cement, and fly ash), byproducts (cement kiln dust, lime kiln dust, and other forms of by product lime), and non–traditional (sulfonated oils, potassium compounds, ammonium chloride, enzymes, polymers) and their combinations, are widely used to improve the properties of expansive soils to meet specific design criterion (Chen, 2012; Cokca, 2001). Soil stabilization using lime is the most widely used method for drying a wet site and improve their engineering behaviour. Historically lime stabilization is in vogue from early Raman period for construction of roads. Cement and lime are consistently used to stabilize soils rapidly for road and airfield applications by military, but recent developments showed several nontraditional stabilizers have been used successfully. There are significant developments in stabilization of soils not for road construction and is not restricted to conventional stabilizers.
With advancements in the applications of stabilizers by deep injection technique, there are several innovative applications of stabilization. But there are certain types of problematic soils which do not respond well to conventional stabilizers. All there is an urgent need to use solid industrial wastes (fly ash, GGBS, rice husk ash) to reduce the cost of their disposal apart from reducing the land area requirement and environmental issues. This also can reduce the cost of stabilization. The use of these wastes requires knowledge about the soils as well as the properties of waste materials. Efforts made to stabilize different problematic soils such as expansive soils with different types of binders and binder combinations and optimize their proportions are presented in this paper.
Fundamental Behind Clay Behavior
The behavior of fine grained soils is much more complex and variable compared to coarse grained soils. The fine grained soil systems undergoes more volume changes, i.e. swell or shrink on wetting and drying, and is influenced by ionic concentration or composition. Very often fine grained soils exhibit unexpected properties/behavior. Four such cases – (1) The aging of quick clay after sampling, in which the remolded strength increased in samples maintained at constant water content; (2) time effects in freshly densified or deposited sand, in which natural sand deposits can lose strength if disturbed but regain strength over time periods of weeks to months; (3) apparently sound lime-stabilized soil that swells and disintegrates starting a few years after construction; and (4) the failure of excess pore pressures to dissipate as predicted during the consolidation of soft clays – are described by Mitchell (1986) and reinforced that soils are not inert materials and they can change with time and sensitive to environmental changes. To understand and also overcome the problems created by unexpected changes in soil behavior, it is necessary to consider the factors that influence their behavior, not only to deal with them and even forecast well in advance. An attempt is made to present briefly the factors that control the behavior of fine grained soils more particularly those of the clays.
Three mechanisms for clay mineral formation (inheritance, neo-formation and transformation) operating in three geological environments (weathering, sedimentary, and diagenetic-hydrothermal) yield nine possibilities for the origin of clay minerals in nature. The mineralogy of clays neoformed in the weathering environment is a function of solution chemistry, with the most dilute solutions favoring formation of the least soluble clays. After erosion and transportation, these clays may be deposited on the ocean floor in a lateral sequence that depends on floccule size. Clays undergo little reaction in the ocean, except for ion exchange and the neoformation of smectite; therefore, most clay found on the ocean floor is inherited from adjacent continents. Upon burial and heating, however, dioctahedral smectite reacts in the diagenetic environment to yield mixed-layer illite-smectite, and finally illite. With uplift and weathering, the cycle begins again.
Clay Mineral Structure
Clay minerals are layer silicates that are formed usually as products of chemical weathering of other silicate minerals at the earth’s surface (Wilson, 1999). They are found most often in shales, the most common type of sedimentary rock. In cool, dry, or temperate climates, clay minerals are fairly stable and are an important component of soil. There are many types of known clay minerals (Fig. 1) (Madejová, 2003; Mitchell, 1986).
Some of the more common types are:
Kaolinite: This clay mineral is the weathering product of feldspars. It has a white, powdery appearance. Because kaolinite is electrically balanced, its ability of adsorb ions is less than that of other clay minerals.
Illite: Resembles muscovite in mineral composition, only finer-grained. It is the weathering product of feldspars and felsic silicates.
Chlorite: This clay mineral is the weathering product of mafic silicates and is stable in cool, dry, or temperate climates. It occurs along with illite in mid-western soils. It is also found in some metamorphic rocks, such as chlorite schist.
Vermiculite: This clay mineral has the ability to adsorb water, but not repeatedly.
Smectite: This clay mineral is the weathering product of mafic silicates, and is stable in arid, semi-arid, or temperate climates. It was formerly known as montmorillonite. Smectite has the ability to adsorb large amounts of water, forming a water-tight barrier, and the chemical industry. There are two main varieties of smectite, a) sodium smectite (the high-swelling form of smectite, which can adsorb up to 18 layers of water molecules between layers of clay) and b) calcium smectite (the low-swelling form of smectite adsorbs less water than does sodium smectite).
Attapulgite: This mineral actually resembles the amphiboles more than it does clay minerals, but has a special property that smectite lacks – as a drilling fluid, it stable in salt water environments. When drilling for offshore oil, conventional drilling mud falls apart in the presence of salt water.
In humid tropical climates, clay minerals are unstable and break down under intense chemical weathering to become hydrated oxides of aluminum (bauxite) and iron (goethite).
Schematic structure of most common clay minerals are given in Fig. 1.
Interstratification in Phyllosilicates
The properties and compositions of some mineral colloids are intermediate between those of well-defined minerals such as kaolinite and smectite as described in above. Such minerals are referred to as mixed-layer or interstratified minerals. Most mixed-layer clays contain smectite or vermiculite as a swelling component (Srodon, 1999). Partial removal of interlayer potassium from micas or of interlayer hydroxide from chlorite is one way that interstratification minerals can form in soils (April, 1980). Other possibilities include i) fixation of adsorbed K+ by some vermiculite layers to yield mica-like layers, and ii) the formation of hydroxide interlayers to produce chlorite- like layers. Interstratification of layers can be regular or random  as given below:
1) A periodic alternation of layers of two types A and B refers to regular or ordered mixed-layers: ABABAB…. or AABAABAAB…
2) A random alteration of each type of layer corresponds to irregular and randomly mixed-layered clays: AABABBBAAABAABA.
Illite-smectite is the most interesting mixed-layer clays (Fig. 1). They are quite ubiquitous, well known from a chemical standpoint and exhibit a mineralogical variation that responds to pressure–temperature variations during diagenesis.
Clay minerals act as “chemical sponges” which hold water and dissolved plant nutrients weathered from other minerals (Al-Ani and Sarapää, 2008). This results from the presence of unbalanced electrical charges on the surface of clay grains, such that some surfaces are positively charged (and thus attract negatively charged ions), while other surfaces are negatively charged (attract positively charged ions). Clay minerals also have the ability to attract water molecules. Because this attraction is a surface phenomenon, it is called adsorption (which is different from absorption because the ions and water are not attracted deep inside the clay grains) (Al-Ani and Sarapää, 2008). Clay minerals resemble the micas in chemical composition, except they are very fine grained, usually microscopic. Like the micas, clay minerals are shaped like flakes with irregular edges and one smooth side.
The composition of individual particles in an element of soil is an important characteristic of soil. This belief is true as far as a fundamental understanding of soil behaviour is concerned. The nature and arrangement of the atom in soil particle– i.e., composition– has a significant influence on the permeability, compressibility, and strength and stress transmission in soils, especially in fine-grained soils. Thus, it is needed to study soil composition if it is needed to understand the fundamentals of clay behaviour and particularly the dependence of this behaviour on time, pressure, and environment.
Stabilization of Soils
The stabilization or, treatment of problematic soil generally refers the conversion of soil particles to rigid granular materials that can resist the internal swelling pressure of the clay, and restriction of the movement of moisture within the soil to control the seasonal variations (Al–Rawas & Goosen, 2006). The long-term performance of any construction project depends on the soundness of the underlying soils. Unstable soils can create significant problems for pavements or structures. Hence, there is a great need to stabilize these unstable soils to improve its engineering properties due to limited financial resources and to utilize the locally available soil to the full extent. Soil stabilization means the improvement of stability or bearing power of the soil by the use of controlled compaction, proportioning and/or the addition of suitable admixture or stabilizers. Broadly, it refers to any chemical or mechanical treatment given to a mass of soil to improve or maintain its engineering properties. Soil stabilization is widely used in connection with road, pavement and foundation construction to improve the Strength, Volume stability and Durability of the soil. The basic principles of soil stabilization consists of evaluating the properties of given soil, deciding the lacking property of soil and choose effective and economical method of soil stabilization and designing the stabilized soil mix for intended stability and durability values. Regardless of the purpose for stabilization, the desired result is the creation of a soil material or soil system that will remain in place under the design use conditions for the design life of the project.
Soil stabilization finds its way to a number of applications like increase of bearing capacity for foundation requirements, improvement of sub-grade properties for road constructions, increasing stability of slopes, embankment constructions etc. Weak compressible soils deposits are often found near the mouths of rivers, along the perimeters of bays, and beneath swamps or lagoons. Soil deposits with high organic content are often found in these low lying types of locations and can be especially troublesome for construction activities. Their compressibility needs to be decreased and strength increased to perform construction activities on such soils.
A successful construction of highways requires the construction of a structure that is capable of carrying the imposed traffic loads. One of the most important layers of the road is the actual foundation, or subgrade. Where the subgrade is founded in an inherently weak soil, this material is typically then removed and replaced with a stronger granular material. This “remove and replace” technique can be both costly and time consuming. An alternative to the “remove and replace” option is to chemically stabilize the host material. This eliminates the requirement to replace the material, and ensures the engineering characteristics and performance of the host material is enhanced to allow for its use within the pavement structure. However, method of stabilization particularly with chemical stabilizers depends mainly on the mineralogical and chemical composition of soils. Proper precautions should be required prior to use of different chemical stabilizers alone or in the combination of various waste binders such as fly ash and GGBS. The stabilization of some of the challenging soils and their mechanism are explained briefly in the following sections.
A pozzolan is a siliceous material, which will react with lime to form cementitious compounds. For production of cementitious compounds, silica in amorphous or non crystalline form is more effective. Lime, cement and fly ash have been extensively used to improve the properties of expansive soils. While lime reacts with soil silica to produce cementitious compounds, these compounds are readily available in Portland cement. However, cement is mainly used as pre-treatment before lime stabilization of expansive soils. Fly ashes often contain both lime and reactive silica and produce cementitious compounds in the presence water. There are three different types of fly ashes from the consideration of pozzolanic nature – self pozzolanic, pozzolanic and non pozzolanic. The reactive silica content of fly ash plays an important role in the pozzolanic properties of fly ash. Fly ashes with good reactive silica content and sufficient lime content are called self pozzolanic (Sivapullaiah et al. 1998; Sivapullaiah et al. 2000). Fly ashes containing reactive silica but in adequate lime content are pozzolanic and develop good strength with addition of lime. Methods are available for optimizing lime content (Sivapullaiah et al., 1996). Fly ashes without reactive silica cannot develop strength even with addition lime and are called non pozzolanic.
Strength of Fly Ash and Fly Ash-Lime and Fly ash-Lime-Gypsum Stabilization
Silica, alumina and secondary oxides of calcium, magnesium, iron and sulfur are the most common minerals found in fly ash. Further, fly ash generally contains silty sized particles and with addition of fly ash to soil causes replacement of clayey sized particles of soils. This results an improvement in gradation of soil matrix and thereby, improving strength of soil with increase in fly ash content (Class “F) in combination with lime and gypsum (Fig. 2). Calcium oxide is the important chemical component of fly ash for soil modification. Calcium oxides in presence of water dissociates into calcium cations and hydroxide anions. On dissociation calcium cations pozzolanically react with chemical compounds found in soil to form cementitious materials. In pozzolanic reactions siliceous material reacts in presence of moisture and calcium to form compounds having cementitious properties. The more the calcium cations is available the greater the degree of soil modification. Silica, alumina, magnesium and iron oxide are the non calcium oxides found in fly ash are inert material. But these oxides can react in presence of sufficient amount of calcium cations to form cementitious materials. The formation of cementitious compounds leads to the improvement in strength in expansive soils (Fig. 2). However, addition of small gypsum to lime treated soil with fly ash causes acceleration in pozzolanic reactions and thereby drastically increase in short term strength (Sivapullaiah and Jha, 2014). The addition of gypsum to soil causes formation of ettringite crystals along with cementitious compounds, which reinforce the soil matrix together and thereby, enhancing the strength of soil at shorter curing periods (Fig. 3).
The method of mixing of fly ash with soil is shown in Fig. 4.
Strength of Expansive soil with waste Granulated Blast Furnace Slag (GGBS)
GGBS is an industrial waste obtained by quenching molten iron blast-furnace slag in water or steam. GGBS, with an annual production of about 15 Mt, is mainly used in concrete manufacturing plants for partial replacement of cement in concrete. However, this utilization rate for GGBS only amounts to 55% (Singh et al., 2008).
The unconfined compressive strength (UCS) of expansive soil (Fig. 5) increases with the addition of small amount of GGBS up to about 10% GGBS and remains constant up about 40% and thereafter decreases with further increase in GGBS content. The variations in strength can be explained by the factors: 1. Decrease in the cohesion of the soil due to addition of frictional materials; 2. Improved strength due cementation of pozzolanic compounds produced; 3. The variations in compaction parameters as the soil GGBS mixtures are compacted to their respective Proctor’s optimum conditions; 4. Coating of GGBS by soil particles (Sharma and Sivapullaiah, 2012).
Fly ash has been used successfully in several projects to improve the compressive and shear strength soils for stabilising bases or subgrades, to stabilize backfill to reduce lateral earth pressures and to stabilize embankments to improve slope stability. Typically stabilized soil depths vary between 15 and 46 cms. The compressive strength of fly ash treated soils is dependent on:
– In-place soil properties
– Delay time
– Moisture content at time of compaction
– Fly ash addition ratio
Another important reason for using fly ash over cement in stabilisation is its advantages of delay time, which is time time lapsed between wet mixing and compaction (sivapullaiah et al 1998).
Ground granulated blast furnace slag amended fly ash as an expansive soil stabilizer
Both fly ash and GGBS possesses reactive silica and lime. Though both are pozzolanic materials, for the reaction products to gain maximum strength an optimum ratio of reactive silica to lime is required. Thus the attempt is made to mix two pozzolanic materials which individually have different amounts of reactive silica and lime. The possibility is that one component present in excess in one material will be utilized by the other which is in need of the same and vice-versa. However, there is a wide variation in the chemical properties of fly ash and GGBS. Fly ash is low in calcium oxide content but rich in silica and alumina while GGBS is relatively high in calcium oxide. The combination of these two materials can be more beneficial when used as a stabilizing agent than using them individually. Each can provide sufficient lime or silica to support pozzolanic reaction, thereby requiring lower amounts of chemical activators. Studies relating to the alkali activation of slag/fly ash mixtures in blended cements and concretes have been carried out by few researchers (Bijen and Waltje, 1989; Puertas et al., 2000; Shi and Day, 1999).
Fig. 6 shows the variation of the unconfined compressive strength (UCS) of treated expansive soil (without lime) for different curing periods in relation to the percentage of the binder (mixture of fly ash and GGBS) (Sharma and Sivapullaiah, 2016). It can be seen that strength increases up to 20% of the binder content and then decreases thereafter. This trend is similar for all the curing periods. For the same percentage of binder, the strength of the samples was found to be directly proportional to the curing period. The formation of cementitious compounds [such as calcium–silicate–hydrates (C–S–H), calcium–aluminate–hydrates (C–A–H), and calcium-aluminium–silicate–hydrates (C–A–S–H)] in the soil–binder matrix is responsible for the increase in the unconfined compressive strength of the stabilized soil (Fig. 7 and 8). However, the addition of binder beyond 20% gave a reduction in strength. For stabilized soils with greater binder contents, the pozzolanic reaction does not occur and the added binder particles act as unbonded particles that reduce the overall strength of the material (Bell, 1996; Kate, 2005). However, the strength of the stabilized soil with high binder contents is still greater than that of untreated soil.
Fly ash and GGBS (constituents of the binder) have very little self-hardening properties in the absence of any chemical activators such as lime or cement (Kaniraj and Havanagi, 1999). This is best illustrated in Fig. 8 where 1% lime is added along with the binder. The samples attain significant strength and the gain depends on the amount of the binder and curing period (Fig. 9). The addition of lime enhances the pozzolanic reaction that provides extra strength to the soil–binder mixture due to the formation of a cemented matrix (Fig. 7 and 8). The strength of the samples is again found to decrease slightly beyond 20% of binder content. However, unlike in the earlier case where the strength continuously decreases, the sample regains strength after the addition of 30% of binder. This implies that the addition of lime helps in binding the unbonded silt particles that were responsible for strength reduction. Therefore, based on the unconfined compressive strength behaviour with and without lime, the addition of 20% binder is recommended as an optimum content to effectively stabilize this expansive soil.
Several industrial solid waste materials can be advantageously be used to improve the properties of problematic soils.
They also can be used to improve the properties of soil beyond what can be achieved with conventional binders.
Fly ash has been widely used to stabilize soils for construction of pavements and embankments.
Fly ash has also advantages in stabilisation of soils containing sulphate etc.
Fly ash has favorable delay in curing period.
GGBS can also be used to stabilize soils. Use of combinations of Fly ash and GGBS reduces lime requirement for fly ash in stabilizing soils.
– Al-Ani T., Sarapää O. (2008). “Clay and clay mineralogy.” Physical–Chemical Properties and Industrial Uses Espoo: Geological Survey of Finland.
– Al-Rawas, A. A.,Goosen, M. F. (Eds.) (2006). “Expansive soils: recent advances in characterization and treatment.” Taylor & Francis.
– April R. H. (1980). “Regularly interstratified chlorite/vermiculite in contact metamorphosed red beds, Newark Group, Connecticut Valley.” Clays and Clay Minerals 28 (1):1-11.
– Bell, F. (1996). “Lime stabilization of clay minerals and soils.” Eng. Geol. 42 (4), 223–237.
– Bijen, J., Waltje, H. (1989). “Alkali activated slag–fly ash cements.” In: Proceedings of 3rd International Conference on the Use of Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Trondheim, Norway, SP114- 76, pp. 1566–1578.
– Chen F. H. (1975). Foundations on expansive soils, Elsevier.
– Cokca E. (2001). “Use of class c fly ashes for the stabilization of an expansive soil.” J. Geotech Geoenviron Eng. 127(7):568–573.
– Kaniraj, S.R., Havanagi, V.G. (1999). “Compressive strength of cement stabilized fly ash–soil mixtures.” Cem. Concr. Res. 29 (5), 673–677.
– Kate, J. (2005). “Strength and volume change behavior of expansive soils treated with fly ash.” In: Proceedings of Geo-Frontiers-2005: Innovations in Grouting and Soil Improvements, Austin, Texas, pp. 24–26.
– Madejová J. (2003). “FTIR techniques in clay mineral studies.” Vibrational spectroscopy 31 (1):1-10.
– Mitchell, J. K. (1986). “Practical problems from surprising soil behavior.” Journal of Geotechnical Engineering, 112(3), 259–289.
– Puertas, F., Martinez-Ramirez, S., Alonso, S., Vazquez, T. (2000). “Alkali activated fly ash/slag cements: strength behaviour and hydration products.” Cem. Concr. Res. 30 (10), 625–1632.
– Sharma, A. K., & Sivapullaiah, P. V. (2012). “Improvement of strength of expansive soil with waste granulated blast furnace slag.” In Geo Congress.
– Sharma, A. K., & Sivapullaiah, P. V. (2016). “Ground granulated blast furnace slag amended fly ash as an expansive soil stabilizer.” Soils and Foundations, 56(2), 205-212.
– Shi, C., Day, R. (1999). “Early strength development and hydration of alkali activated blast furnace slag/fly ash blends.” Adv. Cem. Res. 11 (4), 189–196.
– Singh, S., Tripathy, D.P., Ranjith, P. (2008). “Performance evaluation of cement stabilized fly ash–GBFS mixes as a highway construction material.” Waste Manag. 28 (8), 1331–1337.
– Sivapullaiah, P. V., Jha, A. K. (2014). “Gypsum induced strength behaviour of fly ash-lime stabilized expansive soil.” Geotechnical and Geological Engineering, 32(5), 1261-1273.
– Sivapullaiah, P. V., Prashanth, J. P., Sridharan, A., Narayana, B. V. (1998). “Reactive silica and strength of fly ashes.” Geotechnical & Geological Engineering, 16(3), 239-250.
– Sivapullaiah, P.V., Prashanth, J.P. Sridharan, A (1998), Delay in compaction and importance of lime fixation point on strength and compaction characteristics of soil, Ground Improvement, 2, 1998, 27-32.
– Sivapullaiah, P. V., Sridharan, A., Raju, K. V. B. (2000). “Role of amount and type of clay in the lime stabilization of soils.” Proceedings of the ICE-Ground Improvement 4(1): 37-45.
– Sivapullaiah, P.V., Prashant, J.P., Sridharan, A. (1996). “Effect of fly ash on the index properties of BC soil.” Soils Found., 36(1): 97–103.
– Srodon J. (1999). “Nature of mixed-layer clays and mechanisms of their formation and alteration.” Annual Review of Earth and Planetary Sciences 27 (1):19-53
– Vali H., Koster H. (1986). “Expanding behaviour, structural disorder, regular and random irregular interstratification of 2: 1 layer-silicates studied by high-resolution images of transmission electron microscopy.” Clay Miner 21 (5):827.
– Wilson M. (1999). “The origin and formation of clay minerals in soils: past, present and future perspectives.” Clay Minerals 34 (1):7-7.