*2.1. Ash Utilization for Soil Stabilization*

The factors on which soil stabilization with ash depend on are: The type of ash and its characteristics, characteristics of the soil to be stabilized, the percentage of fly ash, the time period between wetting of the mixture and compaction and soil water content at the time of compaction.

According to ASTM C618 [13], fly ash types are class C and class F. This classification mainly depends on the content of SiO2, Al2O3 and Fe2O3—minimum percentage of SiO2 + Al2O3 + Fe2O3 for class F fly ash is 70%, and for class C is 50%. The percentage of sulfur trioxide (SO3) is max 5% for both ash classes (fly ash with a sulfate content greater than 10% may cause soils to expand more than desired [14]). According to EN 14227-4 [15], fly ash is classified into calcareous (type W, equivalent to ASTM class C), and siliceous (type V, equivalent to ASTM class F).

Class C fly ash is mainly produced from lignite or subbituminous coal. This coal has a higher content of calcium carbonate, so class C fly ash is rich in calcium (more than 20% CaO), resulting in the self-cementing characteristics. Studies concerning fly ash utilization for soil stabilization indicate that class C fly ash is an effective and economical stabilizer for broad engineering applications [14,16–23].

Class F fly ash is mainly produced from burning anthracite or bituminous coal. This class of fly ash has pozzolanic properties, but has no self-cementing characteristics due to its lower CaO content (less than 10%). According to [24], class F fly ash should be used in soil stabilization with the addition of cementitious agent (lime, lime kiln dust, cement and cement kiln dust). However, there are researches indicating that this fly ash can effectively improve some engineering properties of soil (unconfined compressive strength (UCS), California bearing ratio (CBR) and swell potential) without activators [25–29].

According to [14,17,30], the optimal fly ash content for soil stabilization is in the range from 10% to 30%, depending on soil and ash type. Recent studies have shown that compaction properties and the strength of the mixture decreases with the increase in compaction delay time, which is a consequence of the loss of established cement bonds between particles and lower density [14,16]. According to the same research, it is proposed to carry out compaction within two hours after mixing. Water content of the soil during compaction has a major impact on density and strength of the mixture. According to [14,16], the water content for achieving the maximum strength is typically the optimal water content or up to 8% lower than the optimal.

The effects of applying fly ash for soil stabilization are the reduction in the plasticity and soil swell potential and increasing the soil strength and CBR values. The size of fly ash particles is commonly larger than the clay particles, thus the addition of fly ash changes the grain size distribution of the clay and reduces the liquid limit. The chemical composition of ash and treated soil also affects the Atterberg limits. Reduction of plasticity of the clay soil leads to a decrease in swell potential. Çoçka [17] as well as Nalbantoglu and Gucbilmez [30] have found that plasticity and swelling potential decrease with the increase in the content of class C fly ash. Ramadas et al. [26] have analyzed the characteristics of three expansive soils with the addition of class F fly ash of 0–50%, which resulted in significant decrease in liquid limit, swelling pressure and potential.

The increase in strength is the main reason for the use of fly ash for soil stabilization [24]. The California bearing ratio value is the primary parameter in the evaluation of suitability of fly ash stabilized soil utilization in road construction [14,16,20,22,31]. Clays generally have low CBR, and that makes them inappropriate for the use in base layers of pavements. Zia and Fox [32] have found that CBR values of loess increased five times by the addition of 10% class C fly ash. By adding 20% of self-cementing fly ash to fine-grained soil, White et al. [22] obtained CBR values that correspond to well-compacted gravel (~75%). Acosta et al. [18] investigated different soil types with very low CBR values (0–5%) and by the addition of 18% class C fly ash, achieved a significant increase in CBR values (20–56%). Vuki´cevi´c et al. [29] analyzed the effect of class F fly ash on the strength of expansive clay. The highest increase in strength was obtained with the addition of 15% fly ash. The CBR value increased almost three times.

Increase of fly ash stabilized soil strength is a time-dependent process. The study of White et al. [22] on self-cementing fly ash showed a rapid increase in strength during the first 7 to 28 days, after which the slow down trend was registered due to prolonged pozzolanic reactions.

#### *2.2. Ash Utilization as a Material for Embankment*

Ash has been used for many years in construction as fill material in road construction, embankment construction and land reclamation [33]. Low compacted unit weight of fly ash makes it very suitable material in embankment construction.

Class F fly ash is more often applied as the material for embankments and backfills, in comparison with the class C fly ash [34], because of self-cementing characteristics of C class fly ash, which hardens within 2–4 h after the addition of water [35].

The important engineering properties of ash for its utilization in roads construction are: The moisture–density relationship, shear strength and compressibility.

Fly ash has a lower compacted density compared to traditional materials, which leads to smaller applied load and settlement of the subsoil. DiGioia et al. [36] have investigated the maximum dry density and the optimum water content for Western Pennsylvania class F fly ash and Western USA class C fly ash. Values of the maximum dry density varied from 11.9 to 18.7 kN/m3 and values of the optimum water content from 13% to 32%. They concluded that the large variations were due to different physical and chemical characteristics of the ashes, which in the turn depend on the source of coal and the condition of coal combustion.

Shear strength tests on compacted ash specimens indicate that ash strength is mostly generated by internal friction [37]. Class F fly ash has a friction angle usually in the range of 26◦ to 42◦ [38]. Kim et al. [37] conducted tests on a mixture of fly ash and bottom ash and obtained friction angles in the range of 28–48◦, which is in the rank of the shear strength of dense sandy soil.

There is not much published data for the California bearing ratio of ash. According to [39], CBR for class F fly ash ranges between 6.8% and 13.5% in the soaked conditions, and between 10.8% and 15.4% in the unsoaked conditions. For natural soils, CBR values normally range between 3% and 15% (fine-grained soils), 10–40% (sand and sandy soils) and 20–80% for gravels and gravelly soils [40].

Generally, technical standards prescribe that embankment must have small compressibility to reduce roadway settlements. The compressibility can be expressed through compression index Cc and recompression index Cr or through compressibility (constrained) modulus Mv (Cm). Kaniraj and Gayathri [41] carried out consolidation tests on the specimens of class F fly ash from the Dadri thermal power plant (New Delhi, India). They found that the compression indices Cc of the specimens were 0.041 or 0.084, depending on level of effective stress. The average recompression index Cr was 0.008. For the fly ash from the Rajghat thermal power plant (New Delhi), reported Cc and Cr were 0.072 and 0.017, respectively [42]. For the fly ashes from USA and Canada, McLaren and DiGioia [43] found the mean value of 0.13 for Cc. Kim and Prezzi [44] determined the tangent constrained modulus Cm at vertical stresses, from zero to 200 kPa, which is the range of stresses expected in highway embankments. The fly ash used in the study was class F, from three power plants in Indiana (USA). The obtained values were compared with the tangent constrained modulus available in the literature for compacted sand at different densities (at relative compaction of 99% and 85%). Specifically, the values of constrained moduli for the tested fly ashes (in the range of 10 MPa to 30 MPa at stress level 100–200 kPa) correspond to the lower end of the sand moduli range. This indicates that for the same compaction levels, the fly ashes are slightly more compressible than sand.

#### **3. Materials**

#### *3.1. Soil*

Soil was sampled from the location Radljevo, municipality Ub, Serbia. Based on the modified Unified Soil Classification System, the tested soil was high plasticity (CH) clay. Nevertheless, due to demonstrated shortcomings of the Casagrande chart [45], as an alternative, the authors used a new classification approach proposed in [46]. Moreno-Maroto and Alonso-Azcárate [46] classified clay by PI/LL (Plasticity Index vs. Liquid Limit) ratio. The PI/LL ratio for the tested clay was 0.38, which characterized the used material as moderately or slightly clayey soil. The maximum toughness, Tmax, parameter that best represents plasticity [47], estimated by the Moreno-Maroto and Alonso-Azcárate [46] equation was 5.54, which indicates a low influence of clay minerals. Basic physical properties of CH clay are given in Table 1. The tested soil had a low to moderate swelling potential, with a swell deformation of 2.2% [48], which makes it generally unusable for most engineering purposes.


**Table 1.** Physical properties of the high plasticity (CH) clay.

Note: Testing methods are described in Section 4.

#### *3.2. Ash and Slag*

In the scope of this paper, the following waste materials from Serbian power plants were used:

(1) KOL FA—fly ash from electrostatic precipitators in thermal power plant "Kolubara";

(2) KOS FA—fly ash from electrostatic precipitators in thermal power plants "Kostolac A" and "Kostolac B";

(3) KOS AB—ash and slag mixture from the landfills of thermal power plants "Kostolac A" and "Kostolac B";

(4) TENT A—ash and slag mixture from the landfill of thermal power plant "Nikola Tesla A";

(5) TENT B—fly ash from the silos in thermal power plant "Nikola Tesla B".

Basic physical properties of tested waste materials are given in Table 2.


**Table 2.** Physical properties of tested waste materials.

Note: Testing methods are described in Section 4.

According to the standard ASTM C618 [13], the used materials belonged to class F. Chemical composition of all waste materials within this paper is given in Table 3.


**Table 3.** Chemical composition of the used waste materials.

Note: Presented values may not entirely represent the tested material, since the chemical composition of the coal used in the power plants can change over time.
