1. Introduction
Fly ash is an industrial by-product generated during the combustion of coal in thermal power plants [
1,
2]. It is generated in large amounts in many countries [
3]. Over 65% of the produced fly ash is disposed of in landfills [
4]. If the fly ash, as a waste material, is not managed well, it can lead to serious environmental and health problems [
4,
5]. However, many characteristics of fly ash such as low compressibility, high shear resistance, high strength and pozzolanic characteristics offer it an important role in improving the properties of soil in geotechnical applications [
2,
6]. The stabilization of soft soils with the addition of fly ash not only addresses the environmental issues of disposal of fly ash but can also provide technological solutions for soil improvement [
2].
Therefore, the stabilization of different types of soil with fly ash has encouraged various researchers to carry out experimental or/and field studies. The available literature on clay soils stabilized with fly ash is summarized in
Table 1. These investigations have generally pointed out that the inclusion of fly ash can improve the soil structure and characteristics in many aspects, including strength, stiffness, permeability, and compressibility.
Seyrek [
17] conducted Atterberg limits, compaction, swelling potential, swelling pressure tests, and unconfined compressive strength (UCS) tests to analyze the effects of class C and class F fly ash on both high plasticity clay (CH) and low plasticity clay (CL) at 1, 7, and 28 days of curing periods. They showed that the plasticity index of both types of fly ash stabilized soil decreased with the addition of 20% fly ash. However, beyond 20% fly ash content, an increase in plasticity index was observed. The results from the compaction tests showed that the maximum dry unit weight of the soil decreased, and optimum moisture content increased with the addition of fly ash. The amount of swelling and swelling pressure of the soil decreased significantly by increasing the fly ash content. However, the changes became insignificant beyond 25% class C fly ash. For the CH soil, adding 30% class F fly ash gave similar results in terms of reduction in swelling compared to 10% class C fly ash. The peak UCS values for the samples with 28 days of curing were found to be 657 kPa and 915.5 kPa with 25% class F fly ash and 30% class C fly ash, respectively. It was concluded that class C fly ash provided remarkable improvement in compressive strength with increasing the curing. Likewise, Phani Kumar and Sharma [
10] assessed plasticity, strength, swelling, and compaction characteristics of clayey soil with the addition of different percentages of low calcium fly ash. Based on their results, the plasticity index and swelling characteristics of the stabilized soil decreased by approximately 50% with 20% fly ash inclusion. On the other hand, the swelling potential was insignificant after the addition of 20% fly ash. Undrained shear strength increased by about 27% with 20% of fly ash inclusion. According to compaction test results, the optimum moisture content decreased by about 25%, and the maximum dry unit weight increased by about 5% with 20% of fly ash inclusion. In a follow-up study, Phanikumar and Sharma [
13] investigated the effects of class C fly ash using oedometer tests and the cylindrical jar method to determine free swell index, swell potential, swelling pressure, and compression index for an expansive and a nonexpansive high plasticity clay. The plasticity indices of the expansive and the nonexpansive clay were found to be 131–53 and 29, respectively. Fly ash contents up to 20% (based on dry weight of the soil) were added to the soil samples. It was found that the free swell index decreased by approximately 50% for the expansive clay with the addition of 20% fly ash based on the cylindrical jar method. The compression index significantly decreased with the addition of fly ash on both expansive and nonexpansive soils. On the other hand, the effect of fly ash in improving the compressibility properties of the expansive clay was greater compared to the nonexpansive clay. Cokca [
7] assessed the effects of four stabilization materials (high calcium and low calcium fly ash, lime, and cement) on the swelling potential of a clay soil. The amounts of lime and cement used were between 0–8%, while the amounts of fly ash used were between 0–25%. Based on the results, the classification of the stabilized soil changed from CH to CL, MH-ML, ML, and CL with the addition of 8% lime, 8% cement, 25% high calcium fly ash, and 25% low calcium fly ash, respectively. The swelling potential of all the samples stabilized with fly ash, cement, or lime decreased significantly. The highest reduction in swelling potential was 68% for low calcium fly ash. Jose et al. [
18] also investigated the effects of class F fly ash on an expansive soil. The artificial soil (natural soil with bentonite) was used which had a liquid limit of 62%. They reported that the value of free swell index decreased from about 71% to 39% by adding 15% of fly ash. Moreover, the liquid limit of the soil decreased by about 36%, and the compressive strength increased by about 43% with the addition of 15% fly ash.
Prabakar et al. [
8] carried out a number of tests to evaluate compaction, shear strength, California bearing ratio (CBR), and swelling characteristics of soils stabilized with the addition of fly ash ranging from 9 to 46%. They considered three different types of soils: CL (inorganic clay with low plasticity), OL (organic soil with low plasticity), and MH (inorganic silt with high plasticity). The results showed that the dry density of all soil types decreased between 15% and 20% by adding the fly ash. The shear strength of all the soil types increased nonlinearly with the fly ash content. The swelling potential of the soil also decreased, and the CBR value of the soil improved with the addition of fly ash. Sezer et al. [
9] studied the effects of high lime fly ash with different percentages and curing times by applying compaction, UCS, and shear strength tests on a high plasticity clay soil. The fly ash contents used were 0, 5, 10, and 15% of dry weight of the soil, and the curing times were 1, 7, 28, and 90 days. They reported that the maximum dry density decreased, while the optimum moisture content increased with the addition of fly ash. The unconfined compressive strength, cohesion, and friction angle improved with the addition of fly ash. According to the UCS results, the strength parameters of the soil improved inconsiderably after 28 days. The optimal amount of fly ash to stabilize the soil was found to be 15%. Senol et al. [
11] conducted compaction, UCS, and CBR tests on class C fly ash stabilized clay soil to investigate the suitability in pavement design. Tests were conducted after seven days of curing in normal room temperature on mixtures of the soil with between 10–20% class C fly ash. It was concluded that the inclusion of fly ash provides a significant improvement in the UCS and CBR values. Therefore, it was highly recommended that fly ash stabilized soil can be used as soft subgrade material in the field. Brooks [
15] investigated the effects of class C fly ash and rice husk ash on clay soil by conducting UCS, CBR, compaction, and swell-shrinkage tests. According to the UCS test results, failure stress increased approximately 106% with 25% fly ash content. However, beyond 25%, the failure stress decreased with further increase in the fly ash content. The swelling potential of the stabilized soil decreased, and 25% was recommended as the optimum fly ash content for stabilization of the clay soil. Bin-Shafique et al. [
14] carried out cyclic wetting-drying and freeze-thaw tests to study the long-term performance of low- and high- plasticity clay soils stabilized with class C fly ash. They carried out UCS, plasticity index, and vertical swell tests on soil samples stabilized with the fly ash ranging from 0–20%. It was shown that the UCS of both types of soil improved by a factor of four, and the swelling potential and plasticity decreased about 75% and 50%, respectively, with 20% fly ash content. The wetting-drying cycles with saline water or tap water did not have an impact on strength parameters, swelling potential, and plasticity. However, strength parameters decreased about 40% for CH and 20% for CL after freeze-thaw cycles. Nevertheless, for both soil types, after the freeze-thaw cycles the fly ash-stabilized soil still had a significantly greater strength than the unstabilized soil (control sample).
Edil et al. [
12] analyzed the effects of class C fly ash on six inorganic soils (CL and CH) and one organic soil (OH) by applying CBR and resilient modulus (M
r) tests. They reported, on the basis of the test results, that the inclusion of fly ash (10–30%) provides a significant increase in the CBR and M
r parameters of the inorganic soils. The CBR values increased four and eight times with the addition of 10% and 18% fly ash, respectively. On the other hand, the effect of fly ash on organic soil was insignificant. Tastan et al. [
16] investigated the effects of fly ash in stabilization of an organic soil. Resilient modulus (M
r) and UCS tests were carried out on soil stabilized with 10%, 20%, and 30% fly ash. The results indicated that the different fly ash contents and soil types significantly affected the effectiveness of the stabilization. For example, the UCS increased about 400 kPa with addition of fly ash to two types of clayey soil with organic content less than 10%. However, by adding the same amount of fly ash, the UCS of an organic sandy silty peat with 27% organic content only increased 100 kPa.
The review of the literature suggests that fly ash stabilization of soil has great potential for improving the mechanical and physical properties of geomaterials. Common tests used for the study of fly ash stabilized soils are UCS, free swell index, consistency limits, and CBR tests. However, little information is available on shear and consolidation behavior of fly ash stabilized clay soils. Furthermore, previous studies did not fully clarify the different effects of using class C fly ash and class F fly ash on shear, consolidation, and microstructural behavior of stabilized soils. Therefore, there is a need to improve the fundamental understanding of how class C and class F fly ash affect the overall shear, consolidation, and microstructural behavior on soil samples. Shear strength parameters play an important role in estimating the bearing capacity of soils and in the assessment of the stability of geotechnical structures, while consolidation parameters allow the analysis of settlement behavior of soils.
This study presents a comparison of class C and class F fly ash on the stabilization of a clay soil (kaolinite). Their effects are evaluated through a program of laboratory tests including compaction, UCS, consolidated-undrained (CU) triaxial, one-dimensional consolidation (oedometer) tests, and scanning electron microscopy (SEM) analysis. The outcome of the study will improve the current understanding of the effects of class C and class F fly ash on the mechanical behavior and microstructural characteristics of stabilized soils, help determine and compare the suitability of class C and class F fly ash as alternative soil stabilizing agents and encourage the utilization of fly ash to reduce the environmental impacts of fly ash disposal. Another focus will be to identify the optimal amounts of the two types of fly ash for the stabilization of clay soil.