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Article

Comparative Study of the Effects of Conventional, Waste, and Alternative Materials on the Geomechanical Properties of Clayey Soil in the Chemical Soil Stabilisation Technique

by
Elefterija Zlatanović
1,*,
Nemanja Marinković
1,
Zoran Bonić
1,
Nikola Romić
1,
Snežana Djorić-Veljković
1,
Dušan Cvetković
1 and
Dragan Djordjević
2
1
Faculty of Civil Engineering and Architecture of Niš, University of Niš, Aleksandra Medvedeva 14, 18000 Niš, Serbia
2
Faculty of Sciences and Mathematics of Niš, University of Niš, Višegradska 33, 18000 Niš, Serbia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(14), 6249; https://doi.org/10.3390/app14146249
Submission received: 14 June 2024 / Revised: 8 July 2024 / Accepted: 9 July 2024 / Published: 18 July 2024
(This article belongs to the Section Civil Engineering)
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Abstract

:
This paper presents an extensive comparative analysis of the experimental results of chemical stabilisation of clayey soil in laboratory conditions by comparing the effects of adding conventional stabilisers (lime, cement binder), stabilisers that can be considered as waste material (fly ash, rock flour), as well as alternative chloride-based materials (ferric chloride, calcium chloride, potassium chloride) on the geomechanical properties of the soil. With the aim of determining the stabiliser optimal content in the mixture with the soil, in the first part of the research, the effects of stabilisation of clayey soil of medium plasticity using the considered stabilisers with different percentage share on the change in uniaxial compressive strength (UCS) and pH value of the soil at different time intervals after the treatment were analysed. In the second part of the research, additional tests were conducted on soil samples with optimal content for each of the considered stabilisers by monitoring changes in the physical and mechanical properties of the soil. These include Atterberg’s limits (liquid limit and plasticity limit), modulus of compressibility in the oedometer, California bearing ratio (CBR), and swelling potential at different time intervals after the chemical treatment to determine the durability of stabilisation effects. The results of the conducted research reveal that each of the conventional, waste, and alternative materials considered as chemical stabilisers contributes to the improvement of the geomechanical properties of the clayey soil, primarily in terms of increasing the bearing capacity and reducing the swelling of the treated soil.

1. Introduction

It is a very common situation that during the construction of large infrastructure facilities, it is necessary to use local soil material, which, in its natural conditions, does not meet the criteria required for construction purposes. Accordingly, there is a need to apply some soil stabilisation techniques. One of the oldest, but also the most commonly used, techniques is chemical soil stabilisation, which involves the mechanical mixing of a single stabiliser (or a combination of stabilisers) with the soil. In addition to conventional chemical stabilisers, waste products such as fly ash, limestone powder waste, rice husks, polypropylene fibres, recycled textile fibres, lignin fibres from the paper industry, and alike can be used as stabilisers [1,2,3,4]. More recently, many researchers are looking for alternative materials that would be suitable in both economic and environmental aspects, such as chlorides or silica-based additives [5,6,7,8].
The success of applying chemical stabilisation is reflected, first of all, through improvements in the physical and mechanical properties of the soil. In addition, added stabiliser(s) make(s) the soil more resistant to water. The presence of water can induce apparent cohesion in sandy soils, whereas in clayey soils, it increases their tendency to swell and shear [8,9,10].
By comprehensively reviewing the available literature, it can be concluded that the research so far has mostly concentrated on the effects of a specific stabiliser or on the effects of a combination of two stabilisers of different types. What is observed is the lack of a comprehensive comparative analysis that would include a wider range of stabilisers of different types and their individual effects on a broad range of geomechanical soil properties.
Lime and cement have been successfully used for decades, and many authors classify them as “conventional stabilisers”. However, their production involves the consumption of a large amount of natural raw materials and fossil fuels. Therefore, any reduction in the consumption of cement and lime contributes to a reduction in the consumption of these resources (natural raw materials and fossil fuels), thereby reducing CO2 emission. Since for a long time there has been a trend of reusing industrial waste as a secondary raw material in construction engineering, fly ash and rock flour are a separate group of stabilisers used in this study and called “waste materials”. The third group includes chemical reagents that do not yet have a commercial use in geotechnics, and studies on their effects on soil properties are current worldwide. The application of new chemical stabilisers has opened up new possibilities for solving the challenges posed by clayey soils, offering more sustainable, cost-effective, and environment-friendly solutions for construction industry projects. Thus, in a separate third group, chloride-based compounds called “alternative stabilisers” have been studied, such as ferric chloride, calcium chloride, and potassium chloride.
Taking all aforementioned into consideration, this paper presents extensive research on the chemical stabilisation of clayey soil by considering a variety of materials as stabilisers: conventional stabilisers (lime, cement), waste materials (fly ash, rock flour), as well as alternative chloride-based reagents (ferric chloride, calcium chloride, potassium chloride), with an aim of identifying and comparing the individual effects of each on a variety of very important geomechanical properties of the soil. To achieve the abovementioned goal, in the first part of the conducted research, for each of the mentioned reagents, soil samples were treated with three different percentages of reagents with respect to the dry weight of the soil sample, after which the change in uniaxial compressive strength (UCS) and the change in pH value were monitored at time intervals of 3, 7, and 28 days after the treatment. Based on these results, the optimal content of each of the reagents in the mixture with clayey soil was determined. In the second part of this research, on the soil samples prepared with the optimal stabiliser content for each of the considered stabilisers, additional tests were carried out in terms of monitoring changes in the values of Atterberg’s limits (liquid limit and plasticity limit) and the associated plasticity index, compressibility modulus (Mv, Eoed), California bearing ratio (CBR), and swelling over time since the chemical treatment.

2. Materials and Methods Considered in the Research

2.1. Materials Considered in the Research

2.1.1. Natural Clayey Soil

During construction of the E-80 Highway in the south-eastern part of the Republic of Serbia, a landslide occurred near the location Crvena Reka. Clayey soil samples for laboratory tests used in the present research were taken from the depth of the registered slip surface during the construction of a curtain of bored piles designed as part of remediation measures.
The properties of the natural clayey soil used in this research are given in Table 1. Given its mineralogical composition, among clay minerals, illite (11%), montmorillonite (15%), and clinochlore (10%) are present, whereas kaolinite is not identified. With regard to other minerals, calcite CaCO3 (39%) and quartz SiO2 (25%) are predominating. The results of this research should indicate the possibility of reusing the clayey soil material after mixing with a chemical stabiliser.

2.1.2. Chemical Stabilisers

The stabilisers analysed in this research are grouped into three categories: conventional stabilisers—cement and lime, waste materials—fly ash and rock flour, and alternative chloride-based stabilisers—ferric chloride, calcium chloride, and potassium chloride.
When it comes to conventional stabilisers that have been used in civil engineering and geotechnics for a long time, adding the optimal content of stabiliser contributes to a significant increase in the strength, stability, and load-bearing capacity of the soil, while on the other hand, it reduces soil plasticity, water permeability, and swelling [11,12]. In addition, conventional stabilisers contribute to improving soil resistance to adverse climatic and hydrological factors [13].
On the other hand, waste materials have a huge potential for reuse and are successfully applied in other civil engineering fields. They can be used independently but also in combination with conventional stabilisers, which, in the literature, is referred to as a complex stabilised mixture—CSM [14].
Research on chemical stabilisation of expansive soils shows that lime can be replaced by reagents based on chloride compounds due to their easy solubility in water, their more efficient mixing with the soil, and the fact that they provide sufficient amounts of cations for cation exchange [15].
Taking all this into account, the primary goal of this research was to examine the potential of each of the abovementioned reagents alone (without combining them mutually) in terms of improving the geomechanical properties of clayey soils using the chemical stabilisation technique.
  • Conventional stabilisers
Among conventional stabilisers, lime stands out as a versatile and economical solution for chemical soil stabilisation, primarily due to its ability to neutralise acidity (by increasing the soil pH value) and improve the engineering properties of natural soil. In engineering practice, lime is most often used to stabilise fractions of clayey soils of medium to high plasticity [16]. Lime is also successfully applied to coarse-grained soils with a sufficient content of clay fractions.
Within the stabilisation process, a reaction between lime (calcium hydroxide Ca(OH)2) and clay minerals takes place, the intensity of which depends on the mineralogical composition of the clayey soil (silica SiO2, alumina Al2O3). As illustrated in Figure 1, in the soil, calcium hydroxide (lime) is partially separated into Ca2+ ions and 2OH− ions, whereby a cationic exchange is enhanced during which the positively charged ions (cations) of the clayey soil particles (H+, Na+, or K+) as weaker bonded ions are replaced with the positively charged ions from the solution phase (calcium from lime (Ca2+)). Increasing the pH value of the soil contributes to the weakening of the bonds within the crystal lattice of soil particles, which facilitates exchange of cations and thus contributes to the better solubility of silica and alumina contained in clay. The reaction induced by the mentioned cation exchange is very fast, so immediately after the addition of lime to the soil, the soil properties change significantly. Finally, there is a long-term reaction of lime with active silica and alumina from clay minerals—the pozzolanic reaction. The silica and alumina break down, forming a new crystalline stage known as hydration, the products of which are calcium silicate hydrates CSH (Ca2+ + 2OH− + SiO2 → CSH) and calcium aluminate hydrates CAH (Ca2+ + 2OH− + Al2O3 → CAH) as gel-like materials with highly binding properties.
For the purpose of determining the optimal lime content, samples of clayey soil with the addition of 3%, 5%, and 7% lime were examined in this research, whereby commercial hydrated lime marked CL 90 S was used, with the following chemical composition: CaO (71.01%), MgO (3.05%), and SiO2 (2.14%).
When it comes to cement, although there are almost no restrictions on its application in soil stabilisation, the stabilisation of coarse-grained and fine-grained soils can be distinguished. In order for the cement binder to be successfully applied during stabilisation, it is necessary that the content of organic ingredients in the soil is at most 2% and that the pH value of the soil is not less than 5.3 [18].
In the case of coarse-grained soil mixed with a cement binder, with the presence of water, there is hydration of the cement, the formation of certain compounds (primarily calcium hydrate and aluminum hydrate), and the binding of the grains. Due to the relatively small amount of cement, covering the grains with the binder cannot be achieved completely, so the soil is highly porous, but despite this, it is still firm and stable.
With a mixture of fine-grained soil and cement, hydration as well as binding of soil particles occurs. As the particles are of smaller size, the cement can cover the grains completely. With larger amounts of cement, a fine cement skeleton is formed, which extends through the soil and improves its mechanical properties. There is also an additional stabilising effect for this type of soil material. During hydration, a certain amount of CaO is released from the cement, which reacts with active silica and alumina from clayey soil minerals, and then, in a longer, slower process, it further contributes to the increase in soil strength. When it comes to the optimal content of cement, a very wide range can be found in the literature; however, it depends primarily on the properties of the natural soil.
The cement HRB E2 was used in this research. The chemical composition of cement is CaO (51.73%), SiO2 (15.20%), Al2O3 (5.13%), Fe2O3 (2.39%), SO3 (1.87%), and MgO (1.02%). This type of cement binder is primarily used to stabilise coarse-grained soil. For this reason, tests were carried out with a higher percentage share of cement binder compared to the percentage share of lime. For the purpose of determining the optimal cement content, samples with 5%, 10%, and 15% cement were tested.
  • Waste materials
With urbanisation and great industrial growth, the operation of various plants generates a huge amount of industry waste, such as fly ash and rock flour.
Fly (electro-filter) ash is produced in thermal power plants, where it is separated from waste gases using filter devices. Fly ash is a material that needs to be disposed of and treated as construction waste, only in cases when it does not belong to the class of hazardous waste, that is, if it does not have increased radioactivity. In civil engineering, its application was initially related exclusively to production of concrete; in recent decades, however, it has a much wider application, increasingly gaining importance. Although it is still most often used for the production of concrete products (approximately 65%), its use is increasingly common for the purpose of soil and waste stabilisation, approximately 7% [19]. Due to its significant application in this century, some of the eminent researchers included fly ash in conventional stabilisers [5,7]. Although it could be used in combination with lime or cement to stabilise soil, a number of research works have confirmed that fly ash can improve soil properties without additional activators [20,21].
In thermal power plants in Serbia, 35 to 40 million tonnes of coal are consumed annually, and around 7 million tonnes of fly ash are produced. This makes up almost 80% of industrial waste in Serbia. Of this amount, only 3% is used in the production of cement, so there is a huge potential for the application of fly ash for the purpose of soil stabilisation during construction on weak soils of lower bearing capacity.
The fly ash used in this research was deposited from the thermal power plant Kostolac, with the chemical composition SiO2 (56.38%), Al2O3 (17.57%), Fe2O3 (10.39%), CaO (7.46%), and MgO (2.13%). The success of soil improvement with fly ash depends on soil properties, the content of fly ash in the soil, the time interval since treatment, and the amount of water at the time of compaction. Some previous studies have shown that the optimal content of fly ash can range from 10% to 30%, depending on the type of soil and fly ash [21,22]. With an aim of determining the optimal content of fly ash, in this research, samples of clayey soil with the addition of 10%, 15%, and 20% fly ash were tested.
Rock flour is waste from stone quarries and is successfully used as a finishing material on highways. It is produced during the extraction of stone and production of crushed stone, whereby about 20% of the rock material is turned into waste material—rock flour. Research has shown that adding rock flour can successfully contribute to the improvement of the geotechnical properties of expansive soils [23] and lateritic soils [24]. The obtained results indicate the possibility of reducing the plasticity and increasing the bearing capacity of the soil with an increase in the content of rock flour.
According to some recent studies [24,25], the optimal content of rock flour is not higher than 15%. Therefore, in the present research, clayey soil samples with the addition of 5%, 10%, and 15% rock flour were tested. The rock flour was deposited from the stone pit Gradac (“Straževica”-Batočina), which is located near Niš. It consists of dolomite (CaMg(CO3)2), which makes up 99% of the rock, whereas secondary minerals include calcite (CaCO3) and muscovite ((KF)2(Al2O3)3(SiO2)6(H2O)).
  • Alternative chloride-based reagents
Chloride-based compounds are categorised as non-traditional or alternative stabilisers. Non-traditional stabilisers are chemical compounds that do not contain a large amount of calcium [26]. Furthermore, these are mostly reagents in liquid state, which are more economical for transport than traditional ones and are diluted with water on site. They can be applied directly to the soil layer before compaction or injected under pressure into deeper layers [27].
Strong electrolytes such as potassium chloride (KCl), calcium chloride (CaCl2), and ferric chloride (FeCl3) were used as alternative stabilisers in the present research. Some recent studies have shown that these electrolytes can be effectively used instead of lime or cement in soil stabilisation, owing to their better solubility, their easier mixing with soil, and the fact that they provide the necessary cations for the cation exchange process [28,29,30].
Regarding the optimal content of chloride-based reagents, it can be found in the literature that sometimes even less than 1% of chloride is the optimal amount required for the stabilisation of mostly expansive soils [26,31,32], whereas certain studies have shown that the optimal content of chlorides can be considered higher than 5% [28]. For the purpose of determining the optimal content, for each of the considered chlorides, soil samples with the addition of 1%, 2%, and 3% reagent were tested.

2.2. Methods of Experimental Research

In the first stage of the conducted experimental research, each of the considered stabilisers was added to the natural clayey soil with three different percentages of share in relation to the dry mass of the soil. Such laboratory-prepared samples were in accordance with the relevant national codes harmonised with the European Norms (EN) with respect to the uniaxial compressive strength (UCS) test (the code [33] ensuring optimal moisture content and compaction using the Proctor test with 600 kNm/m3 energy). The samples were stored in plastic foil until the corresponding test day (3, 7, and 28 days after the treatment) to evaluate the long-term effectiveness of chemical stabilisation of the clayey soil.
Along with the modification of the physical–mechanical properties, this study also examined the changes in the chemical properties of the treated soil, specifically the variations in pH value with analyses conducted 24 h, 3 days, and 28 days after the treatment using a soil-to-water ratio of 1:2.5. The testing procedure involved mixing 10 g of dry soil with 25 mL of distilled water for 10 min. After achieving a clear solution around 30 min later, the pH value was measured by immersing electrodes into the solution.
Within this stage of the investigation, research control was ensured in such a way that each result (UCS and pH value) represents the mean value of three tested samples for each of the seven stabilisers used, conventional (lime and cement), waste (fly ash and rock flour), and alternative (ferric chloride, calcium chloride, and potassium chloride), for each of the three considered contents of stabilisers and for each of the three considered time intervals after the chemical treatment. This means that a total of 378 tests were performed in order to obtain 126 reliable results.
Based on the initial tests, for each of the considered reagents, the optimal content of stabilisers in the mixture with clayey soil was determined.
Within the second stage of experimental research, subsequent tests were then conducted on samples with such determined optimal stabiliser content to assess changes in the Atterberg’s limits, modulus of compressibility (Mv, Eoed), California bearing ratio (CBR), and soil swelling (s). The Atterberg’s limits were determined according to the code [34]. In the oedometer test (in accordance with the code [35]), the modulus of compressibility was evaluated using samples of 20 mm in height and 70 mm in diameter, subjected to a maximum load of 400 kPa after 24 h of saturation. The CBR value, obtained by compaction of samples with optimal moisture content determined by the Proctor test, was also analysed (based on the code [36]). Prior to CBR testing, the samples were immersed in water for 96 h, and soil swelling was measured. Oedometer tests were conducted after 3, 7, and 28 days of chemical stabilisation, whereas CBR tests were carried out 7 and 28 days after the addition of the stabiliser.
As it was the case in the first stage, here again the control of the research was provided in such a way that each result (Atterberg’s limits, Eoed, CBR, and swelling) represents the mean value of three tested samples for each of the seven stabilisers studied with their optimal content and for each of the considered time intervals after the chemical treatment, indicating that a total of 210 tests were performed to obtain 70 reliable results.
The above-described experimental research and corresponding tests were conducted at the Laboratory for Geotechnics of the Faculty of Civil Engineering and Architecture of the University of Niš.
The specified mix design table (Table 2) summarises the mixture specimens with the proposed stabilising materials and the corresponding percentage shares considered in the study.
In this research, the examined specimens, treated with the selected chemical stabilisers, were prepared with an optimal moisture content of 18.5%, which was obtained by Proctor test for untreated soil. This enabled the comparability of the achieved results regarding the effects of different types and different percentages of stabilisers in the mixture with clayey soil.

3. Research Results—Analysis and Discussion

3.1. Uniaxial Compressive Strength (UCS)

To examine the change in the uniaxial compressive strength (UCS), different percentage shares were used for each of the considered reagents with the aim of determining the optimal content of reagents in the mixture with clayey soil. Figure 2, Figure 3 and Figure 4 present the results obtained for each of the tested reagents with three different percentage shares, which were selected based on a review of the literature. The samples were examined 3, 7, and 28 days after the chemical treatment. The values shown in the diagrams represent the mean value of the ultimate load at the failure of three samples.

3.1.1. Effects of Conventional Stabilisers on UCS

Of the conventional reagents, lime and cement were considered in this study. In accordance with the explanations given in Section 2.2, lime was added to the natural clayey soil with a share of 3%, 5%, and 7% relative to the dry weight of the soil, whereas, with respect to cement, samples of the clayey soil mixture with a percentage share of 5%, 10%, and 15% were prepared. The obtained results revealed that the addition of traditional reagents contributes to a multifold increase in the UCS value after the chemical treatment of clayey soil (Figure 2).
With the addition of lime, a significant increase in the UCS value was achieved after the chemical treatment. Regardless of the percentage of the added stabiliser, a significant improvement in this soil property was observed over time. Namely, the tests conducted 3 and 7 days after the chemical treatment resulted in similar UCS values, whereas 28 days after the treatment the registered UCS values were significantly higher—with regard to 3% added lime, 3 days after the chemical treatment the UCS value increased twice (from a value of 205 kPa in the natural condition of the soil to 435 kPa), 7 days after the treatment the UCS value increased 2.5 times (to 533 kPa), and 28 days after the treatment the increase in UCS value was fourfold (896 kPa). An almost identical trend of increasing UCS values over time was observed in samples with 5% (529 kPa, 667 kPa, 1145 kPa) and 7% added lime (622 kPa, 715 kPa, 1178 kPa).
What was also observed was that a particularly significant improvement in this soil property was achieved when the lime content in the clay mixture was increased from 3% to 5%, whereas the increase to 7% did not induce a more pronounced increase in the value of UCS compared to the results for the case of 5% lime addition. This result is consistent with the research findings of other studies [37,38], according to which soil mixed with lower lime content shows a higher UCS value compared to soil mixed with higher contents of lime. Thus, 28 days after the chemical treatment, the value of UCS with the addition of 3% lime increased almost four times (896 kPa), with the addition of 5% lime the increase was 5.5 times (1145 kPa), whereas the value of UCS with the addition of 7% lime was almost six times higher than the initial natural soil value (1178 kPa). This result points to the conclusion that 5% can be considered the optimal lime content.
In contrast to lime, with the addition of a cement binder, a trend of increasing UCS values was registered both with the increase in the percentage share of the binder and over time elapsed since the chemical treatment. For the time period after 3 days with the addition of 5% cement, there was an increase in the UCS value by 1.5 times (307 kPa), with the addition of 10% cement the improvement was 2.5 times (511 kPa), whereas with the addition of 15% cement the value increased by almost four times (776 kPa) in relation to the natural soil condition (205 kPa). A similar trend of increase, which is almost linear, was also registered in the tests carried out 7 and 28 days after the treatment with the addition of cement binder. The maximum value was registered by adding 15% cement (1425 kPa) 28 days after the treatment (increasing the UCS value almost seven times), which is at the same time the maximum UCS value is registered by comparing all the materials (conventional, waste, and alternative) considered as stabilisers in this research. This indicates that the optimal content with regard to this stabiliser is 15% for this specific study. However, the trend of constant increase in UCS value with increasing cement content for all the considered time intervals imposes the necessity for further research on the effects of using a higher percentage of cement in order to precisely determine the optimal stabiliser content in the clay mixture.
By comparing these two types of conventional stabilisers, it was observed that for all test time intervals after the chemical treatment of clayey soil, better results are achieved with the addition of 3% lime than with the addition of 5% cement binder. By comparing the results obtained for the case of addition of 5% lime and for the case of addition of 10% cement binder, it can be observed that almost matching UCS values were achieved. The addition of 15% cement contributed to slightly higher UCS values compared to the addition of 7% lime.
Finally, it could be concluded that when stabilising clayey soil, from the aspect of this soil property, the use of lime is more rational than cement.

3.1.2. Effects of Waste-Based Stabilisers on UCS

Among waste materials, fly ash (by varying the content in the mixture with clayey soil in the amount of 10%, 15%, and 20%) and rock flour (with 5%, 10%, and 15% content) were considered as chemical stabilisers.
Addition of fly ash/rock flour resulted in a weaker improvement compared to conventional stabilisers (Figure 3). What is also noticeable is that the change in UCS value is more pronounced with the increase in the content of added waste material (percentage share of fly ash/rock flour in the mixture), whereas the time interval that has passed since the chemical treatment did not have a significant effect.
The maximum UCS values obtained with the addition of fly ash in the amount of 10% (307 kPa) and 20% (342 kPa) were lower than those with the addition of 15% fly ash (371 kPa). This result points to the conclusion that the optimal content of fly ash can be considered 15%. The UCS value obtained by the test performed 3 days after the treatment did not change significantly with further passage of time.
A similar trend of change in UCS value was observed on samples with the addition of rock flour. Namely, compared to the natural soil state (205 kPa), the maximum improvement (279 kPa) was achieved with the addition of 10% rock flour 7 days after the treatment. Similar results were obtained after 3 days (268 kPa) and after 28 days since the treatment (275 kPa). The addition of 5% and 15% rock flour did not contribute to a significant increase in the UCS value. This finding points to the conclusion that 10% can be considered the optimal content of rock flour.

3.1.3. Effects of Alternative Chloride-Based Stabilisers on UCS

Chloride-based reagents—ferric chloride, calcium chloride, and potassium chloride—were considered as alternative stabilisers, and with the aim of determining the optimal content of this type of reagent, clayey soil samples with the addition of 1%, 2%, and 3% reagent were tested.
For all the considered chloride-based reagents, the measured UCS values were significantly lower than the values achieved with the addition of conventional stabilisers; their application, however, certainly resulted in an increase in soil strength, which is in the range of values achieved with the application of stabilisers based on waste materials (Figure 4).
With the addition of 1% ferric chloride, the test results obtained 3, 7, and 28 days after the treatment are quite similar, whereas, with the addition of 2% and 3% stabiliser, the UCS values increased more significantly with time. It was also observed that with the addition of 3% ferric chloride, higher UCS values were achieved compared to the samples with 1% of this stabiliser added. However, compared to the natural state (205 kPa), the maximum improvement was registered with the addition of 2% ferric chloride (369 kPa).
On the other hand, with the addition of calcium chloride or potassium chloride, the samples treated with 1% stabiliser resulted in higher UCS values than the samples with the addition of 3% stabiliser. It was noted that the values after 28 days with the addition of 1% calcium chloride (371 kPa) and 2% calcium chloride (391 kPa) are very close. Even closer UCS values were registered with the addition of 1% potassium chloride (337 kPa) and 2% potassium chloride (342 kPa). It should be noted that with the addition of 1% potassium chloride after 7 days since the treatment, a higher value (321 kPa) was obtained than with the addition of 2% of this stabiliser (297 kPa).
In the case of each of the considered chloride-based stabilisers, the maximum value of UCS was obtained with the addition of 2% stabiliser 28 days after the treatment, which leads to the conclusion that the optimal content of chloride as a reagent in the soil mixture can be considered 2%.
For the sake of comparison and determining the best-performing binder type, a graphic interpretation showing the UCS performance for the natural clayey soil and the best-performing soils using conventional, waste-based, and chloride-based stabilisers is presented in Figure 5. Conventional stabilisers proved to be the most effective stabilisers in terms of improving the uniaxial compressive strength of the clayey soil, between which cement stands out slightly.

3.2. pH Value

The optimal stabiliser content refers to the percentage of a stabiliser in the soil mixture that provides the most effective stabilisation of soil of weak properties in its natural conditions. Furthermore, the optimal content of stabiliser contributes to the increase in load-bearing capacity, i.e., reduced plasticity, of the soil. Some of the studies have shown that in the case of applying lime as a stabiliser, the most effective outcome of the stabiliser is achieved at a soil pH value of 12.4 [39,40,41]. With suitable external conditions, a pozzolanic reaction develops between the stabiliser and the soil, in which the stabiliser causes a high pH value of the environment and thus dissolves silicon and aluminium compounds of the clayey soil.
The changes in the pH value of the treated soil over time for each of the considered stabilisers and for the corresponding percentage share of the stabiliser in the mixture with clayey soil are given in Table 3. Conventional stabilisers and stabilisers based on waste materials used in this research showed the ability to increase the pH value of clayey soil, which initiates pozzolanic reactions. In the first 24 h after mixing with the stabiliser, an increase in pH values was noted, after which the values remained constant or slightly decreased. These results clearly indicate the potential of stabilisers to cause permanent chemical changes in soil. High pH values are associated with the dissolution of primary minerals, in particular, clay minerals, which further leads to the formation of secondary minerals. It is also noticeable that the suggested optimal pH value of 12.4, which results in the strongest effect of the stabiliser, except in the case of lime, was also achieved in the case of cement (thus, in the case of conventional stabilisers), whereas in the case of stabilisers based on waste materials, it was still somewhat lower.
On the other hand, adding a chloride-based stabiliser reduced the pH value of the treated soil compared to the natural state of the soil. This result is due to the fact that only the solution of calcium chloride in water is alkaline (pH ≈ 8.5), whereas the solutions of ferric chloride and potassium chloride are acidic (pH < 7.0).

3.3. Optimal Stabiliser Content

Based on the obtained results with regard to both the UCS values and the pH values, the optimal content for each of the considered stabilisers was determined. These values of the optimal content of each stabiliser were used for the purposes of additional tests, the results of which are presented and elaborated on in the subsequent parts.
It was determined that the optimal content of lime as a stabiliser is 5%, for rock flour it is 10%, and with the addition of fly ash it is 15%, whereas, in the case of chloride (ferric chloride, calcium chloride, or potassium chloride) it attains 2%. Regarding cement, a trend of constant increase in UCS value with increasing cement content was observed for all the considered time intervals after the treatment. Therefore, for the purposes of additional tests, the highest cement content of 15% analysed in this research was considered optimal. The aforementioned finding, however, imposes the need for further research on the optimal cement content, including percentages in the mixture with clayey soil higher than the considered 15%. The higher cement content, on the other hand, entails the question of the economic justification of the implementation of the procedure.

3.4. Atterberg’s Limits (LL and PL) and Plasticity Index (PI)

For each of the considered stabilisers, the change in the Atterberg’s limits (the liquid limit (LL) and the plastic limit (PL)) was registered on the samples prepared with the optimal content of the stabiliser in the mixture, and on their basis, the corresponding value of the plasticity index (PI) was calculated as the difference between LL and PL. The results are shown in Figure 6 and Figure 7.
Based on Figure 6, in which the lower part of the columns (solid colour) shows the measured values of LL, it is concluded that there were no significant changes in the LL value with the addition of conventional stabilisers, as well as waste materials, regardless of the time interval of soil testing after the treatment. In the case of each of the considered alternative chloride-based stabilisers, the LL value decreased after 28 days since the treatment. Compared to the natural state of the soil (LL = 49%), the maximum decrease in LL value was registered with the addition of 2% ferric chloride after 28 days (LL = 42%), whereas the maximum increase in LL value was achieved with the addition of 15% fly ash after 7 days (LL = 53%).
On the other hand, an increase in PL values was registered in all the considered cases of stabilisers, as presented in Figure 6, in which the upper part of the columns (dotted colour) represents the measured values of PL. It is also noted that for each of the considered stabilisers, the greatest increase in PL value was achieved after 28 days since the treatment. The maximum increase in the value of PL compared to the natural state of the soil (PL = 23%) was registered with the addition of the optimal content of lime in the amount of 5% (PL = 34%), i.e., cement binder in the amount of 15% (PL = 35%).
Regarding the PI value, the greatest change was registered by adding the optimal content of cement binder, in which case 28 days after the chemical treatment the PI value decreased from the initial 26% to 13% (Figure 7). A significant decrease in the value of PI after 28 days since the treatment was also registered in the case of adding the optimal content of lime (PI = 18%), as well as in the case of application of the considered chlorides (PI < 16%). By adding waste materials, the value of PI did not change significantly, that is, the values of LL and values of PL are similar to the values in the natural soil state. It can be assumed that the reason for this result is the absence of chemical reactions between soil and waste materials. On the other hand, the addition of traditional stabilisers or chlorides affects the formation of compounds that absorb water from the soil and thus reduces its moisture content.

3.5. Modulus of Compressibility (Mv, Eoed)

The modulus of compressibility (Mv, Eoed) was determined for four load levels (50 kPa, 100 kPa, 200 kPa, and 400 kPa) in the oedometric test. The obtained values of Eoed at the maximum load level of 400 kPa are shown in Figure 8. The samples were tested at time intervals of 3, 7, and 28 days after the treatment of the clayey soil with the optimal content of each of the considered stabilisers.
Based on the given graphical interpretation of the results, it can be concluded that each of the considered stabilisers contributed to the increase in the value of Eoed. The greatest increase in the value of Eoed is achieved by adding a conventional stabiliser—lime (38.392 MPa) or cement (44.07 MPa), whereby the value increased by about 3 times compared to the natural soil condition (12.945 MPa).
With regard to the waste material as a stabiliser, the effect on improving this soil property is somewhat smaller, with an increase in the Eoed value of about 1.5 times in the case of rock flour and about 2 times in the case of fly ash.
Considering the alternative materials as stabilisers, the highest value was obtained with the addition of calcium chloride (an increase of about 2 times), with the addition of ferric chloride the value was increased about 1.5 times, whereas potassium chloride had a minor contribution to the improvement of the natural soil property.
It is also noticeable that in the cases of lime and calcium chloride, the value of Eoed achieved 28 days after the treatment was lower than the value obtained after 7 days.
Lastly, it can be concluded that from the aspect of this soil property, conventional stabilisers resulted in the greatest effect, between which cement stands out slightly. However, given the optimal stabiliser content, the use of lime is more rational than cement.

3.6. California Bearing Ratio (CBR) and Swelling (s)

One of the most common reasons for replacing soil material in engineering practice is the low value of the California bearing ratio (CBR), which is typical for fine-grained (clayey) soils. The results of this research, presented in Figure 9, show that each of the considered materials as stabilisers—conventional, as well as waste and alternative—contributed to the improvement of the CBR value of the soil 7 and 28 days after the chemical treatment. Nevertheless, the greatest difference in effects between traditional stabilisers and all other stabilisers used in this research was observed precisely with regard to this property of the soil. Namely, compared to the soil in its natural state (CBR = 2.71%), the greatest improvement was registered with the addition of traditional stabilisers. With the addition of lime, an increase of seven times (CBR = 18.97%) was registered, and with the addition of cement binder, up to 8.5 times (CBR = 22.97%) 28 days after the treatment in both cases. The smallest increase in CBR value was registered by adding rock flour (CBR = 4.22%), i.e., potassium chloride (CBR = 3.94%). Considering that in the road construction for the subgrade at roadbed, according to the technical conditions a CBR value greater than 3% is required, the obtained results indicate the fact that each of considered stabilisers according to this criterion can find their application for the stabilisation of clayey soil for the given purposes.
Therefore, by comparing all the examined stabilisers, it can be concluded that from the aspect of CBR, conventional stabilisers resulted in the highest increase, between which cement stands out slightly. Considering the optimal stabiliser content, however, the use of lime is more rational than cement.
According to the CBR test standard, samples are immersed in water for 96 h before testing in laboratory conditions, during which soil swelling (s) is measured. All analysed stabilisers contributed to a significant reduction in soil swelling (Figure 10), in particular 28 days after the soil chemical treatment. In addition, all the considered stabilisers contributed to the reduction of swelling below 1.75% as early as 7 days after the treatment. The most significant reduction after 7 days since the treatment was registered with the addition of cement, whereas after 28 days, the greatest reduction in swelling was shown by the soil sample treated with the optimal amount of lime (reduction up to fifteen times). The minor effect on reducing swelling was achieved by adding rock flour, i.e., potassium chloride (reduction up to five times).
In the end, it can be concluded that with regard to soil swelling, the greatest effect was achieved by conventional stabilisers, although the difference compared to the other considered stabilisers is not so pronounced, as was the case with the previously discussed geotechnical properties of the soil.

4. Concluding Remarks

The effects of a variety of materials using the chemical stabilisation technique on the improvement of geomechanical properties of clayey soil were investigated and compared, considering conventional materials (lime and cement), waste materials (fly ash and rock flour), as well as alternative materials—chloride-based compounds (ferric chloride, calcium chloride, and potassium chloride), along with the determination of the optimal content of a single additive in the soil mixture and the durability of the effects of the implemented soil chemical stabilisation. The most important conclusions are as follows:
  • All the considered stabilisers contributed to the improvement of the uniaxial compressive strength of the clayey soil, among which conventional stabilisers proved to be the most effective.
  • The results of the change in pH value revealed that conventional stabilisers are the most successful in reducing soil acidity, a slight increase in the pH value was registered with the addition of waste materials, whereas with the use of chloride the pH value of the natural soil decreased.
  • It was determined that the optimal content of stabiliser for the treated clayey soil in the case of applying chlorides attains 2%, in the case of lime is 5%, for rock flour is 10%, with the addition of flying ash is 15%, whereas for cement is found to be higher than the considered 15%, which on the other hand raises the question of the economic justification of the procedure with this binder.
  • None of the examined stabilisers contributed to a significant change in the LL value, whereas each of them contributed to an increase in the PL value, which ultimately led to a reduction in the plasticity index of the treated clayey soil.
  • The best results in terms of increasing the value of the compressibility modulus were achieved with the addition of conventional stabilisers, although the other considered stabilisers also contributed to the reduced compressibility of the treated soil compared to the natural soil material.
  • In the case of all the considered stabilisers, CBR values are higher than 3%, which is of great benefit when it comes to road construction, whereby the greatest improvement was achieved in the case of stabilisation with conventional stabilisers.
  • Each of the considered stabilisers contributed to the reduction of soil swelling, whereby the greatest reduction was achieved by the addition of conventional stabilisers.
After conducting extensive laboratory tests, the obtained results presented in this paper confirmed that the improvement of the geomechanical properties of the clayey soil can be successfully achieved with the use of traditional stabilisers, but also with the use of waste materials and chloride-based compounds. Each of the considered stabilisers, added to the natural clayey soil, would enable its application for various practical purposes in the field of construction.
Lastly, it should be emphasised that the results presented in this paper refer to local clayey soil. Therefore, there is a need for more comprehensive further research, which will analyse soil samples from a wider area, as a larger number of tests on a greater variety of clayey soils will result in more reliable findings.

Author Contributions

All authors have contributed to all major parts of the presented research (conceptualisation, methodology, investigation, resources, data curation, formal analysis, writing—original draft preparation, writing—review and editing, visualisation, and supervision); project administration, E.Z. and Z.B.; funding acquisition, E.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the support of the Science Fund of the Republic of Serbia in the scope of the scientific–research project “A New Concept in Improvement of Geotechnical Properties of Ground—Chemical Electrokinetic Treatment of Soils (ElectroSoil)”, Grant No. 7742530.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this research are included in the article; further inquiries may be directed to the corresponding author.

Acknowledgments

The authors of the paper hereby express their sincere gratitude to Nebojša Davidović, retired from the Faculty of Civil Engineering and Architecture of Niš, for his selfless and wholehearted help and useful suggestions in conducting this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 06249 g001
Figure 1. A schematic presentation of cation exchange and stabilisation mechanism by adding lime to a clayey soil [17].
Figure 1. A schematic presentation of cation exchange and stabilisation mechanism by adding lime to a clayey soil [17].
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Figure 2. Change in the soil UCS value for the considered conventional stabilisers with increasing stabiliser content and with time after the treatment.
Figure 2. Change in the soil UCS value for the considered conventional stabilisers with increasing stabiliser content and with time after the treatment.
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Figure 3. Change in the soil UCS value for the considered waste materials with increasing stabiliser content and with time after the treatment.
Figure 3. Change in the soil UCS value for the considered waste materials with increasing stabiliser content and with time after the treatment.
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Figure 4. Change in the soil UCS value for the considered alternative materials (chlorides) with increasing stabiliser content and with time after the treatment.
Figure 4. Change in the soil UCS value for the considered alternative materials (chlorides) with increasing stabiliser content and with time after the treatment.
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Figure 5. Comparison of the effects of the examined conventional, waste, and alternative materials on the soil UCS value.
Figure 5. Comparison of the effects of the examined conventional, waste, and alternative materials on the soil UCS value.
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Figure 6. Change in the soil Atterberg’s limits over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
Figure 6. Change in the soil Atterberg’s limits over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
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Figure 7. Change in the soil PI value over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
Figure 7. Change in the soil PI value over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
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Figure 8. Change in the soil Eoed value over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
Figure 8. Change in the soil Eoed value over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
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Figure 9. Change in the soil CBR value over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
Figure 9. Change in the soil CBR value over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
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Figure 10. Change in the soil swelling over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
Figure 10. Change in the soil swelling over time given the optimal content of the considered conventional, waste, and alternative materials as stabilisers.
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Table 1. Properties of the clayey soil used in the present research.
Table 1. Properties of the clayey soil used in the present research.
Property of SoilSymbol (Unit)Value
Particle DensityGs (-)2.705
Grain Size DistributionGravel (%)1.2
Sand (%)4.9
Silt (%)40.6
Clay (%)53.3
Coefficient of UniformityCu (-)8.0
Coefficient of CurvatureCc (-)2.0
USCS Soil ClassificationSymbol (-)CL
Maximum Dry DensityMDD (g/cm3)1.903
Optimal Moisture ContentOMC (%)18.5
Uniaxial Compressive StrengthUCS (kPa)205
Liquid LimitLL (%)49
Plastic LimitPL (%)23
Plasticity IndexPI (%)26
pH ValuepH (-)9.5
Modulus of CompressibilityMv (MPa)12.945
California Bearing RatioCBR (%)2.71
Swellings (%)2.91
Table 2. Summary of the tested samples with the corresponding percentage share of studied stabilisers.
Table 2. Summary of the tested samples with the corresponding percentage share of studied stabilisers.
Mixture SpecimensPercentage Share of a Stabiliser in the Mixture
Soil + Lime3%5%7%
Soil + Cement5%10%15%
Soil + Rock Flour
Soil + Fly Ash10%15%20%
Soil + Ferric Chloride1%2%3%
Soil + Calcium Chloride
Soil + Potassium Chloride
Table 3. Change in the soil pH value after the treatment with the analysed stabilisers.
Table 3. Change in the soil pH value after the treatment with the analysed stabilisers.
Soil ConditionspH Value
After 24 hAfter 3 DaysAfter 28 Days
Soil in natural conditions9.59.59.5
Soil after chemical stabilisationLime3%12.112.012.0
5%12.512.512.4
7%12.812.812.7
Cement5%11.611.611.6
10%12.012.012.0
15%12.412.412.4
Fly ash10%11.211.211.0
15%11.611.311.3
20%12.011.811.6
Rock flour5%10.510.510.2
10%10.910.810.5
15%11.111.110.6
Ferric chloride1%9.59.59.2
2%9.49.49.2
3%9.49.39.0
Calcium chloride1%9.29.29.2
2%9.09.09.0
3%9.09.09.0
Potassium chloride1%9.59.59.5
2%9.59.59.5
3%9.49.49.4
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Zlatanović, E.; Marinković, N.; Bonić, Z.; Romić, N.; Djorić-Veljković, S.; Cvetković, D.; Djordjević, D. Comparative Study of the Effects of Conventional, Waste, and Alternative Materials on the Geomechanical Properties of Clayey Soil in the Chemical Soil Stabilisation Technique. Appl. Sci. 2024, 14, 6249. https://doi.org/10.3390/app14146249

AMA Style

Zlatanović E, Marinković N, Bonić Z, Romić N, Djorić-Veljković S, Cvetković D, Djordjević D. Comparative Study of the Effects of Conventional, Waste, and Alternative Materials on the Geomechanical Properties of Clayey Soil in the Chemical Soil Stabilisation Technique. Applied Sciences. 2024; 14(14):6249. https://doi.org/10.3390/app14146249

Chicago/Turabian Style

Zlatanović, Elefterija, Nemanja Marinković, Zoran Bonić, Nikola Romić, Snežana Djorić-Veljković, Dušan Cvetković, and Dragan Djordjević. 2024. "Comparative Study of the Effects of Conventional, Waste, and Alternative Materials on the Geomechanical Properties of Clayey Soil in the Chemical Soil Stabilisation Technique" Applied Sciences 14, no. 14: 6249. https://doi.org/10.3390/app14146249

APA Style

Zlatanović, E., Marinković, N., Bonić, Z., Romić, N., Djorić-Veljković, S., Cvetković, D., & Djordjević, D. (2024). Comparative Study of the Effects of Conventional, Waste, and Alternative Materials on the Geomechanical Properties of Clayey Soil in the Chemical Soil Stabilisation Technique. Applied Sciences, 14(14), 6249. https://doi.org/10.3390/app14146249

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