ABSTRACT
The contamination of soil due to petrochemical activities and leakage of hydrocarbon organic materials is not only an environmental issue but also a geotechnical problem. One suitable and economical solution for improving the geotechnical parameters of soils contaminated with these substances is soil stabilization through artificial cementation. In this experimental study, the influence of geopolymer cement based on g-granulated blast-furnace slag) GGBFS) at percentages of 0, 5, and 10, along with the alkaline activator sodium hydroxide at concentrations of 4, 6, and 8 molars, on the improvement of the kaolinite clay contaminated with glycerol organic substance at 3, 6, and 9 percentages has been investigated. The soil samples, including clean soil, contaminated soil, geopolymer-soil, and geopolymer-contaminated soil, were prepared and subjected to unconfined compressive strength (UCS) tests after 7 days of curing. The initial results indicated that UCS of glycerol-contaminated kaolinite clay decreases compared to the clean kaolinite. The reduction increases with an increase in the contaminant content. After improvement, the UCS of uncontaminated kaolinite increases with an increase in the slag percentage for every alkaline solute concentration. In such a way that the geopolymer cement stabilized kaolinite containing 10% slag and 6 molar sodium hydroxide solution showed 441% increase in UCS compared to the uncontaminated kaolinite. In the glycerol-contaminated samples, the highest increase in past improvement UCS occurred using a geopolymer containing 10% slag activated with 6 molar sodium hydroxide. For kaolinite contaminated with 3%, 6%, and 9% glycerol, it approximately increased UCS about 489%, 446%, and 402% compared to the clean soil, respectively. The results of scanning electron macroscopy confirmed the obtained results of present study.

Introduction

In many cases, soil contaminated with chemicals will undergo changes in its physical and mechanical properties. Soil pollution usually arises from landfill sites, surface water contaminated with underlying layers, leakage from pipelines and underground storage tanks, industrial waste, fertilizers, and pesticides (Karkush and Abdulkareem, 2017). These changes can lead to an increase in settlement of structures built on these soils, decrease in the bearing capacity of foundations, prolongation of construction operations, increased construction costs, and an increase in the risk of the structural failure (Du et al., 2021; Rahman, 2010.( Various researchers have studied the behavior of contaminated soils and proposed methods for their remediation and improvement. For instance, Zanjarani and Hamidi (2014) investigations have been conducted on the geotechnical behavior of kaolinite clay soil contaminated with two types of petroleum contaminants. Based on these studies, it was determined that as the contamination percentage increases, the compressibility of the contaminated soil increases compared to the clean soil, but the soil's consolidation coefficient and permeability decrease with the increase in the percentage of contaminants. Cement stabilization can be used to improve contaminated soil, involving the mixing of contaminated soil with cementitious materials such as Portland cement, lime, or other cementitious base materials. In general, this process improves soil properties such as Unconfined Compressive Strength (UCS), cohesion, and internal friction angle by forming calcium silicate hydrate (C-S-H) bonds. The resulting cementitious mortar covers soil particles and bonds them together (Shah et al., 2003). Estabragh et al. (2016) conducted laboratory experiments to investigate the mechanical properties of clay soil contaminated with 3%, 6%, and 9% glycerol and blended with 3% and 6% Portland cement type I. Based on the obtained results, improvement of the mechanical parameters was observed through soil remediation with Portland cement. Additionally, Jahani et al. (2023) explored the possibility of remediating CL type clay soil contaminated with glycerol solution using activated blast furnace slag (GGBFS) with magnesium oxide (MgO) and a mixture of MgO and cement. The results indicated that GGBFS activation by adding MgO or a mixture of MgO and cement ultimately led to improved performance, enhancing the compressive strength and increasing the peak strength and stiffness at mid-strength (E50) of contaminated soil samples by 50%. Scanning Electron Microscopy (SEM) images also revealed that chemical reactions between soil particles and additives has occurred.
On a global scale, there is high potential for transitioning towards the use of greener cementitious materials. Therefore, in recent years the researchers have focused on producing geopolymer cements, which exhibit suitable compatibility with the environment. Geopolymer cement offers advantages such as lower production costs, ease of accessibility, significantly reduced carbon dioxide emissions, and optimized mechanical properties. The use and production of geopolymer cement address issues arising from increased waste and byproducts generated by industries and mines, thereby reducing their environmental and health concerns. The production and use of Portland cement mitigate environmental drawbacks, including increased carbon dioxide emissions from cement production. According to the United States Geological Survey, global clinker production reached 3.57 billion tons in 2014. Producing each ton of Portland cement clinker results in approximately 900 kilograms of carbon dioxide emission. This level of production means that the cement industry accounts for 6 to 7 percent of human-produced carbon dioxide emissions (Khatib, 2016).
In this research, the main objective is to investigate the mechanical behavior and strength characteristics of the clean and glycerol-contaminated kaolinite by using an environmental friendly geopolymer cement produced based on blast furnace slag activated by sodium hydroxide. The optimum contents and the rate of increase in compressive strength will be evaluated and interpreted after soil improvement.

Methodology
The materials used in this study include kaolinite clay, blast furnace slag at 0, 5, and 10 weight percent of dry soil, water, and sodium hydroxide at concentrations of 4, 6, and 8 molars as an alkaline activator with high pH, as well as an organic matter glycerol contaminant, which were used in preparing the samples. To prepare soil-geopolymer and contaminated soil-geopolymer samples, various amounts of slag (0, 5, and 10 percent by weight of dry soil) were mixed with three concentrations of alkaline solution (4, 6, and 8 molars) to achieve an optimum moisture content of 17 percent. To produce geopolymer cement based on blast furnace slag, the slag was initially mixed with sodium hydroxide solution at specific concentrations and optimum moisture content. Since one of the main objectives in soil improvement studies is to evaluate the mechanical behavior and soil strength, and to create identical conditions to compare the strength of samples produced in the present study, all samples were compacted at the maximum dry unit weight and optimum moisture content achieved through the static compaction method. Accordingly, the soil was poured in three layers into a mold with a diameter of 38 millimeters and a height of 76 millimeters. Then, each layer was compacted using a loading device at a speed of 1.5 millimeters per minute. Uniaxial compressive strength tests were conducted on the samples according to ASTM D1633-07 standard. According to this standard, the samples were kept out of the storage conditions (moist environment) two hours before being placed in the loading device. Then, they were subjected to complete consolidation conditions under loading at a rate of 1 millimeter per minute (Kumar et al., 2007, and Estabragh et al., 2016). Additionally, scanning electron microscopy (SEM) was used to observe the microstructure of the samples under different conditions.

Conclusion
According to the results, the uniaxial compressive strength of kaolinite without contamination increases with an increase in the blast furnace slag content at different concentrations of sodium hydroxide solution. The highest increase in uniaxial compressive strength compared to untreated kaolinite is related to the geopolymer cement consisting of 10% slag and 6 molar sodium hydroxide solution. In this case, a uniaxial compressive strength of 2710 kPa was obtained, indicating 441% increase compared to the untreated sample. According to the results, the uniaxial compressive strength of kaolin soil is 493 kPa. Glycerol addition to the natural kaolinite decreases the uniaxial compressive strength of the contaminated soil. This trend continues with an increase in the glycerol content, so that with the addition of 9% of glycerol to the soil, the uniaxial compressive strength decreases to 254 kPa. In other words, adding 9% glycerol to the soil results in 48% reduction of the final strength compared to the natural soil. Adding geopolymer cement to the clay soil increases the final compressive strength of the samples. With an increase in the geopolymer cement percentage, the final amount of compressive strength increases. According to the results, an increase in the geopolymer cement percentage also increases the final strength of the soil contaminated with various glycerol contents. Furthermore, the highest increase in uniaxial compressive strength compared to kaolinite with 3, 6, and 9% contaminant is related to the geopolymer containing 10% slag and 6 molar sodium hydroxide solution, which are approximately 489, 446, and 402%, respectively. Comparing the results obtained for contaminated and uncontaminated kaolinite treated with geopolymer cement with the findings of Hamidi and Hajimohammadi (2023(, the more and better performance of geopolymer cement observed compared to the lime and Portland cement in kaolinite soil stabilization. The results of scanning electron microscopy revealed the effective performance of geopolymer in bonding soils particles and decrease of void ratio during an artificial cementation process.

Keywords
Kaolinite clay; Glycerol; Geopolymer cement; GGBFS; Sodium hydroxide