# Investigation of Cement and Fly Ash on the Improvement of Fine Sand Soil
## 1 Introduction  
Due to population growth and urbanization, construction on unstable soils plagued by problems like liquefaction, excessive settlement, low bearing capacity, swelling, etc., has become necessary. For this reason, soil improvement methods have gained importance to enhance unsuitable areas in recent years. Soil improvement techniques are categorized as surface and deep soil, mechanical and chemical, or additive and non-additive improvement. Chemical remediation involves injecting or mixing active chemical compounds like fly ash, Portland cement, sodium chloride, lime, or viscoelastic substances such as bitumen [1]. However, in recent years, many studies have been performed to investigate ecological solutions to stabilize soils. Alkaline-activated cements are popular eco-friendly binders [2; 3; 4]; another one is fly ash, both of which have been considered as alternatives to Portland cement in geotechnical applications. Improving geotechnical properties of claye soils with fly ash has been studied for a while in geotechnical engineering. The recent studies on stabilizing silt and clay soils by fly ash were gathered in a paper as a review and it was shown that fly ash can be used as a substitute material to improve the mechanical properties of fine-grained soils [5]. However, studies on the improvement of sand soils with fly ash have been limited; [PERSON] and [PERSON] [6] added fly ash and cement to sand and silt soils and found that cement had a greater effect than fly ash. [PERSON] [7] investigated the impact of fly ash on the CBR values of six types of soils and found that CBR values increased with increasing fly ash content, with the highest increase observed in silty sand soil and the lowest increase seen in high plasticity soil. [PERSON] et al. [8], [PERSON] and [PERSON] [9], [PERSON] and [PERSON] [10] conducted unconfined compression tests on sandy soils and found that fly ash increased the unconfined compressive strength of the soil by the amount of increased fly ash. [PERSON] et al. [11] studied the effect of fly ash and cement on sandy soil's optimum water content and unconfinedcompressive strength, and the highest strength was obtained by adding 25% cement and 30% fly ash after 28 days curing, while the lowest strength was obtained by adding 5% cement and 30% fly ash after 1 day curing. [PERSON] et al. [12; 13] investigated the impact of fly ash on fine sand soil's compaction behavior and CBR values by adding 5%, 10% and 15% F class fly ash with 3% Portland cement. The addition of cement decreased the optimum water content, while the addition of fly ash increased it. Both the curing time and fly ash amount had significant effects on the soil's bearing capacity according to compaction test results; [PERSON] et al. [14] found that adding fly ash to fine sand soils improved strength, with higher amounts of fly ash and longer curing times leading to increase the strength. [PERSON] et al. [15] studied the effect of fly ash on compaction and shear behavior of coarse-grained soils and found that increasing fly ash content and maximum grain size led to an increase in maximum dry density and decreased optimum water content. Additionally, they stated that the shear strength values increased with increasing fly ash content and maximum grain size. The curing conditions and curing duration are also important subjects to discuss regarding the mechanical strength of stabilized soil. It is known that moisture content affects the mechanical properties of soils by which the increment in water content reduces the mechanical properties of the soil. Some recent studies such as [PERSON] and [PERSON] [16] and [PERSON] et al. [17] focused on the curing duration and curing conditions on the mechanical strength of stabilized soil samples. [PERSON] et al. [12] indicated that cement is an effective activator for sandy soil improvement in their study in which they used 3% cement with 5-20% fly ash.