## 1 Introduction  
Population increase and space restrictions make soil enhancement essential to provide a strong subsurface layer to facilitate infrastructure, buildings and railway construction. High plasticity soil causes collapses in the foundations, road networks and water systems, so it is considered a massive challenge in geotechnical engineering. Moreover, the development of infrastructural and industrial sectors consumed a large number of building materials resulting in a detrimental effect on the environment. Therefore, the need of having a stable and strong subgrade soil layer to carry the applied loads is a challenging task that brought the idea of soil stabilization. The most widespread binders, cement and lime, were used in soft soil stabilization during the past decades due to their efficiency in enhancing soil strength, reducing settlement [1, 2] and controlling shrinkage and swelling [3]. Although they are effective in improving the geotechnical properties of soil, there are some disadvantages accompanying these binders such as cost and environmental effects. For 1 ton of cement production, 1.5-ton of raw materials will be consumed, accompanied by 5.6 GJ/ton energy consumption and approximately 0.95 tons of CO\({}_{2}\) release [4, 5, 6], while lime production releases around 0.86 tons of CO\({}_{2}\) per ton of lime [7]. Moreover, about 7% of all greenhouse gases in the atmosphere are CO\({}_{2}\) emissions caused by cement manufacturing [8, 9]. At the same time, waste disposal--mainly fly ash from plants--has greatly increased in recent years and the majority of the waste is disposed of conventionally: landfills [10]. In recent years, researchers began to look for binders that can provide a solution for waste disposal, replace the disadvantages of calcium-based binders and be environmentally friendly, simultaneously. This developed the idea of what is termed geopolymer to be considered as the next generation. Geopolymer (also described as alkali-activated materials) is an inorganic polymer formed of Al and Si ions alternately tetrahedrally connected with shared oxygen atoms. The general formula for the description of the chemical composition of geopolymers is Mn[-(SiO\({}_{2}\))\({}_{x}\)-AlO\({}_{2}\)]_n_.wH\({}_{2}\)O, in which z can be 1, 2 or 3, M represents the alkali cation (i.e., K or Na) and \(n\) is polymerization degree. Consequently, the structure of geopolymer can be polysialate (-Si-O-Al-O-), polysialate-siloxo (-Si-O-Al-O-Si-O-) or polysialate-disiloxo (-Si-O-Al-O-Si-O-Si-O-), based on the silica amount in the precursor [11]. The geopolymerization process can be summarized as follows [12; 13]: dissolving and breaking of the reactive aluminum and silicate chains in the aluminosilicate material (precursor) because of the pH increase generated by the activator solution, and formation of monomers. Subsequently, polycondensation reactions take place to accumulate and precipitate the resulting products to form an amorphous three-dimensional structure which will tend to crystalize. An enormous amount of geotechnical research has shown the effectiveness of using geopolymers produced from different aluminosilicate materials in soil stabilization such as metakaolin [14; 15], olivine [16], class F fly ash [17], class C fly ash [18] and granulated blast furnace slag [19; 20]. Between the different aluminosilicate precursors, palm oil fuel ash (POFA) utilization merits more recognition due to its high potentiality to produce geopolymer and its abundance as a waste, especially in Southeast Asian countries such as Malaysia [21]. In very limited attempts, POFA, which is agricultural waste, has been employed in geopolymer production for soft soil stabilization purposes. [PERSON] et al. [22] investigated the use of POFA along with potassium hydroxide (KOH) for an alkali activation process to be utilized in clayey soil stabilization. Laboratory tests focused on the shear strength and microstructural tests including scanning electron microscope (SEM) and x-ray diffraction tests. [PERSON] et al.