Stabilization of fill soil improve its physical and mechanical properties and led to better load carrying capacity and enable steeper temporary slope to be formed during construction period. In this study an industrial by-product, fly-ash (FA), was used as precursor to produce geopolymer as chemical stabilizer to improve the physical and mechanical properties of fill soil. Sodium silicate solution (Na2SiO3) and sodium hydroxide (NaOH) at a concentration of 15 molars were combined as the liquid alkaline activator (AA). The effect of different fly ash to alkaline activator ratio (0, 1.0, 1.5, 2.5 and 3.0) on the physical and mechanical properties of treated fill soil were measured using Atterberg limit test and unconfined compressive strength (UCS) test. All sample were tested at 1 day and 3 days of curing period under room temperature. The result indicates that the highest compressive strength is obtained with fly ash to alkaline activator ratio of 2.5 for both 1 day and 3 days of curing. In addition, the plasticity index of the treated sample for all fly ash/alkaline activator were lower than that of the untreated fill soil.
