Nowadays, filling sand is mainly used for roadbed construction. However, this material resource is gradually depleting. At the same time, sand has loose grain structure and small elastic modulus (E ≤ 40Mpa) therefore it fails to meet the technical requirements of roadbed construction. Consequently, roadbed is often embanked with alternative materials such as soil reinforced with inorganic binders, natural aggregates, and macadam. leading to high cost and environmental pollution. Therefore, it is important to determine a new type of roadbed material. This study presents several laboratory tests on the mechanical and physical properties of sand mixed with fly ash in order to find an appropriate mixture for roadbed construction. The test results show that the mixture of sand and fly ash, which contains an optimal ratio of sand/fly ash of 70%/30% with a dry volume value of 1.821g/cm³, not only meets the requirements for roadbed materials, but also utilizes more fly ash. When adopting Geopolymer technology where alkaline solution is added to reinforce sand-fly ash mixture with the ratio of sand/fly ash of 70%/30%, fly ash will be activated by the alkaline solution to produce gelatinous material through a series of reactions. The gelatinous material is characterized by the high-strength and the tightly-linked molecules. Apart from being a suitable material for roadbed construction, sand fly ash material can also be used as a foundation layer for pavement structure