Strength development of Recycled Asphalt Pavement – Fly ash geopolymer as a road construction material

• Evaluation of Fly Ash (FA) based geopolymer stabilized Recycled Asphalt Pavement (RAP).
• Role of L, NaOH/Na2SiO3, FA, and curing temperature and duration are investigated.
• Microstructural development was examined via XRD and SEM analyses.
• UCS of RAP-FA geopolymers and RAP-FA blends were compared with road authorities’ requirements.

This paper investigates the strength development of Recycled Asphalt Pavement (RAP)-Fly Ash (FA) geopolymer as a road construction material. A mixture of sodium hydroxide solution (NaOH) and sodium silicate solution (Na2SiO3) is used as a liquid alkaline activator (L). Unconfined Compression Strength (UCS) is used as an indicator to measure the strength development of RAP-FA geopolymer and RAP-FA blend (without L). The UCS development is analyzed via Scanning Electron Microscopy (SEM), and X-ray Diffraction (XRD) analyses. Test results show that the compacted RAP-FA blend can be used as a base course material as its UCS values meet the specified strength requirements. The UCS of RAP-FA blends increases with time due to the formation of Calcium Silicate Hydrate (C-S-H) and Calcium Aluminate Hydrate (C-A-H), as detected from XRD and SEM analyses. The UCS of RAP-FA geopolymer increases as the NaOH/Na2SiO3 ratio decreases and is higher than those of compacted RAP-FA blends. When the NaOH/Na2SiO3 ratios are less than 90:10. At an early stage of 7 days and room temperature curing, XRD and SEM analyses indicate that low geopolymerization products (N-A-S-H) in RAP-FA geopolymer are detected when only NaOH (NaOH/Na2SiO3 = 100:0) is used as L, hence the UCS of RAP-FA geopolymer at NaOH/Na2SiO3 = 100:0 is lower than that of RAP-FA blends. With increasing curing time and temperature, NaOH solution dissolves more silica and alumina from FA in the geopolymerization reaction, hence the UCS developed with time and temperature. The highly soluble silica from Na2SiO3 incorporates with leached silica and alumina from FA into a N-A-S-H gel which co-exists with C-S-H and C-A-H from RAP and FA reaction. Therefore, the 7-day UCS values of RAP-FA geopolymer increase with decreasing NaOH/Na2SiO3 ratios for both room temperature and 40 °C curing. This research study confirms the potential of RAP-FA blends and RAP-FA geopolymers as an alternative stabilized pavement material.