Mechanical Properties of Fly Ash–Asphalt Emulsion Geopolymer Stabilized Crushed Rock for Sustainable Pavement Base

Abstract

Research on the usage of industrial by-products to improve mechanical properties of low-quality materials in infrastructure applications is of global interest. Two industrial by-products, namely, fly ash (FA) and asphalt emulsion (AE), were used in this research to develop an FA-AE stabilized marginal crushed rock (CR) as a sustainable pavement base material. A liquid alkaline activator was used, comprised of a mixture of sodium hydroxide (NaOH) and sodium silicate hydrate (${\mathrm{Na}}_2{\mathrm{SiO}}_3$ ) at ${\mathrm{Na}}_2{\mathrm{SiO}}_3:\mathrm{NaOH}=50:50$ and NaOH concentration = 5 M. In this research study, the mechanical properties of FA-AE stabilized CR were investigated via unconfined compressive strength (UCS), indirect tensile strength (ITS), flexural strength (FS), indirect tensile resilient modulus (IT $M_r$ ), and indirect tensile fatigue life (ITFL) tests. The UCS of FA geopolymer stabilized CR (without AE) was found to be dependent on the FA content and curing time. For all the curing times studied, the higher FA replacement ratio resulted in higher UCS values. Although the asphalt film enhances particle confinement, it was found to retard the geopolymerization reaction. According to the local road authority, which requires a $\mathrm{UCS}>1.75\mathrm{MPa}$ for a stabilized base course, the addition of AE reduced the UCS of FA-AE geopolymer stabilized CR. To meet the minimum 7-day strength requirement for both low and high-traffic roads, a geopolymer mix was suggested with an FA replacement $\mathrm{ratio}\ge 20\%$ and an $\mathrm{AE}\mathrm{content}\ge 1\%$ . Compared with the cement stabilized CR at a similar UCS value, the FA-AE geopolymer stabilized CR had higher FS, ITS, IT $M_r$ , and ITFL but lower carbon footprints.