Strength and Microstructural Study of Recycled Asphalt Pavement: Slag Geopolymer as a Pavement Base Material

Abstract

The integrated transdisciplinary concepts and technologies for greener and more sustainable innovations for recycling waste materials through geopolymer technique into construction materials were developed in this research. The objective of this research was to study the strength and microstructural development of slag (S)-based geopolymer-stabilized recycled asphalt pavement (RAP) as a pavement base material. An attempt was made to study the influence of a liquid alkaline activator (L=NaOH/Na2SiO3 ratio) on the strength development of RAP-S geopolymers, which was evaluated by an unconfined compressive strength (UCS) test and examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The XRD and SEM analyses results indicated that the main geopolymerization reaction product of RAP-S geopolymers was calcium alumino-silicate hydrate (C-A-S-H). Without Na2SiO3 (L=NaOH/Na2SiO3=1:), the S-geopolymer exhibited a slow geopolymerization reaction. The silica presented in Na2SiO3 was highly soluble; hence, a suitable content of NaOH/Na2SiO3=6:4 can produce RAP+2%S geopolymers as a base material whose 7-day UCS value met the minimum strength requirements specified by the Department of Highways, Thailand. However, when excess Na2SiO3 content (NaOH/Na2SiO3=5:5) was used, the presence of silica retarded the geopolymerization formation rate, leading to UCS reduction. The outcomes of this research were found to be useful for making progress in innovative technologies and applications of geopolymers, which strongly reflected the improvements made in the practical applications of the S-geopolymer as a cleaner production by using the recycled waste material in road work.